-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcredemo program. There are separate text files for the pcregrep and
pcretest commands.
-----------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions


INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a few differences. Some features that appeared in Python and PCRE
       before they appeared in Perl are also available using the  Python  syn-
       tax,  there  is  some  support for one or two .NET and Oniguruma syntax
       items, and there is an option for requesting some  minor  changes  that
       give better JavaScript compatibility.

       Starting with release 8.30, it is possible to compile two separate PCRE
       libraries:  the  original,  which  supports  8-bit  character   strings
       (including  UTF-8  strings),  and a second library that supports 16-bit
       character strings (including UTF-16 strings). The build process  allows
       either  one  or both to be built. The majority of the work to make this
       possible was done by Zoltan Herczeg.

       The two libraries contain identical sets of functions, except that  the
       names  in  the  16-bit  library start with pcre16_ instead of pcre_. To
       avoid over-complication and reduce the documentation maintenance  load,
       most of the documentation describes the 8-bit library, with the differ-
       ences for the 16-bit library described separately in the  pcre16  page.
       References  to  functions or structures of the form pcre[16]_xxx should
       be  read  as  meaning  "pcre_xxx  when  using  the  8-bit  library  and
       pcre16_xxx when using the 16-bit library".

       The  current implementation of PCRE corresponds approximately with Perl
       5.12, including support for UTF-8/16 encoded strings and  Unicode  gen-
       eral  category properties. However, UTF-8/16 and Unicode support has to
       be explicitly enabled; it is not the default. The Unicode tables corre-
       spond to Unicode release 6.0.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative function that matches the same compiled patterns in a  dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.  For a discussion of the two matching algorithms,  see  the
       pcrematching page.

       PCRE  is  written  in C and released as a C library. A number of people
       have written wrappers and interfaces of various kinds.  In  particular,
       Google  Inc.   have  provided a comprehensive C++ wrapper for the 8-bit
       library. This is now included as part of  the  PCRE  distribution.  The
       pcrecpp  page  has  details of this interface. Other people's contribu-
       tions can be found in the Contrib directory at the  primary  FTP  site,
       which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details  of  exactly which Perl regular expression features are and are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern  and pcrecompat pages. There is a syntax summary in the pcresyntax
       page.

       Some features of PCRE can be included, excluded, or  changed  when  the
       library  is  built.  The pcre_config() function makes it possible for a
       client to discover which features are  available.  The  features  them-
       selves  are described in the pcrebuild page. Documentation about build-
       ing PCRE for various operating systems can be found in the  README  and
       NON-UNIX-USE files in the source distribution.

       The  libraries contains a number of undocumented internal functions and
       data tables that are used by more than one  of  the  exported  external
       functions,  but  which  are  not  intended for use by external callers.
       Their names all begin with "_pcre_" or "_pcre16_", which hopefully will
       not  provoke  any name clashes. In some environments, it is possible to
       control which external symbols are exported when a  shared  library  is
       built, and in these cases the undocumented symbols are not exported.


USER DOCUMENTATION

       The  user  documentation  for PCRE comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain text format, all the sections, except  the  pcredemo  sec-
       tion, are concatenated, for ease of searching. The sections are as fol-
       lows:

         pcre              this document
         pcre16            details of the 16-bit library
         pcre-config       show PCRE installation configuration information
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper for the 8-bit library
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command (8-bit only)
         pcrejit           discussion of the just-in-time optimization support
         pcrelimits        details of size and other limits
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API for the 8-bit library
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         pcretest          description of the pcretest testing command
         pcreunicode       discussion of Unicode and UTF-8/16 support

       In addition, in the "man" and HTML formats, there is a short  page  for
       each 8-bit C library function, listing its arguments and results.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

       Putting  an actual email address here seems to have been a spam magnet,
       so I've taken it away. If you want to email me, use  my  two  initials,
       followed by the two digits 10, at the domain cam.ac.uk.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE 16-BIT API BASIC FUNCTIONS

       pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16_extra *pcre16_study(const pcre16 *code, int options,
            const char **errptr);

       void pcre16_free_study(pcre16_extra *extra);

       int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE 16-BIT API STRING EXTRACTION FUNCTIONS

       int pcre16_copy_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_UCHAR16 *buffer, int buffersize);

       int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
            int buffersize);

       int pcre16_get_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_stringnumber(const pcre16 *code,
            PCRE_SPTR16 name);

       int pcre16_get_stringtable_entries(const pcre16 *code,
            PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);

       int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_substring_list(PCRE_SPTR16 subject,
            int *ovector, int stringcount, PCRE_SPTR16 **listptr);

       void pcre16_free_substring(PCRE_SPTR16 stringptr);

       void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);


PCRE 16-BIT API AUXILIARY FUNCTIONS

       pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);

       void pcre16_jit_stack_free(pcre16_jit_stack *stack);

       void pcre16_assign_jit_stack(pcre16_extra *extra,
            pcre16_jit_callback callback, void *data);

       const unsigned char *pcre16_maketables(void);

       int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
            int what, void *where);

       int pcre16_refcount(pcre16 *code, int adjust);

       int pcre16_config(int what, void *where);

       const char *pcre16_version(void);

       int pcre16_pattern_to_host_byte_order(pcre16 *code,
            pcre16_extra *extra, const unsigned char *tables);


PCRE 16-BIT API INDIRECTED FUNCTIONS

       void *(*pcre16_malloc)(size_t);

       void (*pcre16_free)(void *);

       void *(*pcre16_stack_malloc)(size_t);

       void (*pcre16_stack_free)(void *);

       int (*pcre16_callout)(pcre16_callout_block *);


PCRE 16-BIT API 16-BIT-ONLY FUNCTION

       int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
            PCRE_SPTR16 input, int length, int *byte_order,
            int keep_boms);


THE PCRE 16-BIT LIBRARY

       Starting  with  release  8.30, it is possible to compile a PCRE library
       that supports 16-bit character strings, including  UTF-16  strings,  as
       well  as  or instead of the original 8-bit library. The majority of the
       work to make  this  possible  was  done  by  Zoltan  Herczeg.  The  two
       libraries contain identical sets of functions, used in exactly the same
       way. Only the names of the functions and the data types of their  argu-
       ments  and results are different. To avoid over-complication and reduce
       the documentation maintenance load,  most  of  the  PCRE  documentation
       describes  the  8-bit  library,  with only occasional references to the
       16-bit library. This page describes what is different when you use  the
       16-bit library.

       WARNING:  A  single  application can be linked with both libraries, but
       you must take care when processing any particular pattern to use  func-
       tions  from  just one library. For example, if you want to study a pat-
       tern that was compiled with  pcre16_compile(),  you  must  do  so  with
       pcre16_study(), not pcre_study(), and you must free the study data with
       pcre16_free_study().


THE HEADER FILE

       There is only one header file, pcre.h. It contains prototypes  for  all
       the  functions  in  both  libraries,  as  well as definitions of flags,
       structures, error codes, etc.


THE LIBRARY NAME

       In Unix-like systems, the 16-bit library is called libpcre16,  and  can
       normally  be  accesss  by adding -lpcre16 to the command for linking an
       application that uses PCRE.


STRING TYPES

       In the 8-bit library, strings are passed to PCRE library  functions  as
       vectors  of  bytes  with  the  C  type "char *". In the 16-bit library,
       strings are passed as vectors of unsigned 16-bit quantities. The  macro
       PCRE_UCHAR16  specifies  an  appropriate  data type, and PCRE_SPTR16 is
       defined as "const PCRE_UCHAR16 *". In very  many  environments,  "short
       int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
       as "short int", but checks that it really is a 16-bit data type. If  it
       is not, the build fails with an error message telling the maintainer to
       modify the definition appropriately.


STRUCTURE TYPES

       The types of the opaque structures that are used  for  compiled  16-bit
       patterns  and  JIT stacks are pcre16 and pcre16_jit_stack respectively.
       The  type  of  the  user-accessible  structure  that  is  returned   by
       pcre16_study()  is  pcre16_extra, and the type of the structure that is
       used for passing data to a callout  function  is  pcre16_callout_block.
       These structures contain the same fields, with the same names, as their
       8-bit counterparts. The only difference is that pointers  to  character
       strings are 16-bit instead of 8-bit types.


16-BIT FUNCTIONS

       For  every function in the 8-bit library there is a corresponding func-
       tion in the 16-bit library with a name that starts with pcre16_ instead
       of  pcre_.  The  prototypes are listed above. In addition, there is one
       extra function, pcre16_utf16_to_host_byte_order(). This  is  a  utility
       function  that converts a UTF-16 character string to host byte order if
       necessary. The other 16-bit  functions  expect  the  strings  they  are
       passed to be in host byte order.

       The input and output arguments of pcre16_utf16_to_host_byte_order() may
       point to the same address, that is, conversion in place  is  supported.
       The output buffer must be at least as long as the input.

       The  length  argument  specifies the number of 16-bit data units in the
       input string; a negative value specifies a zero-terminated string.

       If byte_order is NULL, it is assumed that the string starts off in host
       byte  order. This may be changed by byte-order marks (BOMs) anywhere in
       the string (commonly as the first character).

       If byte_order is not NULL, a non-zero value of the integer to which  it
       points  means  that  the input starts off in host byte order, otherwise
       the opposite order is assumed. Again, BOMs in  the  string  can  change
       this. The final byte order is passed back at the end of processing.

       If  keep_boms  is  not  zero,  byte-order  mark characters (0xfeff) are
       copied into the output string. Otherwise they are discarded.

       The result of the function is the number of 16-bit  units  placed  into
       the  output  buffer,  including  the  zero terminator if the string was
       zero-terminated.


SUBJECT STRING OFFSETS

       The offsets within subject strings that are returned  by  the  matching
       functions are in 16-bit units rather than bytes.


NAMED SUBPATTERNS

       The  name-to-number translation table that is maintained for named sub-
       patterns uses 16-bit characters.  The  pcre16_get_stringtable_entries()
       function returns the length of each entry in the table as the number of
       16-bit data units.


OPTION NAMES

       There   are   two   new   general   option   names,   PCRE_UTF16    and
       PCRE_NO_UTF16_CHECK,     which     correspond    to    PCRE_UTF8    and
       PCRE_NO_UTF8_CHECK in the 8-bit library. In  fact,  these  new  options
       define the same bits in the options word.

       For  the  pcre16_config() function there is an option PCRE_CONFIG_UTF16
       that returns 1 if UTF-16 support is configured, otherwise  0.  If  this
       option  is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option is
       given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error.


CHARACTER CODES

       In 16-bit mode, when  PCRE_UTF16  is  not  set,  character  values  are
       treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
       that they can range from 0 to 0xffff instead of 0  to  0xff.  Character
       types  for characters less than 0xff can therefore be influenced by the
       locale in the same way as before.  Characters greater  than  0xff  have
       only one case, and no "type" (such as letter or digit).

       In  UTF-16  mode,  the  character  code  is  Unicode, in the range 0 to
       0x10ffff, with the exception of values in the range  0xd800  to  0xdfff
       because  those  are "surrogate" values that are used in pairs to encode
       values greater than 0xffff.

       A UTF-16 string can indicate its endianness by special code knows as  a
       byte-order mark (BOM). The PCRE functions do not handle this, expecting
       strings  to  be  in  host  byte  order.  A  utility   function   called
       pcre16_utf16_to_host_byte_order()  is  provided  to help with this (see
       above).


ERROR NAMES

       The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16  corre-
       spond  to  their  8-bit  counterparts.  The error PCRE_ERROR_BADMODE is
       given when a compiled pattern is passed to a  function  that  processes
       patterns  in  the  other  mode, for example, if a pattern compiled with
       pcre_compile() is passed to pcre16_exec().

       There are new error codes whose names  begin  with  PCRE_UTF16_ERR  for
       invalid  UTF-16  strings,  corresponding to the PCRE_UTF8_ERR codes for
       UTF-8 strings that are described in the section entitled "Reason  codes
       for  invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors
       are:

         PCRE_UTF16_ERR1  Missing low surrogate at end of string
         PCRE_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE_UTF16_ERR3  Isolated low surrogate
         PCRE_UTF16_ERR4  Invalid character 0xfffe


ERROR TEXTS

       If there is an error while compiling a pattern, the error text that  is
       passed  back by pcre16_compile() or pcre16_compile2() is still an 8-bit
       character string, zero-terminated.


CALLOUTS

       The subject and mark fields in the callout block that is  passed  to  a
       callout function point to 16-bit vectors.


TESTING

       The  pcretest  program continues to operate with 8-bit input and output
       files, but it can be used for testing the 16-bit library. If it is  run
       with the command line option -16, patterns and subject strings are con-
       verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
       library  functions  are used instead of the 8-bit ones. Returned 16-bit
       strings are converted to 8-bit for output. If the 8-bit library was not
       compiled, pcretest defaults to 16-bit and the -16 option is ignored.

       When  PCRE  is  being built, the RunTest script that is called by "make
       check" uses the pcretest -C option to discover which of the  8-bit  and
       16-bit libraries has been built, and runs the tests appropriately.


NOT SUPPORTED IN 16-BIT MODE

       Not all the features of the 8-bit library are available with the 16-bit
       library. The C++ and POSIX wrapper functions  support  only  the  8-bit
       library, and the pcregrep program is at present 8-bit only.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREBUILD(3)                                                      PCREBUILD(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of cmake-gui if you are using CMake
       instead of configure to build PCRE.

       There  is  a  lot more information about building PCRE in non-Unix-like
       environments in the file called NON_UNIX_USE, which is part of the PCRE
       distribution.  You  should consult this file as well as the README file
       if you are building in a non-Unix-like environment.

       The complete list of options for configure (which includes the standard
       ones  such  as  the  selection  of  the  installation directory) can be
       obtained by running

         ./configure --help

       The following sections include  descriptions  of  options  whose  names
       begin with --enable or --disable. These settings specify changes to the
       defaults for the configure command. Because of the way  that  configure
       works,  --enable  and --disable always come in pairs, so the complemen-
       tary option always exists as well, but as it specifies the default,  it
       is not described.


BUILDING 8-BIT and 16-BIT LIBRARIES

       By  default,  a  library  called libpcre is built, containing functions
       that take string arguments contained in vectors  of  bytes,  either  as
       single-byte  characters,  or interpreted as UTF-8 strings. You can also
       build a separate library, called libpcre16, in which strings  are  con-
       tained  in  vectors of 16-bit data units and interpreted either as sin-
       gle-unit characters or UTF-16 strings, by adding

         --enable-pcre16

       to the configure command. If you do not want the 8-bit library, add

         --disable-pcre8

       as well. At least one of the two libraries must be built. Note that the
       C++  and  POSIX wrappers are for the 8-bit library only, and that pcre-
       grep is an 8-bit program. None of these are built if  you  select  only
       the 16-bit library.


BUILDING SHARED AND STATIC LIBRARIES

       The  PCRE building process uses libtool to build both shared and static
       Unix libraries by default. You can suppress one of these by adding  one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


C++ SUPPORT

       By  default,  if the 8-bit library is being built, the configure script
       will search for a C++ compiler and C++ header files. If it finds  them,
       it  automatically  builds  the C++ wrapper library (which supports only
       8-bit strings). You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8 and UTF-16 SUPPORT

       To build PCRE with support for UTF Unicode character strings, add

         --enable-utf

       to the configure command.  This  setting  applies  to  both  libraries,
       adding support for UTF-8 to the 8-bit library and support for UTF-16 to
       the 16-bit library. There are no separate options  for  enabling  UTF-8
       and  UTF-16  independently because that would allow ridiculous settings
       such as  requesting  UTF-16  support  while  building  only  the  8-bit
       library.  It  is not possible to build one library with UTF support and
       the other without in the same configuration. (For backwards compatibil-
       ity, --enable-utf8 is a synonym of --enable-utf.)

       Of  itself,  this  setting does not make PCRE treat strings as UTF-8 or
       UTF-16. As well as compiling PCRE with this option, you also have  have
       to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat-
       tern compiling functions.

       If you set --enable-utf when compiling in an EBCDIC  environment,  PCRE
       expects its input to be either ASCII or UTF-8 (depending on the runtime
       option). It is not possible to support both EBCDIC and UTF-8  codes  in
       the  same  version  of  the  library.  Consequently,  --enable-utf  and
       --enable-ebcdic are mutually exclusive.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF support allows the libraries to process character codepoints up  to
       0x10ffff  in the strings that they handle. On its own, however, it does
       not provide any facilities for accessing the properties of such charac-
       ters. If you want to be able to use the pattern escapes \P, \p, and \X,
       which refer to Unicode character properties, you must add

         --enable-unicode-properties

       to the configure command. This implies UTF support, even  if  you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 30K of tables to the
       PCRE library. Only the general category properties such as  Lu  and  Nd
       are supported. Details are given in the pcrepattern documentation.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time compiler support is included in the build by specifying

         --enable-jit

       This  support  is available only for certain hardware architectures. If
       this option is set for an  unsupported  architecture,  a  compile  time
       error  occurs.   See  the pcrejit documentation for a discussion of JIT
       usage. When JIT support is enabled, pcregrep automatically makes use of
       it, unless you add

         --disable-pcregrep-jit

       to the "configure" command.


CODE VALUE OF NEWLINE

       By  default,  PCRE interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You  can compile PCRE to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the  configure  command.  There  is  also  a  --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which causes PCRE to recognize any of the three sequences  CR,  LF,  or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE to recognize any Unicode newline sequence.

       Whatever  line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time  it  is
       conventional to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


POSIX MALLOC USAGE

       When  the  8-bit library is called through the POSIX interface (see the
       pcreposix documentation), additional working storage  is  required  for
       holding  the  pointers  to  capturing substrings, because PCRE requires
       three integers per substring, whereas the POSIX interface provides only
       two.  If  the number of expected substrings is small, the wrapper func-
       tion uses space on the stack, because this is faster  than  using  mal-
       loc()  for each call. The default threshold above which the stack is no
       longer used is 10; it can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, two-byte values are used  for  these
       offsets,  leading  to  a  maximum size for a compiled pattern of around
       64K. This is sufficient to handle all but the most  gigantic  patterns.
       Nevertheless,  some  people do want to process truly enormous patterns,
       so it is possible to compile PCRE to use three-byte or  four-byte  off-
       sets by adding a setting such as

         --with-link-size=3

       to  the  configure command. The value given must be 2, 3, or 4. For the
       16-bit library, a value of 3 is rounded up to 4. Using  longer  offsets
       slows down the operation of PCRE because it has to load additional data
       when handling them.


AVOIDING EXCESSIVE STACK USAGE

       When matching with the pcre_exec() function, PCRE implements backtrack-
       ing  by  making recursive calls to an internal function called match().
       In environments where the size of the stack is limited,  this  can  se-
       verely  limit  PCRE's operation. (The Unix environment does not usually
       suffer from this problem, but it may sometimes be necessary to increase
       the  maximum  stack size.  There is a discussion in the pcrestack docu-
       mentation.) An alternative approach to recursion that uses memory  from
       the  heap  to remember data, instead of using recursive function calls,
       has been implemented to work round the problem of limited  stack  size.
       If you want to build a version of PCRE that works this way, add

         --disable-stack-for-recursion

       to  the  configure  command. With this configuration, PCRE will use the
       pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
       ment  functions. By default these point to malloc() and free(), but you
       can replace the pointers so that your own functions are used instead.

       Separate functions are  provided  rather  than  using  pcre_malloc  and
       pcre_free  because  the  usage  is  very  predictable:  the block sizes
       requested are always the same, and  the  blocks  are  always  freed  in
       reverse  order.  A calling program might be able to implement optimized
       functions that perform better  than  malloc()  and  free().  PCRE  runs
       noticeably more slowly when built in this way. This option affects only
       the pcre_exec() function; it is not relevant for pcre_dfa_exec().


LIMITING PCRE RESOURCE USAGE

       Internally, PCRE has a function called match(), which it calls  repeat-
       edly   (sometimes   recursively)  when  matching  a  pattern  with  the
       pcre_exec() function. By controlling the maximum number of  times  this
       function  may be called during a single matching operation, a limit can
       be placed on the resources used by a single call  to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen-
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as

         --with-match-limit=500000

       to   the   configure  command.  This  setting  has  no  effect  on  the
       pcre_dfa_exec() matching function.

       In some environments it is desirable to limit the  depth  of  recursive
       calls of match() more strictly than the total number of calls, in order
       to restrict the maximum amount of stack (or heap,  if  --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it defaults to the value that  is  set  for  --with-match-limit,  which
       imposes  no  additional constraints. However, you can set a lower limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to the configure command. This value can  also  be  overridden  at  run
       time.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE  uses fixed tables for processing characters whose code values are
       less than 256. By default, PCRE is built with a set of tables that  are
       distributed  in  the  file pcre_chartables.c.dist. These tables are for
       ASCII codes only. If you add

         --enable-rebuild-chartables

       to the configure command, the distributed tables are  no  longer  used.
       Instead,  a  program  called dftables is compiled and run. This outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you are cross compiling, because dftables is run on the local host.  If
       you  need  to  create alternative tables when cross compiling, you will
       have to do so "by hand".)


USING EBCDIC CODE

       PCRE assumes by default that it will run in an  environment  where  the
       character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
       This is the case for most computer operating systems.  PCRE  can,  how-
       ever, be compiled to run in an EBCDIC environment by adding

         --enable-ebcdic

       to the configure command. This setting implies --enable-rebuild-charta-
       bles. You should only use it if you know that  you  are  in  an  EBCDIC
       environment  (for  example,  an  IBM  mainframe  operating system). The
       --enable-ebcdic option is incompatible with --enable-utf.


PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

         --enable-pcregrep-libz
         --enable-pcregrep-libbz2

       to the configure command. These options naturally require that the rel-
       evant  libraries  are installed on your system. Configuration will fail
       if they are not.


PCREGREP BUFFER SIZE

       pcregrep uses an internal buffer to hold a "window" on the file  it  is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The size of the buffer is controlled by  a  parameter
       whose default value is 20K. The buffer itself is three times this size,
       but because of the way it is used for holding "before" lines, the long-
       est  line  that  is guaranteed to be processable is the parameter size.
       You can change the default parameter value by adding, for example,

         --with-pcregrep-bufsize=50K

       to the configure command. The caller of pcregrep can, however, override
       this value by specifying a run-time option.


PCRETEST OPTION FOR LIBREADLINE SUPPORT

       If you add

         --enable-pcretest-libreadline

       to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licensed, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

       Setting  this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some  extra
       configuration  may  be necessary. The INSTALL file for libreadline says
       this:

         "Readline uses the termcap functions, but does not link with the
         termcap or curses library itself, allowing applications which link
         with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


SEE ALSO

       pcreapi(3), pcre16, pcre_config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 07 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREMATCHING(3)                                                PCREMATCHING(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() and pcre16_exec() functions. These work in the same was  as
       Perl's matching function, and provide a Perl-compatible matching opera-
       tion. The just-in-time (JIT) optimization  that  is  described  in  the
       pcrejit documentation is compatible with these functions.

       An  alternative  algorithm  is  provided  by  the  pcre_dfa_exec()  and
       pcre16_dfa_exec() functions; they operate in a different way,  and  are
       not  Perl-compatible. This alternative has advantages and disadvantages
       compared with the standard algorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding  the  current  point  have to be independently
       inspected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  The  matches are returned in decreasing order of length. There is
       an option to stop the algorithm after the first match (which is  neces-
       sarily the shortest) is found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment",  the  result
       will  be the three strings "caterpillar", "cater", and "cat" that start
       at the fifth character of the subject. The algorithm does not automati-
       cally move on to find matches that start at later positions.

       There are a number of features of PCRE regular expressions that are not
       supported by the alternative matching algorithm. They are as follows:

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
       ungreedy quantifiers are treated in exactly the same way. However, pos-
       sessive  quantifiers can make a difference when what follows could also
       match what is quantified, for example in a pattern like this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and  PCRE's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, back references within the  pat-
       tern are not supported, and cause errors if encountered.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5.  Because  many  paths  through the tree may be active, the \K escape
       sequence, which resets the start of the match when encountered (but may
       be  on  some  paths  and not on others), is not supported. It causes an
       error if encountered.

       6. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always -1.

       7.  The  \C  escape  sequence, which (in the standard algorithm) always
       matches a single data unit, even in UTF-8 or UTF-16 modes, is not  sup-
       ported  in these modes, because the alternative algorithm moves through
       the subject string one character (not data unit) at  a  time,  for  all
       active paths through the tree.

       8.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using  the alternative matching algorithm provides the following advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically  found,  and  in particular, the longest match is found. To find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2.  Because  the  alternative  algorithm  scans the subject string just
       once, and never needs to backtrack (except for lookbehinds), it is pos-
       sible  to  pass  very  long subject strings to the matching function in
       several pieces, checking for partial matching each time. Although it is
       possible  to  do multi-segment matching using the standard algorithm by
       retaining partially matched substrings, it  is  more  complicated.  The
       pcrepartial  documentation  gives  details of partial matching and dis-
       cusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1. It is substantially slower than  the  standard  algorithm.  This  is
       partly  because  it has to search for all possible matches, but is also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE NATIVE API BASIC FUNCTIONS

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

       void pcre_free_study(pcre_extra *extra);

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);


PCRE NATIVE API AUXILIARY FUNCTIONS

       pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);

       void pcre_jit_stack_free(pcre_jit_stack *stack);

       void pcre_assign_jit_stack(pcre_extra *extra,
            pcre_jit_callback callback, void *data);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       const char *pcre_version(void);

       int pcre_pattern_to_host_byte_order(pcre *code,
            pcre_extra *extra, const unsigned char *tables);


PCRE NATIVE API INDIRECTED FUNCTIONS

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


PCRE 8-BIT AND 16-BIT LIBRARIES

       From  release  8.30,  PCRE  can  be  compiled as a library for handling
       16-bit character strings as  well  as,  or  instead  of,  the  original
       library that handles 8-bit character strings. To avoid too much compli-
       cation, this document describes the 8-bit versions  of  the  functions,
       with only occasional references to the 16-bit library.

       The  16-bit  functions  operate in the same way as their 8-bit counter-
       parts; they just use different  data  types  for  their  arguments  and
       results, and their names start with pcre16_ instead of pcre_. For every
       option that has UTF8 in its name (for example, PCRE_UTF8), there  is  a
       corresponding 16-bit name with UTF8 replaced by UTF16. This facility is
       in fact just cosmetic; the 16-bit option names define the same bit val-
       ues.

       References to bytes and UTF-8 in this document should be read as refer-
       ences to 16-bit data  quantities  and  UTF-16  when  using  the  16-bit
       library,  unless specified otherwise. More details of the specific dif-
       ferences for the 16-bit library are given in the pcre16 page.


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       are  also some wrapper functions (for the 8-bit library only) that cor-
       respond to the POSIX regular expression  API,  but  they  do  not  give
       access  to  all  the functionality. They are described in the pcreposix
       documentation. Both of these APIs define a set of C function  calls.  A
       C++ wrapper (again for the 8-bit library only) is also distributed with
       PCRE. It is documented in the pcrecpp page.

       The native API C function prototypes are defined  in  the  header  file
       pcre.h,  and  on Unix-like systems the (8-bit) library itself is called
       libpcre. It can normally be accessed by adding -lpcre  to  the  command
       for  linking an application that uses PCRE. The header file defines the
       macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release
       numbers  for the library. Applications can use these to include support
       for different releases of PCRE.

       In a Windows environment, if you want to statically link an application
       program  against  a  non-dll  pcre.a  file, you must define PCRE_STATIC
       before including pcre.h or pcrecpp.h, because otherwise  the  pcre_mal-
       loc()   and   pcre_free()   exported   functions   will   be   declared
       __declspec(dllimport), with unwanted results.

       The  functions  pcre_compile(),  pcre_compile2(),   pcre_study(),   and
       pcre_exec()  are used for compiling and matching regular expressions in
       a Perl-compatible manner. A sample program that demonstrates  the  sim-
       plest  way  of  using them is provided in the file called pcredemo.c in
       the PCRE source distribution. A listing of this program is given in the
       pcredemo  documentation, and the pcresample documentation describes how
       to compile and run it.

       Just-in-time compiler support is an optional feature of PCRE  that  can
       be built in appropriate hardware environments. It greatly speeds up the
       matching performance of  many  patterns.  Simple  programs  can  easily
       request  that  it  be  used  if available, by setting an option that is
       ignored when it is not relevant. More complicated programs  might  need
       to     make    use    of    the    functions    pcre_jit_stack_alloc(),
       pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to  control
       the  JIT  code's  memory  usage.   These functions are discussed in the
       pcrejit documentation.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble,  is  also provided. This uses a different algorithm for the match-
       ing. The alternative algorithm finds all possible matches (at  a  given
       point  in  the  subject), and scans the subject just once (unless there
       are lookbehind assertions). However, this  algorithm  does  not  return
       captured  substrings.  A description of the two matching algorithms and
       their advantages and disadvantages is given in the  pcrematching  docu-
       mentation.

       In  addition  to  the  main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

       The function pcre_maketables() is used to  build  a  set  of  character
       tables   in   the   current   locale  for  passing  to  pcre_compile(),
       pcre_exec(), or pcre_dfa_exec(). This is an optional facility  that  is
       provided  for  specialist  use.  Most  commonly,  no special tables are
       passed, in which case internal tables that are generated when  PCRE  is
       built are used.

       The  function  pcre_fullinfo()  is used to find out information about a
       compiled pattern. The function pcre_version() returns a  pointer  to  a
       string containing the version of PCRE and its date of release.

       The  function  pcre_refcount()  maintains  a  reference count in a data
       block containing a compiled pattern. This is provided for  the  benefit
       of object-oriented applications.

       The  global  variables  pcre_malloc and pcre_free initially contain the
       entry points of the standard malloc()  and  free()  functions,  respec-
       tively. PCRE calls the memory management functions via these variables,
       so a calling program can replace them if it  wishes  to  intercept  the
       calls. This should be done before calling any PCRE functions.

       The  global  variables  pcre_stack_malloc  and pcre_stack_free are also
       indirections to memory management functions.  These  special  functions
       are  used  only  when  PCRE is compiled to use the heap for remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function.  See  the  pcrebuild  documentation  for details of how to do
       this. It is a non-standard way of building PCRE, for  use  in  environ-
       ments  that  have  limited stacks. Because of the greater use of memory
       management, it runs more slowly. Separate  functions  are  provided  so
       that  special-purpose  external  code  can  be used for this case. When
       used, these functions are always called in a  stack-like  manner  (last
       obtained,  first freed), and always for memory blocks of the same size.
       There is a discussion about PCRE's stack usage in the  pcrestack  docu-
       mentation.

       The global variable pcre_callout initially contains NULL. It can be set
       by the caller to a "callout" function, which PCRE  will  then  call  at
       specified  points during a matching operation. Details are given in the
       pcrecallout documentation.


NEWLINES

       PCRE supports five different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or any Unicode newline sequence. The Unicode newline sequences
       are the three just mentioned, plus the single characters  VT  (vertical
       tab,  U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each of the first three conventions is used by at least  one  operating
       system  as its standard newline sequence. When PCRE is built, a default
       can be specified.  The default default is LF, which is the  Unix  stan-
       dard.  When  PCRE  is run, the default can be overridden, either when a
       pattern is compiled, or when it is matched.

       At compile time, the newline convention can be specified by the options
       argument  of  pcre_compile(), or it can be specified by special text at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter or pair of characters that indicate a line break". The choice  of
       newline  convention  affects  the  handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF  is a recognized line ending sequence, the match position advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

       The  choice of newline convention does not affect the interpretation of
       the \n or \r escape sequences, nor does  it  affect  what  \R  matches,
       which is controlled in a similar way, but by separate options.


MULTITHREADING

       The  PCRE  functions  can be used in multi-threading applications, with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

       The compiled form of a regular expression is not altered during  match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.

       If the just-in-time optimization feature is being used, it needs  sepa-
       rate  memory stack areas for each thread. See the pcrejit documentation
       for more details.


SAVING PRECOMPILED PATTERNS FOR LATER USE

       The compiled form of a regular expression can be saved and re-used at a
       later  time,  possibly by a different program, and even on a host other
       than the one on which  it  was  compiled.  Details  are  given  in  the
       pcreprecompile  documentation,  which  includes  a  description  of the
       pcre_pattern_to_host_byte_order() function. However, compiling a  regu-
       lar  expression  with one version of PCRE for use with a different ver-
       sion is not guaranteed to work and may cause crashes.


CHECKING BUILD-TIME OPTIONS

       int pcre_config(int what, void *where);

       The function pcre_config() makes it possible for a PCRE client to  dis-
       cover which optional features have been compiled into the PCRE library.
       The pcrebuild documentation has more details about these optional  fea-
       tures.

       The  first  argument  for pcre_config() is an integer, specifying which
       information is required; the second argument is a pointer to a variable
       into  which  the  information  is placed. The returned value is zero on
       success, or the negative error code PCRE_ERROR_BADOPTION if  the  value
       in  the  first argument is not recognized. The following information is
       available:

         PCRE_CONFIG_UTF8

       The output is an integer that is set to one if UTF-8 support is  avail-
       able;  otherwise  it  is  set  to  zero. If this option is given to the
       16-bit  version  of  this  function,  pcre16_config(),  the  result  is
       PCRE_ERROR_BADOPTION.

         PCRE_CONFIG_UTF16

       The output is an integer that is set to one if UTF-16 support is avail-
       able; otherwise it is set to zero. This value should normally be  given
       to the 16-bit version of this function, pcre16_config(). If it is given
       to the 8-bit version of this function, the result is  PCRE_ERROR_BADOP-
       TION.

         PCRE_CONFIG_UNICODE_PROPERTIES

       The  output  is  an  integer  that is set to one if support for Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_JIT

       The output is an integer that is set to one if support for just-in-time
       compiling is available; otherwise it is set to zero.

         PCRE_CONFIG_JITTARGET

       The  output is a pointer to a zero-terminated "const char *" string. If
       JIT support is available, the string contains the name of the architec-
       ture  for  which the JIT compiler is configured, for example "x86 32bit
       (little endian + unaligned)". If JIT  support  is  not  available,  the
       result is NULL.

         PCRE_CONFIG_NEWLINE

       The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The four values  that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and -1 for ANY.  Though they are derived from ASCII,  the  same  values
       are returned in EBCDIC environments. The default should normally corre-
       spond to the standard sequence for your operating system.

         PCRE_CONFIG_BSR

       The output is an integer whose value indicates what character sequences
       the  \R  escape sequence matches by default. A value of 0 means that \R
       matches any Unicode line ending sequence; a value of 1  means  that  \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

         PCRE_CONFIG_LINK_SIZE

       The output is an integer that contains the number  of  bytes  used  for
       internal  linkage  in  compiled  regular  expressions.  For  the  8-bit
       library, the value can be 2, 3, or 4. For the 16-bit library, the value
       is either 2 or 4 and is still a number of bytes. The default value of 2
       is sufficient for all but the most massive patterns,  since  it  allows
       the  compiled  pattern  to  be  up to 64K in size.  Larger values allow
       larger regular expressions to be compiled, at  the  expense  of  slower
       matching.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       The  output  is  an integer that contains the threshold above which the
       POSIX interface uses malloc() for output vectors. Further  details  are
       given in the pcreposix documentation.

         PCRE_CONFIG_MATCH_LIMIT

       The  output is a long integer that gives the default limit for the num-
       ber of internal matching function calls  in  a  pcre_exec()  execution.
       Further details are given with pcre_exec() below.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

       The output is a long integer that gives the default limit for the depth
       of  recursion  when  calling  the  internal  matching  function  in   a
       pcre_exec()  execution.  Further  details  are  given  with pcre_exec()
       below.

         PCRE_CONFIG_STACKRECURSE

       The output is an integer that is set to one if internal recursion  when
       running pcre_exec() is implemented by recursive function calls that use
       the stack to remember their state. This is the usual way that  PCRE  is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on the  heap  instead  of  recursive  function  calls.  In  this  case,
       pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
       blocks on the heap, thus avoiding the use of the stack.


COMPILING A PATTERN

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the two interfaces is that pcre_compile2() has an additional  argument,
       errorcodeptr,  via  which  a  numerical  error code can be returned. To
       avoid too much repetition, we refer just to pcre_compile()  below,  but
       the information applies equally to pcre_compile2().

       The pattern is a C string terminated by a binary zero, and is passed in
       the pattern argument. A pointer to a single block  of  memory  that  is
       obtained  via  pcre_malloc is returned. This contains the compiled code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

       Although  the compiled code of a PCRE regex is relocatable, that is, it
       does not depend on memory location, the complete pcre data block is not
       fully  relocatable, because it may contain a copy of the tableptr argu-
       ment, which is an address (see below).

       The options argument contains various bit settings that affect the com-
       pilation.  It  should be zero if no options are required. The available
       options are described below. Some of them (in  particular,  those  that
       are  compatible with Perl, but some others as well) can also be set and
       unset from within the pattern (see  the  detailed  description  in  the
       pcrepattern  documentation). For those options that can be different in
       different parts of the pattern, the contents of  the  options  argument
       specifies their settings at the start of compilation and execution. The
       PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK,  and
       PCRE_NO_START_OPT options can be set at the time of matching as well as
       at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try to free it. Normally, the offset from the start of the  pattern  to
       the  byte  that  was  being  processed when the error was discovered is
       placed in the variable pointed to by erroffset, which must not be  NULL
       (if  it is, an immediate error is given). However, for an invalid UTF-8
       string, the offset is that of the first byte of the failing character.

       Some errors are not detected until the whole pattern has been  scanned;
       in  these  cases,  the offset passed back is the length of the pattern.
       Note that the offset is in bytes, not characters, even in  UTF-8  mode.
       It may sometimes point into the middle of a UTF-8 character.

       If  pcre_compile2()  is  used instead of pcre_compile(), and the error-
       codeptr argument is not NULL, a non-zero error code number is  returned
       via  this argument in the event of an error. This is in addition to the
       textual error message. Error codes and messages are listed below.

       If the final argument, tableptr, is NULL, PCRE uses a  default  set  of
       character  tables  that  are  built  when  PCRE  is compiled, using the
       default C locale. Otherwise, tableptr must be an address  that  is  the
       result  of  a  call to pcre_maketables(). This value is stored with the
       compiled pattern, and used again by pcre_exec(), unless  another  table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

       This code fragment shows a typical straightforward  call  to  pcre_com-
       pile():

         pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

       The  following  names  for option bits are defined in the pcre.h header
       file:

         PCRE_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all  with  number  255, before each pattern item. For discussion of the
       callout facility, see the pcrecallout documentation.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence  matches.  The choice is either to match only CR, LF, or CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

         PCRE_CASELESS

       If this bit is set, letters in the pattern match both upper  and  lower
       case  letters.  It  is  equivalent  to  Perl's /i option, and it can be
       changed within a pattern by a (?i) option setting. In UTF-8 mode,  PCRE
       always  understands the concept of case for characters whose values are
       less than 128, so caseless matching is always possible. For  characters
       with  higher  values,  the concept of case is supported if PCRE is com-
       piled with Unicode property support, but not otherwise. If you want  to
       use  caseless  matching  for  characters 128 and above, you must ensure
       that PCRE is compiled with Unicode property support  as  well  as  with
       UTF-8 support.

         PCRE_DOLLAR_ENDONLY

       If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE_DOLLAR_ENDONLY option  is  ignored
       if  PCRE_MULTILINE  is  set.   There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

         PCRE_DOTALL

       If this bit is set, a dot metacharacter in the pattern matches a  char-
       acter of any value, including one that indicates a newline. However, it
       only ever matches one character, even if newlines are  coded  as  CRLF.
       Without  this option, a dot does not match when the current position is
       at a newline. This option is equivalent to Perl's /s option, and it can
       be  changed within a pattern by a (?s) option setting. A negative class
       such as [^a] always matches newline characters, independent of the set-
       ting of this option.

         PCRE_DUPNAMES

       If  this  bit is set, names used to identify capturing subpatterns need
       not be unique. This can be helpful for certain types of pattern when it
       is  known  that  only  one instance of the named subpattern can ever be
       matched. There are more details of named subpatterns  below;  see  also
       the pcrepattern documentation.

         PCRE_EXTENDED

       If  this  bit  is  set,  whitespace  data characters in the pattern are
       totally ignored except when escaped or inside a character class. White-
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line,  inclusive,  are  also  ignored.  This is equivalent to Perl's /x
       option, and it can be changed within a pattern by a  (?x)  option  set-
       ting.

       Which  characters  are  interpreted  as  newlines  is controlled by the
       options passed to pcre_compile() or by a special sequence at the  start
       of  the  pattern, as described in the section entitled "Newline conven-
       tions" in the pcrepattern documentation. Note that the end of this type
       of  comment  is  a  literal  newline  sequence  in  the pattern; escape
       sequences that happen to represent a newline do not count.

       This option makes it possible to include  comments  inside  complicated
       patterns.   Note,  however,  that this applies only to data characters.
       Whitespace  characters  may  never  appear  within  special   character
       sequences in a pattern, for example within the sequence (?( that intro-
       duces a conditional subpattern.

         PCRE_EXTRA

       This option was invented in order to turn on  additional  functionality
       of  PCRE  that  is  incompatible with Perl, but it is currently of very
       little use. When set, any backslash in a pattern that is followed by  a
       letter  that  has  no  special  meaning causes an error, thus reserving
       these combinations for future expansion. By  default,  as  in  Perl,  a
       backslash  followed by a letter with no special meaning is treated as a
       literal. (Perl can, however, be persuaded to give an error for this, by
       running  it with the -w option.) There are at present no other features
       controlled by this option. It can also be set by a (?X) option  setting
       within a pattern.

         PCRE_FIRSTLINE

       If  this  option  is  set,  an  unanchored pattern is required to match
       before or at the first  newline  in  the  subject  string,  though  the
       matched text may continue over the newline.

         PCRE_JAVASCRIPT_COMPAT

       If this option is set, PCRE's behaviour is changed in some ways so that
       it is compatible with JavaScript rather than Perl. The changes  are  as
       follows:

       (1)  A  lone  closing square bracket in a pattern causes a compile-time
       error, because this is illegal in JavaScript (by default it is  treated
       as a data character). Thus, the pattern AB]CD becomes illegal when this
       option is set.

       (2) At run time, a back reference to an unset subpattern group  matches
       an  empty  string (by default this causes the current matching alterna-
       tive to fail). A pattern such as (\1)(a) succeeds when this  option  is
       set  (assuming  it can find an "a" in the subject), whereas it fails by
       default, for Perl compatibility.

       (3) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (4) \u matches a lower case "u" character unless it is followed by four
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to match. By default, \u causes a compile time error (Perl
       uses it to upper case the following character).

       (5) \x matches a lower case "x" character unless it is followed by  two
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, as in Perl, a  hexadecimal  number  is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

         PCRE_MULTILINE

       By default, PCRE treats the subject string as consisting  of  a  single
       line  of characters (even if it actually contains newlines). The "start
       of line" metacharacter (^) matches only at the  start  of  the  string,
       while  the  "end  of line" metacharacter ($) matches only at the end of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

       When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These  options  override the default newline definition that was chosen
       when PCRE was built. Setting the first or the second specifies  that  a
       newline  is  indicated  by a single character (CR or LF, respectively).
       Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
       two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
       U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
       (paragraph  separator, U+2029). For the 8-bit library, the last two are
       recognized only in UTF-8 mode.

       The newline setting in the  options  word  uses  three  bits  that  are
       treated as a number, giving eight possibilities. Currently only six are
       used (default plus the five values above). This means that if  you  set
       more  than one newline option, the combination may or may not be sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
       cause an error.

       The only time that a line break in a pattern  is  specially  recognized
       when  compiling  is when PCRE_EXTENDED is set. CR and LF are whitespace
       characters, and so are ignored in this mode. Also, an unescaped #  out-
       side  a  character class indicates a comment that lasts until after the
       next line break sequence. In other circumstances, line break  sequences
       in patterns are treated as literal data.

       The newline option that is set at compile time becomes the default that
       is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

         PCRE_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be  used  for  capturing  (and  they acquire numbers in the usual way).
       There is no equivalent of this option in Perl.

         NO_START_OPTIMIZE

       This is an option that acts at matching time; that is, it is really  an
       option  for  pcre_exec()  or  pcre_dfa_exec().  If it is set at compile
       time, it is remembered with the compiled pattern and assumed at  match-
       ing  time.  For  details  see  the discussion of PCRE_NO_START_OPTIMIZE
       below.

         PCRE_UCP

       This option changes the way PCRE processes \B, \b, \D, \d, \S, \s,  \W,
       \w,  and  some  of  the POSIX character classes. By default, only ASCII
       characters are recognized, but if PCRE_UCP is set,  Unicode  properties
       are  used instead to classify characters. More details are given in the
       section on generic character types in the pcrepattern page. If you  set
       PCRE_UCP,  matching  one of the items it affects takes much longer. The
       option is available only if PCRE has been compiled with  Unicode  prop-
       erty support.

         PCRE_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE_UTF8

       This option causes PCRE to regard both the pattern and the  subject  as
       strings of UTF-8 characters instead of single-byte strings. However, it
       is available only when PCRE is built to include UTF  support.  If  not,
       the  use  of  this option provokes an error. Details of how this option
       changes the behaviour of PCRE are given in the pcreunicode page.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked.  There  is  a  discussion about the validity of
       UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence  is
       found,  pcre_compile()  returns an error. If you already know that your
       pattern is valid, and you want to skip this check for performance  rea-
       sons,  you  can set the PCRE_NO_UTF8_CHECK option.  When it is set, the
       effect of passing an invalid UTF-8 string as a pattern is undefined. It
       may  cause  your  program  to  crash. Note that this option can also be
       passed to pcre_exec() and pcre_dfa_exec(),  to  suppress  the  validity
       checking of subject strings.


COMPILATION ERROR CODES

       The  following  table  lists  the  error  codes than may be returned by
       pcre_compile2(), along with the error messages that may be returned  by
       both  compiling  functions.  Note  that error messages are always 8-bit
       ASCII strings, even in 16-bit mode. As PCRE has developed,  some  error
       codes  have  fallen  out of use. To avoid confusion, they have not been
       re-used.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression is too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is compiled without UTF support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N{name}, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string (specifically UTF-8)
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 in 8-bit non-UTF-8 mode
         52  internal error: overran compiling workspace
         53  internal error: previously-checked referenced subpattern
               not found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced, angle-bracketed, or quoted
               name/number or by a plain number
         58  a numbered reference must not be zero
         59  an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+
         64  ] is an invalid data character in JavaScript compatibility mode
         65  different names for subpatterns of the same number are
               not allowed
         66  (*MARK) must have an argument
         67  this version of PCRE is not compiled with Unicode property
               support
         68  \c must be followed by an ASCII character
         69  \k is not followed by a braced, angle-bracketed, or quoted name
         70  internal error: unknown opcode in find_fixedlength()
         71  \N is not supported in a class
         72  too many forward references
         73  disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
         74  invalid UTF-16 string (specifically UTF-16)

       The numbers 32 and 10000 in errors 48 and 49  are  defaults;  different
       values may be used if the limits were changed when PCRE was built.


STUDYING A PATTERN

       pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

       If  a  compiled  pattern is going to be used several times, it is worth
       spending more time analyzing it in order to speed up the time taken for
       matching.  The function pcre_study() takes a pointer to a compiled pat-
       tern as its first argument. If studying the pattern produces additional
       information  that  will  help speed up matching, pcre_study() returns a
       pointer to a pcre_extra block, in which the study_data field points  to
       the results of the study.

       The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block  also  con-
       tains  other  fields  that can be set by the caller before the block is
       passed; these are described below in the section on matching a pattern.

       If studying the  pattern  does  not  produce  any  useful  information,
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any  of   the   other   fields   to   pcre_exec()   or
       pcre_dfa_exec(), it must set up its own pcre_extra block.

       The second argument of pcre_study() contains option bits. There is only
       one option: PCRE_STUDY_JIT_COMPILE. If this is set,  and  the  just-in-
       time  compiler  is  available,  the  pattern  is  further compiled into
       machine code that executes much faster than  the  pcre_exec()  matching
       function. If the just-in-time compiler is not available, this option is
       ignored. All other bits in the options argument must be zero.

       JIT compilation is a heavyweight optimization. It can  take  some  time
       for  patterns  to  be analyzed, and for one-off matches and simple pat-
       terns the benefit of faster execution might be offset by a much  slower
       study time.  Not all patterns can be optimized by the JIT compiler. For
       those that cannot be handled, matching automatically falls back to  the
       pcre_exec()  interpreter.  For more details, see the pcrejit documenta-
       tion.

       The third argument for pcre_study() is a pointer for an error  message.
       If  studying  succeeds  (even  if no data is returned), the variable it
       points to is set to NULL. Otherwise it is set to  point  to  a  textual
       error message. This is a static string that is part of the library. You
       must not try to free it. You should test the  error  pointer  for  NULL
       after calling pcre_study(), to be sure that it has run successfully.

       When  you are finished with a pattern, you can free the memory used for
       the study data by calling pcre_free_study(). This function was added to
       the  API  for  release  8.20. For earlier versions, the memory could be
       freed with pcre_free(), just like the pattern itself. This  will  still
       work  in  cases  where  PCRE_STUDY_JIT_COMPILE  is  not used, but it is
       advisable to change to the new function when convenient.

       This is a typical way in which pcre_study() is used (except that  in  a
       real application there should be tests for errors):

         int rc;
         pcre *re;
         pcre_extra *sd;
         re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
         sd = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options */
           &error);        /* set to NULL or points to a message */
         rc = pcre_exec(   /* see below for details of pcre_exec() options */
           re, sd, "subject", 7, 0, 0, ovector, 30);
         ...
         pcre_free_study(sd);
         pcre_free(re);

       Studying a pattern does two things: first, a lower bound for the length
       of subject string that is needed to match the pattern is computed. This
       does not mean that there are any strings of that length that match, but
       it does guarantee that no shorter strings match. The value is  used  by
       pcre_exec()  and  pcre_dfa_exec()  to  avoid  wasting time by trying to
       match strings that are shorter than the lower bound. You can  find  out
       the value in a calling program via the pcre_fullinfo() function.

       Studying a pattern is also useful for non-anchored patterns that do not
       have a single fixed starting character. A bitmap of  possible  starting
       bytes  is  created. This speeds up finding a position in the subject at
       which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
       values less than 256.)

       These  two optimizations apply to both pcre_exec() and pcre_dfa_exec().
       However, they are not used by pcre_exec()  if  pcre_study()  is  called
       with  the  PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is
       successful.  The  optimizations  can  be  disabled   by   setting   the
       PCRE_NO_START_OPTIMIZE    option    when    calling    pcre_exec()   or
       pcre_dfa_exec(). You might want to do this  if  your  pattern  contains
       callouts  or (*MARK) (which cannot be handled by the JIT compiler), and
       you want to make use of these facilities in cases where matching fails.
       See the discussion of PCRE_NO_START_OPTIMIZE below.


LOCALE SUPPORT

       PCRE  handles  caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by  character  value.  When running in UTF-8 mode, this applies only to
       characters with codes less than 128. By  default,  higher-valued  codes
       never match escapes such as \w or \d, but they can be tested with \p if
       PCRE is built with Unicode character property  support.  Alternatively,
       the  PCRE_UCP  option  can  be  set at compile time; this causes \w and
       friends to use Unicode property support instead of built-in tables. The
       use of locales with Unicode is discouraged. If you are handling charac-
       ters with codes greater than 128, you should either use UTF-8 and  Uni-
       code, or use locales, but not try to mix the two.

       PCRE  contains  an  internal set of tables that are used when the final
       argument of pcre_compile() is  NULL.  These  are  sufficient  for  many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The internal tables can always be overridden by tables supplied by  the
       application that calls PCRE. These may be created in a different locale
       from the default. As more and more applications change  to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are  built by calling the pcre_maketables() function,
       which has no arguments, in the relevant locale. The result can then  be
       passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
       example, to build and use tables that are appropriate  for  the  French
       locale  (where  accented  characters  with  values greater than 128 are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

       The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
       if you are using Windows, the name for the French locale is "french".

       When  pcre_maketables()  runs,  the  tables are built in memory that is
       obtained via pcre_malloc. It is the caller's responsibility  to  ensure
       that  the memory containing the tables remains available for as long as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern,  and the same tables are used via this pointer by pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It is possible to pass a table pointer or NULL (indicating the  use  of
       the  internal  tables)  to  pcre_exec(). Although not intended for this
       purpose, this facility could be used to match a pattern in a  different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       The pcre_fullinfo() function returns information about a compiled  pat-
       tern.  It replaces the pcre_info() function, which was removed from the
       library at version 8.30, after more than 10 years of obsolescence.

       The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
       pattern.  The second argument is the result of pcre_study(), or NULL if
       the pattern was not studied. The third argument specifies  which  piece
       of  information  is required, and the fourth argument is a pointer to a
       variable to receive the data. The yield of the  function  is  zero  for
       success, or one of the following negative numbers:

         PCRE_ERROR_NULL           the argument code was NULL
                                   the argument where was NULL
         PCRE_ERROR_BADMAGIC       the "magic number" was not found
         PCRE_ERROR_BADENDIANNESS  the pattern was compiled with different
                                   endianness
         PCRE_ERROR_BADOPTION      the value of what was invalid

       The  "magic  number" is placed at the start of each compiled pattern as
       an simple check against passing an arbitrary memory pointer. The  endi-
       anness error can occur if a compiled pattern is saved and reloaded on a
       different host. Here is a typical call of  pcre_fullinfo(),  to  obtain
       the length of the compiled pattern:

         int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           sd,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

       The  possible  values for the third argument are defined in pcre.h, and
       are as follows:

         PCRE_INFO_BACKREFMAX

       Return the number of the highest back reference  in  the  pattern.  The
       fourth  argument  should  point to an int variable. Zero is returned if
       there are no back references.

         PCRE_INFO_CAPTURECOUNT

       Return the number of capturing subpatterns in the pattern.  The  fourth
       argument should point to an int variable.

         PCRE_INFO_DEFAULT_TABLES

       Return  a pointer to the internal default character tables within PCRE.
       The fourth argument should point to an unsigned char *  variable.  This
       information call is provided for internal use by the pcre_study() func-
       tion. External callers can cause PCRE to use  its  internal  tables  by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return information about the first data unit of any matched string, for
       a non-anchored pattern. (The name of this option refers  to  the  8-bit
       library,  where data units are bytes.) The fourth argument should point
       to an int variable.

       If there is a fixed first value, for example, the  letter  "c"  from  a
       pattern  such  as (cat|cow|coyote), its value is returned. In the 8-bit
       library, the value is always less than 256; in the 16-bit  library  the
       value can be up to 0xffff.

       If there is no fixed first value, and if either

       (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
       branch starts with "^", or

       (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

       -1  is  returned, indicating that the pattern matches only at the start
       of a subject string or after any newline within the  string.  Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

         PCRE_INFO_FIRSTTABLE

       If  the pattern was studied, and this resulted in the construction of a
       256-bit table indicating a fixed set of values for the first data  unit
       in  any  matching string, a pointer to the table is returned. Otherwise
       NULL is returned. The fourth argument should point to an unsigned  char
       * variable.

         PCRE_INFO_HASCRORLF

       Return  1  if  the  pattern  contains any explicit matches for CR or LF
       characters, otherwise 0. The fourth argument should  point  to  an  int
       variable.  An explicit match is either a literal CR or LF character, or
       \r or \n.

         PCRE_INFO_JCHANGED

       Return 1 if the (?J) or (?-J) option setting is used  in  the  pattern,
       otherwise  0. The fourth argument should point to an int variable. (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

         PCRE_INFO_JIT

       Return 1 if the pattern was  studied  with  the  PCRE_STUDY_JIT_COMPILE
       option,  and just-in-time compiling was successful. The fourth argument
       should point to an int variable. A return value of  0  means  that  JIT
       support  is  not available in this version of PCRE, or that the pattern
       was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT
       compiler could not handle this particular pattern. See the pcrejit doc-
       umentation for details of what can and cannot be handled.

         PCRE_INFO_JITSIZE

       If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
       option,  return  the  size  of  the JIT compiled code, otherwise return
       zero. The fourth argument should point to a size_t variable.

         PCRE_INFO_LASTLITERAL

       Return the value of the rightmost literal data unit that must exist  in
       any  matched  string, other than at its start, if such a value has been
       recorded. The fourth argument should point to an int variable. If there
       is no such value, -1 is returned. For anchored patterns, a last literal
       value is recorded only if it follows something of variable length.  For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

         PCRE_INFO_MINLENGTH

       If the pattern was studied and a minimum length  for  matching  subject
       strings  was  computed,  its  value is returned. Otherwise the returned
       value is -1. The value is a number of characters, which in  UTF-8  mode
       may  be  different from the number of bytes. The fourth argument should
       point to an int variable. A non-negative value is a lower bound to  the
       length  of  any  matching  string. There may not be any strings of that
       length that do actually match, but every string that does match  is  at
       least that long.

         PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       PCRE  supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of  each  entry;  both  of  these  return  an int value. The entry size
       depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
       a pointer to the first entry of the table. This is a pointer to char in
       the 8-bit library, where the first two bytes of each entry are the num-
       ber  of  the capturing parenthesis, most significant byte first. In the
       16-bit library, the pointer points to 16-bit data units, the  first  of
       which  contains  the  parenthesis  number. The rest of the entry is the
       corresponding name, zero terminated.

       The names are in alphabetical order. Duplicate names may appear if  (?|
       is used to create multiple groups with the same number, as described in
       the section on duplicate subpattern numbers in  the  pcrepattern  page.
       Duplicate  names  for  subpatterns with different numbers are permitted
       only if PCRE_DUPNAMES is set. In all cases  of  duplicate  names,  they
       appear  in  the table in the order in which they were found in the pat-
       tern. In the absence of (?| this is the  order  of  increasing  number;
       when (?| is used this is not necessarily the case because later subpat-
       terns may have lower numbers.

       As a simple example of the name/number table,  consider  the  following
       pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
       set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named subpatterns, so the table has  four  entries,  and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code  to  extract  data from named subpatterns using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OKPARTIAL

       Return  1  if  the  pattern  can  be  used  for  partial  matching with
       pcre_exec(), otherwise 0. The fourth argument should point  to  an  int
       variable.  From  release  8.00,  this  always  returns  1,  because the
       restrictions that previously applied  to  partial  matching  have  been
       lifted.  The  pcrepartial documentation gives details of partial match-
       ing.

         PCRE_INFO_OPTIONS

       Return a copy of the options with which the pattern was  compiled.  The
       fourth  argument  should  point to an unsigned long int variable. These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings at the start of the pattern itself. In
       other words, they are the options that will be in force  when  matching
       starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with
       the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,
       and PCRE_EXTENDED.

       A  pattern  is  automatically  anchored by PCRE if all of its top-level
       alternatives begin with one of the following:

         ^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

         PCRE_INFO_SIZE

       Return  the size of the compiled pattern in bytes (for both libraries).
       The fourth argument should point to a size_t variable. This value  does
       not  include  the  size  of  the  pcre  structure  that  is returned by
       pcre_compile(). The value that is passed as the argument  to  pcre_mal-
       loc()  when pcre_compile() is getting memory in which to place the com-
       piled data is the value returned by this option plus the  size  of  the
       pcre  structure. Studying a compiled pattern, with or without JIT, does
       not alter the value returned by this option.

         PCRE_INFO_STUDYSIZE

       Return the size in bytes of the data block pointed to by the study_data
       field  in  a  pcre_extra  block.  If pcre_extra is NULL, or there is no
       study data, zero is returned. The fourth argument  should  point  to  a
       size_t  variable. The study_data field is set by pcre_study() to record
       information that will speed  up  matching  (see  the  section  entitled
       "Studying a pattern" above). The format of the study_data block is pri-
       vate, but its length is made available via this option so that  it  can
       be  saved  and  restored  (see  the  pcreprecompile  documentation  for
       details).


REFERENCE COUNTS

       int pcre_refcount(pcre *code, int adjust);

       The pcre_refcount() function is used to maintain a reference  count  in
       the data block that contains a compiled pattern. It is provided for the
       benefit of applications that  operate  in  an  object-oriented  manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.   It is changed only by calling this function, whose action is to
       add the adjust value (which may be positive or  negative)  to  it.  The
       yield of the function is the new value. However, the value of the count
       is constrained to lie between 0 and 65535, inclusive. If the new  value
       is outside these limits, it is forced to the appropriate limit value.

       Except  when it is zero, the reference count is not correctly preserved
       if a pattern is compiled on one host and then  transferred  to  a  host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       The  function pcre_exec() is called to match a subject string against a
       compiled pattern, which is passed in the code argument. If the  pattern
       was  studied,  the  result  of  the study should be passed in the extra
       argument. You can call pcre_exec() with the same code and  extra  argu-
       ments  as  many  times as you like, in order to match different subject
       strings with the same pattern.

       This function is the main matching facility  of  the  library,  and  it
       operates  in  a  Perl-like  manner. For specialist use there is also an
       alternative matching function, which is described below in the  section
       about the pcre_dfa_exec() function.

       In  most applications, the pattern will have been compiled (and option-
       ally studied) in the same process that calls pcre_exec().  However,  it
       is possible to save compiled patterns and study data, and then use them
       later in different processes, possibly even on different hosts.  For  a
       discussion about this, see the pcreprecompile documentation.

       Here is an example of a simple call to pcre_exec():

         int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       If  the  extra argument is not NULL, it must point to a pcre_extra data
       block. The pcre_study() function returns such a block (when it  doesn't
       return  NULL), but you can also create one for yourself, and pass addi-
       tional information in it. The pcre_extra block contains  the  following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         void *executable_jit;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;
         unsigned char **mark;

       In  the  16-bit  version  of  this  structure,  the mark field has type
       "PCRE_UCHAR16 **".

       The flags field is a bitmap that specifies which of  the  other  fields
       are set. The flag bits are:

         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_EXECUTABLE_JIT
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES
         PCRE_EXTRA_MARK

       Other  flag  bits should be set to zero. The study_data field and some-
       times the executable_jit field are set in the pcre_extra block that  is
       returned  by pcre_study(), together with the appropriate flag bits. You
       should not set these yourself, but you may add to the block by  setting
       the other fields and their corresponding flag bits.

       The match_limit field provides a means of preventing PCRE from using up
       a vast amount of resources when running patterns that are not going  to
       match,  but  which  have  a very large number of possibilities in their
       search trees. The classic example is a pattern that uses nested  unlim-
       ited repeats.

       Internally,  pcre_exec() uses a function called match(), which it calls
       repeatedly (sometimes recursively). The limit  set  by  match_limit  is
       imposed  on the number of times this function is called during a match,
       which has the effect of limiting the amount of  backtracking  that  can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string.

       When pcre_exec() is called with a pattern that was successfully studied
       with  the  PCRE_STUDY_JIT_COMPILE  option, the way that the matching is
       executed is entirely different. However, there is still the possibility
       of  runaway  matching  that  goes  on  for a very long time, and so the
       match_limit value is also used in this case (but in a different way) to
       limit how long the matching can continue.

       The  default  value  for  the  limit can be set when PCRE is built; the
       default default is 10 million, which handles all but the  most  extreme
       cases.  You  can  override  the  default by suppling pcre_exec() with a
       pcre_extra    block    in    which    match_limit    is    set,     and
       PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The match_limit_recursion field is similar to match_limit, but  instead
       of limiting the total number of times that match() is called, it limits
       the depth of recursion. The recursion depth is a  smaller  number  than
       the  total number of calls, because not all calls to match() are recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting the recursion depth limits the amount of  machine  stack  that
       can  be used, or, when PCRE has been compiled to use memory on the heap
       instead of the stack, the amount of heap memory that can be used.  This
       limit  is not relevant, and is ignored, if the pattern was successfully
       studied with PCRE_STUDY_JIT_COMPILE.

       The default value for match_limit_recursion can be  set  when  PCRE  is
       built;  the  default  default  is  the  same  value  as the default for
       match_limit. You can override the default by suppling pcre_exec()  with
       a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

       The  callout_data  field is used in conjunction with the "callout" fea-
       ture, and is described in the pcrecallout documentation.

       The tables field  is  used  to  pass  a  character  tables  pointer  to
       pcre_exec();  this overrides the value that is stored with the compiled
       pattern. A non-NULL value is stored with the compiled pattern  only  if
       custom  tables  were  supplied to pcre_compile() via its tableptr argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's  internal  tables  to be used. This facility is helpful when re-
       using patterns that have been saved after compiling  with  an  external
       set  of  tables,  because  the  external tables might be at a different
       address when pcre_exec() is called. See the  pcreprecompile  documenta-
       tion for a discussion of saving compiled patterns for later use.

       If  PCRE_EXTRA_MARK  is  set in the flags field, the mark field must be
       set to point to a suitable variable. If the pattern contains any  back-
       tracking  control verbs such as (*MARK:NAME), and the execution ends up
       with a name to pass back, a pointer to the  name  string  (zero  termi-
       nated)  is  placed  in  the  variable pointed to by the mark field. The
       names are within the compiled pattern; if you wish  to  retain  such  a
       name  you must copy it before freeing the memory of a compiled pattern.
       If there is no name to pass back, the variable pointed to by  the  mark
       field  is  set  to NULL. For details of the backtracking control verbs,
       see the section entitled "Backtracking control" in the pcrepattern doc-
       umentation.

   Option bits for pcre_exec()

       The  unused  bits of the options argument for pcre_exec() must be zero.
       The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,    PCRE_NOTEOL,    PCRE_NOTEMPTY,   PCRE_NOTEMPTY_ATSTART,
       PCRE_NO_START_OPTIMIZE,  PCRE_NO_UTF8_CHECK,   PCRE_PARTIAL_SOFT,   and
       PCRE_PARTIAL_HARD.

       If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
       option,  the   only   supported   options   for   JIT   execution   are
       PCRE_NO_UTF8_CHECK,   PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,  and
       PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is  not
       supported.  If an unsupported option is used, JIT execution is disabled
       and the normal interpretive code in pcre_exec() is run.

         PCRE_ANCHORED

       The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
       matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
       turned out to be anchored by virtue of its contents, it cannot be  made
       unachored at matching time.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence matches. The choice is either to match only CR, LF,  or  CRLF,
       or  to  match  any Unicode newline sequence. These options override the
       choice that was made or defaulted when the pattern was compiled.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override  the  newline  definition  that  was  chosen  or
       defaulted  when the pattern was compiled. For details, see the descrip-
       tion of pcre_compile()  above.  During  matching,  the  newline  choice
       affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
       ters. It may also alter the way the match position is advanced after  a
       match failure for an unanchored pattern.

       When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
       set, and a match attempt for an unanchored pattern fails when the  cur-
       rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
       explicit matches for  CR  or  LF  characters,  the  match  position  is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing  at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string,  because  it  con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those  characters,  or  one  of the \r or \n escape sequences. Implicit
       matches such as [^X] do not count, nor does \s (which includes  CR  and
       LF in the characters that it matches).

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

         PCRE_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before it. Setting this without PCRE_MULTILINE (at compile time)
       causes circumflex never to match. This option affects only  the  behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option  affects only the behaviour of the dollar metacharacter. It does
       not affect \Z or \z.

         PCRE_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

         PCRE_NOTEMPTY_ATSTART

       This is like PCRE_NOTEMPTY, except that an empty string match  that  is
       not  at  the  start  of  the  subject  is  permitted. If the pattern is
       anchored, such a match can occur only if the pattern contains \K.

       Perl    has    no    direct    equivalent    of    PCRE_NOTEMPTY     or
       PCRE_NOTEMPTY_ATSTART,  but  it  does  make a special case of a pattern
       match of the empty string within its split() function, and  when  using
       the  /g  modifier.  It  is  possible  to emulate Perl's behaviour after
       matching a null string by first trying the match again at the same off-
       set  with  PCRE_NOTEMPTY_ATSTART  and  PCRE_ANCHORED,  and then if that
       fails, by advancing the starting offset (see below) and trying an ordi-
       nary  match  again. There is some code that demonstrates how to do this
       in the pcredemo sample program. In the most general case, you  have  to
       check  to  see  if the newline convention recognizes CRLF as a newline,
       and if so, and the current character is CR followed by LF, advance  the
       starting offset by two characters instead of one.

         PCRE_NO_START_OPTIMIZE

       There  are a number of optimizations that pcre_exec() uses at the start
       of a match, in order to speed up the process. For  example,  if  it  is
       known that an unanchored match must start with a specific character, it
       searches the subject for that character, and fails  immediately  if  it
       cannot  find  it,  without actually running the main matching function.
       This means that a special item such as (*COMMIT) at the start of a pat-
       tern  is  not  considered until after a suitable starting point for the
       match has been found. When callouts or (*MARK) items are in use,  these
       "start-up" optimizations can cause them to be skipped if the pattern is
       never actually used. The start-up optimizations are in  effect  a  pre-
       scan of the subject that takes place before the pattern is run.

       The  PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly causing performance to suffer,  but  ensuring  that  in  cases
       where  the  result is "no match", the callouts do occur, and that items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position  in  the  subject  string. If PCRE_NO_START_OPTIMIZE is set at
       compile time, it cannot be unset at matching time.

       Setting PCRE_NO_START_OPTIMIZE can change the  outcome  of  a  matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When  this  is  compiled, PCRE records the fact that a match must start
       with the character "A". Suppose the subject  string  is  "DEFABC".  The
       start-up  optimization  scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the  pat-
       tern  must  match the current starting position, which in this case, it
       does. However, if the same match  is  run  with  PCRE_NO_START_OPTIMIZE
       set,  the  initial  scan  along the subject string does not happen. The
       first match attempt is run starting  from  "D"  and  when  this  fails,
       (*COMMIT)  prevents  any  further  matches  being tried, so the overall
       result is "no match". If the pattern is studied,  more  start-up  opti-
       mizations  may  be  used. For example, a minimum length for the subject
       may be recorded. Consider the pattern

         (*MARK:A)(X|Y)

       The minimum length for a match is one  character.  If  the  subject  is
       "ABC",  there  will  be  attempts  to  match "ABC", "BC", "C", and then
       finally an empty string.  If the pattern is studied, the final  attempt
       does  not take place, because PCRE knows that the subject is too short,
       and so the (*MARK) is never encountered.  In this  case,  studying  the
       pattern  does  not  affect the overall match result, which is still "no
       match", but it does affect the auxiliary information that is returned.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8  string is automatically checked when pcre_exec() is subsequently
       called.  The value of startoffset is also checked  to  ensure  that  it
       points  to  the start of a UTF-8 character. There is a discussion about
       the validity of UTF-8 strings in the pcreunicode page.  If  an  invalid
       sequence   of   bytes   is   found,   pcre_exec()   returns  the  error
       PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
       truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
       both cases, information about the precise nature of the error may  also
       be  returned (see the descriptions of these errors in the section enti-
       tled Error return values from pcre_exec() below).  If startoffset  con-
       tains a value that does not point to the start of a UTF-8 character (or
       to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.

       If you already know that your subject is valid, and you  want  to  skip
       these    checks    for   performance   reasons,   you   can   set   the
       PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
       do  this  for the second and subsequent calls to pcre_exec() if you are
       making repeated calls to find all  the  matches  in  a  single  subject
       string.  However,  you  should  be  sure  that the value of startoffset
       points to the start of a character (or the end of  the  subject).  When
       PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
       subject or an invalid value of startoffset is undefined.  Your  program
       may crash.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These  options turn on the partial matching feature. For backwards com-
       patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A  partial
       match  occurs if the end of the subject string is reached successfully,
       but there are not enough subject characters to complete the  match.  If
       this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
       matching continues by testing any remaining alternatives.  Only  if  no
       complete  match  can be found is PCRE_ERROR_PARTIAL returned instead of
       PCRE_ERROR_NOMATCH. In other words,  PCRE_PARTIAL_SOFT  says  that  the
       caller  is  prepared to handle a partial match, but only if no complete
       match can be found.

       If PCRE_PARTIAL_HARD is set, it overrides  PCRE_PARTIAL_SOFT.  In  this
       case,  if  a  partial  match  is found, pcre_exec() immediately returns
       PCRE_ERROR_PARTIAL, without  considering  any  other  alternatives.  In
       other  words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important that an alternative complete match.

       In both cases, the portion of the string that was  inspected  when  the
       partial match was found is set as the first matching string. There is a
       more detailed discussion of partial and  multi-segment  matching,  with
       examples, in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The  subject string is passed to pcre_exec() as a pointer in subject, a
       length in bytes in length, and a starting byte offset  in  startoffset.
       If  this  is  negative  or  greater  than  the  length  of the subject,
       pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting  offset  is
       zero,  the  search  for a match starts at the beginning of the subject,
       and this is by far the most common case. In UTF-8 mode, the byte offset
       must  point  to  the start of a UTF-8 character (or the end of the sub-
       ject). Unlike the pattern string, the subject may contain  binary  zero
       bytes.

       A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre_exec() again after a previous  suc-
       cess.   Setting  startoffset differs from just passing over a shortened
       string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When  applied  to the string "Mississipi" the first call to pcre_exec()
       finds the first occurrence. If pcre_exec() is called  again  with  just
       the  remainder  of  the  subject,  namely  "issipi", it does not match,
       because \B is always false at the start of the subject, which is deemed
       to  be  a  word  boundary. However, if pcre_exec() is passed the entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence  of "iss" because it is able to look behind the starting point to
       discover that it is preceded by a letter.

       Finding all the matches in a subject is tricky  when  the  pattern  can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first  trying  the  match  again  at  the   same   offset,   with   the
       PCRE_NOTEMPTY_ATSTART  and  PCRE_ANCHORED  options,  and  then  if that
       fails, advancing the starting  offset  and  trying  an  ordinary  match
       again. There is some code that demonstrates how to do this in the pcre-
       demo sample program. In the most general case, you have to check to see
       if  the newline convention recognizes CRLF as a newline, and if so, and
       the current character is CR followed by LF, advance the starting offset
       by two characters instead of one.

       If  a  non-zero starting offset is passed when the pattern is anchored,
       one attempt to match at the given offset is made. This can only succeed
       if  the  pattern  does  not require the match to be at the start of the
       subject.

   How pcre_exec() returns captured substrings

       In general, a pattern matches a certain portion of the subject, and  in
       addition,  further  substrings  from  the  subject may be picked out by
       parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
       this  is  called "capturing" in what follows, and the phrase "capturing
       subpattern" is used for a fragment of a pattern that picks out  a  sub-
       string.  PCRE  supports several other kinds of parenthesized subpattern
       that do not cause substrings to be captured.

       Captured substrings are returned to the caller via a vector of integers
       whose  address is passed in ovector. The number of elements in the vec-
       tor is passed in ovecsize, which must be a non-negative  number.  Note:
       this argument is NOT the size of ovector in bytes.

       The  first  two-thirds of the vector is used to pass back captured sub-
       strings, each substring using a pair of integers. The  remaining  third
       of  the  vector is used as workspace by pcre_exec() while matching cap-
       turing subpatterns, and is not available for passing back  information.
       The  number passed in ovecsize should always be a multiple of three. If
       it is not, it is rounded down.

       When a match is successful, information about  captured  substrings  is
       returned  in  pairs  of integers, starting at the beginning of ovector,
       and continuing up to two-thirds of its length at the  most.  The  first
       element  of  each pair is set to the byte offset of the first character
       in a substring, and the second is set to the byte offset of  the  first
       character  after  the end of a substring. Note: these values are always
       byte offsets, even in UTF-8 mode. They are not character counts.

       The first pair of integers, ovector[0]  and  ovector[1],  identify  the
       portion  of  the subject string matched by the entire pattern. The next
       pair is used for the first capturing subpattern, and so on.  The  value
       returned by pcre_exec() is one more than the highest numbered pair that
       has been set.  For example, if two substrings have been  captured,  the
       returned  value is 3. If there are no capturing subpatterns, the return
       value from a successful match is 1, indicating that just the first pair
       of offsets has been set.

       If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

       If the vector is too small to hold all the captured substring  offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function returns a value of zero. If neither the actual string  matched
       not  any captured substrings are of interest, pcre_exec() may be called
       with ovector passed as NULL and ovecsize as zero. However, if the  pat-
       tern  contains  back  references  and  the ovector is not big enough to
       remember the related substrings, PCRE has to get additional memory  for
       use  during matching. Thus it is usually advisable to supply an ovector
       of reasonable size.

       There are some cases where zero is returned  (indicating  vector  over-
       flow)  when  in fact the vector is exactly the right size for the final
       match. For example, consider the pattern

         (a)(?:(b)c|bd)

       If a vector of 6 elements (allowing for only 1 captured  substring)  is
       given with subject string "abd", pcre_exec() will try to set the second
       captured string, thereby recording a vector overflow, before failing to
       match  "c"  and  backing  up  to  try  the second alternative. The zero
       return, however, does correctly indicate that  the  maximum  number  of
       slots (namely 2) have been filled. In similar cases where there is tem-
       porary overflow, but the final number of used slots  is  actually  less
       than the maximum, a non-zero value is returned.

       The pcre_fullinfo() function can be used to find out how many capturing
       subpatterns there are in a compiled  pattern.  The  smallest  size  for
       ovector  that  will allow for n captured substrings, in addition to the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It is possible for capturing subpattern number n+1 to match  some  part
       of the subject when subpattern n has not been used at all. For example,
       if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2 is not. When this happens, both values in  the  offset  pairs  corre-
       sponding to unused subpatterns are set to -1.

       Offset  values  that correspond to unused subpatterns at the end of the
       expression are also set to -1. For example,  if  the  string  "abc"  is
       matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
       matched. The return from the function is 2, because  the  highest  used
       capturing  subpattern  number  is 1, and the offsets for for the second
       and third capturing subpatterns (assuming the vector is  large  enough,
       of course) are set to -1.

       Note:  Elements  in  the first two-thirds of ovector that do not corre-
       spond to capturing parentheses in the pattern are never  changed.  That
       is,  if  a pattern contains n capturing parentheses, no more than ovec-
       tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements  (in
       the first two-thirds) retain whatever values they previously had.

       Some  convenience  functions  are  provided for extracting the captured
       substrings as separate strings. These are described below.

   Error return values from pcre_exec()

       If pcre_exec() fails, it returns a negative number. The  following  are
       defined in the header file:

         PCRE_ERROR_NOMATCH        (-1)

       The subject string did not match the pattern.

         PCRE_ERROR_NULL           (-2)

       Either  code  or  subject  was  passed as NULL, or ovector was NULL and
       ovecsize was not zero.

         PCRE_ERROR_BADOPTION      (-3)

       An unrecognized bit was set in the options argument.

         PCRE_ERROR_BADMAGIC       (-4)

       PCRE stores a 4-byte "magic number" at the start of the compiled  code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an  environment  with the other endianness. This is the error that PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_OPCODE (-5)

       While running the pattern match, an unknown item was encountered in the
       compiled  pattern.  This  error  could be caused by a bug in PCRE or by
       overwriting of the compiled pattern.

         PCRE_ERROR_NOMEMORY       (-6)

       If a pattern contains back references, but the ovector that  is  passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE gets a block of memory at the start of matching to  use  for  this
       purpose.  If the call via pcre_malloc() fails, this error is given. The
       memory is automatically freed at the end of matching.

       This error is also given if pcre_stack_malloc() fails  in  pcre_exec().
       This  can happen only when PCRE has been compiled with --disable-stack-
       for-recursion.

         PCRE_ERROR_NOSUBSTRING    (-7)

       This error is used by the pcre_copy_substring(),  pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

         PCRE_ERROR_MATCHLIMIT     (-8)

       The backtracking limit, as specified by  the  match_limit  field  in  a
       pcre_extra  structure  (or  defaulted) was reached. See the description
       above.

         PCRE_ERROR_CALLOUT        (-9)

       This error is never generated by pcre_exec() itself. It is provided for
       use  by  callout functions that want to yield a distinctive error code.
       See the pcrecallout documentation for details.

         PCRE_ERROR_BADUTF8        (-10)

       A string that contains an invalid UTF-8 byte sequence was passed  as  a
       subject,  and the PCRE_NO_UTF8_CHECK option was not set. If the size of
       the output vector (ovecsize) is at least 2,  the  byte  offset  to  the
       start  of  the  the invalid UTF-8 character is placed in the first ele-
       ment, and a reason code is placed in the  second  element.  The  reason
       codes are listed in the following section.  For backward compatibility,
       if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8  char-
       acter   at   the   end   of   the   subject  (reason  codes  1  to  5),
       PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The UTF-8 byte sequence that was passed as a subject  was  checked  and
       found  to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
       value of startoffset did not point to the beginning of a UTF-8  charac-
       ter or the end of the subject.

         PCRE_ERROR_PARTIAL        (-12)

       The  subject  string did not match, but it did match partially. See the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       This code is no longer in  use.  It  was  formerly  returned  when  the
       PCRE_PARTIAL  option  was used with a compiled pattern containing items
       that were  not  supported  for  partial  matching.  From  release  8.00
       onwards, there are no restrictions on partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This error is given if the value of the ovecsize argument is negative.

         PCRE_ERROR_RECURSIONLIMIT (-21)

       The internal recursion limit, as specified by the match_limit_recursion
       field  in  a  pcre_extra  structure (or defaulted) was reached. See the
       description above.

         PCRE_ERROR_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

         PCRE_ERROR_BADOFFSET      (-24)

       The value of startoffset was negative or greater than the length of the
       subject, that is, the value in length.

         PCRE_ERROR_SHORTUTF8      (-25)

       This  error  is returned instead of PCRE_ERROR_BADUTF8 when the subject
       string ends with a truncated UTF-8 character and the  PCRE_PARTIAL_HARD
       option  is  set.   Information  about  the  failure  is returned as for
       PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this  case,  but
       this  special error code for PCRE_PARTIAL_HARD precedes the implementa-
       tion of returned information; it is retained for backwards  compatibil-
       ity.

         PCRE_ERROR_RECURSELOOP    (-26)

       This error is returned when pcre_exec() detects a recursion loop within
       the pattern. Specifically, it means that either the whole pattern or  a
       subpattern  has been called recursively for the second time at the same
       position in the subject string. Some simple patterns that might do this
       are  detected  and faulted at compile time, but more complicated cases,
       in particular mutual recursions between two different subpatterns, can-
       not be detected until run time.

         PCRE_ERROR_JIT_STACKLIMIT (-27)

       This  error  is  returned  when a pattern that was successfully studied
       using the PCRE_STUDY_JIT_COMPILE option is being matched, but the  mem-
       ory  available  for  the  just-in-time  processing  stack  is not large
       enough. See the pcrejit documentation for more details.

         PCRE_ERROR_BADMODE (-28)

       This error is given if a pattern that was compiled by the 8-bit library
       is passed to a 16-bit library function, or vice versa.

         PCRE_ERROR_BADENDIANNESS (-29)

       This  error  is  given  if  a  pattern  that  was compiled and saved is
       reloaded on a host with  different  endianness.  The  utility  function
       pcre_pattern_to_host_byte_order() can be used to convert such a pattern
       so that it runs on the new host.

       Error numbers -16 to -20 and -22 are not used by pcre_exec().

   Reason codes for invalid UTF-8 strings

       This section applies only  to  the  8-bit  library.  The  corresponding
       information for the 16-bit library is given in the pcre16 page.

       When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
       UTF8, and the size of the output vector (ovecsize) is at least  2,  the
       offset  of  the  start  of the invalid UTF-8 character is placed in the
       first output vector element (ovector[0]) and a reason code is placed in
       the  second  element  (ovector[1]). The reason codes are given names in
       the pcre.h header file:

         PCRE_UTF8_ERR1
         PCRE_UTF8_ERR2
         PCRE_UTF8_ERR3
         PCRE_UTF8_ERR4
         PCRE_UTF8_ERR5

       The string ends with a truncated UTF-8 character;  the  code  specifies
       how  many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
       characters to be no longer than 4 bytes, the  encoding  scheme  (origi-
       nally  defined  by  RFC  2279)  allows  for  up to 6 bytes, and this is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE_UTF8_ERR6
         PCRE_UTF8_ERR7
         PCRE_UTF8_ERR8
         PCRE_UTF8_ERR9
         PCRE_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the  character  do  not have the binary value 0b10 (that is, either the
       most significant bit is 0, or the next bit is 1).

         PCRE_UTF8_ERR11
         PCRE_UTF8_ERR12

       A character that is valid by the RFC 2279 rules is either 5 or 6  bytes
       long; these code points are excluded by RFC 3629.

         PCRE_UTF8_ERR13

       A  4-byte character has a value greater than 0x10fff; these code points
       are excluded by RFC 3629.

         PCRE_UTF8_ERR14

       A 3-byte character has a value in the  range  0xd800  to  0xdfff;  this
       range  of code points are reserved by RFC 3629 for use with UTF-16, and
       so are excluded from UTF-8.

         PCRE_UTF8_ERR15
         PCRE_UTF8_ERR16
         PCRE_UTF8_ERR17
         PCRE_UTF8_ERR18
         PCRE_UTF8_ERR19

       A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it  codes
       for  a  value that can be represented by fewer bytes, which is invalid.
       For example, the two bytes 0xc0, 0xae give the value 0x2e,  whose  cor-
       rect coding uses just one byte.

         PCRE_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary value 0b10 (that is, the most significant bit is 1 and the  sec-
       ond  is  0). Such a byte can only validly occur as the second or subse-
       quent byte of a multi-byte character.

         PCRE_UTF8_ERR21

       The first byte of a character has the value 0xfe or 0xff. These  values
       can never occur in a valid UTF-8 string.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       Captured  substrings  can  be  accessed  directly  by using the offsets
       returned by pcre_exec() in  ovector.  For  convenience,  the  functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list() are provided for extracting captured substrings  as  new,
       separate,  zero-terminated strings. These functions identify substrings
       by number. The next section describes functions  for  extracting  named
       substrings.

       A  substring that contains a binary zero is correctly extracted and has
       a further zero added on the end, but the result is not, of course, a  C
       string.   However,  you  can  process such a string by referring to the
       length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not adequate for handling strings containing binary zeros, because  the
       end of the final string is not independently indicated.

       The  first  three  arguments  are the same for all three of these func-
       tions: subject is the subject string that has  just  been  successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were  captured  by  the match, including the substring that matched the
       entire regular expression. This is the value returned by pcre_exec() if
       it  is greater than zero. If pcre_exec() returned zero, indicating that
       it ran out of space in ovector, the value passed as stringcount  should
       be the number of elements in the vector divided by three.

       The  functions pcre_copy_substring() and pcre_get_substring() extract a
       single substring, whose number is given as  stringnumber.  A  value  of
       zero  extracts  the  substring that matched the entire pattern, whereas
       higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
       string(),  the  string  is  placed  in buffer, whose length is given by
       buffersize, while for pcre_get_substring() a new  block  of  memory  is
       obtained  via  pcre_malloc,  and its address is returned via stringptr.
       The yield of the function is the length of the  string,  not  including
       the terminating zero, or one of these error codes:

         PCRE_ERROR_NOMEMORY       (-6)

       The  buffer  was too small for pcre_copy_substring(), or the attempt to
       get memory failed for pcre_get_substring().

         PCRE_ERROR_NOSUBSTRING    (-7)

       There is no substring whose number is stringnumber.

       The pcre_get_substring_list()  function  extracts  all  available  sub-
       strings  and  builds  a list of pointers to them. All this is done in a
       single block of memory that is obtained via pcre_malloc. The address of
       the  memory  block  is returned via listptr, which is also the start of
       the list of string pointers. The end of the list is marked  by  a  NULL
       pointer.  The  yield  of  the function is zero if all went well, or the
       error code

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

       When any of these functions encounter a substring that is unset,  which
       can  happen  when  capturing subpattern number n+1 matches some part of
       the subject, but subpattern n has not been used at all, they return  an
       empty string. This can be distinguished from a genuine zero-length sub-
       string by inspecting the appropriate offset in ovector, which is  nega-
       tive for unset substrings.

       The  two convenience functions pcre_free_substring() and pcre_free_sub-
       string_list() can be used to free the memory  returned  by  a  previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively. They do nothing more than  call  the  function  pointed  to  by
       pcre_free,  which  of course could be called directly from a C program.
       However, PCRE is used in some situations where it is linked via a  spe-
       cial   interface  to  another  programming  language  that  cannot  use
       pcre_free directly; it is for these cases that the functions  are  pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
       subpattern of that name.

       Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

       Most    of    the    arguments   of   pcre_copy_named_substring()   and
       pcre_get_named_substring() are the same  as  those  for  the  similarly
       named  functions  that extract by number. As these are described in the
       previous section, they are not re-described here. There  are  just  two
       differences:

       First,  instead  of a substring number, a substring name is given. Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to  the compiled pattern. This is needed in order to gain access to the
       name-to-number translation table.

       These functions call pcre_get_stringnumber(), and if it succeeds,  they
       then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
       ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
       behaviour may not be what you want (see the next section).

       Warning: If the pattern uses the (?| feature to set up multiple subpat-
       terns with the same number, as described in the  section  on  duplicate
       subpattern  numbers  in  the  pcrepattern page, you cannot use names to
       distinguish the different subpatterns, because names are  not  included
       in  the compiled code. The matching process uses only numbers. For this
       reason, the use of different names for subpatterns of the  same  number
       causes an error at compile time.


DUPLICATE SUBPATTERN NAMES

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
       subpatterns are not required to be unique. (Duplicate names are  always
       allowed  for subpatterns with the same number, created by using the (?|
       feature. Indeed, if such subpatterns are named, they  are  required  to
       use the same names.)

       Normally, patterns with duplicate names are such that in any one match,
       only one of the named subpatterns participates. An example is shown  in
       the pcrepattern documentation.

       When    duplicates   are   present,   pcre_copy_named_substring()   and
       pcre_get_named_substring() return the first substring corresponding  to
       the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
       (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
       function  returns one of the numbers that are associated with the name,
       but it is not defined which it is.

       If you want to get full details of all captured substrings for a  given
       name,  you  must  use  the pcre_get_stringtable_entries() function. The
       first argument is the compiled pattern, and the second is the name. The
       third  and  fourth  are  pointers to variables which are updated by the
       function. After it has run, they point to the first and last entries in
       the  name-to-number  table  for  the  given  name.  The function itself
       returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
       there  are none. The format of the table is described above in the sec-
       tion entitled Information about a pattern above.  Given all  the  rele-
       vant  entries  for the name, you can extract each of their numbers, and
       hence the captured data, if any.


FINDING ALL POSSIBLE MATCHES

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which stops when it finds the first match, starting at a given point in
       the subject. If you want to find all possible matches, or  the  longest
       possible  match,  consider using the alternative matching function (see
       below) instead. If you cannot use the alternative function,  but  still
       need  to  find all possible matches, you can kludge it up by making use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.  When your callout function is called, extract and save the  cur-
       rent  matched  substring.  Then  return  1, which forces pcre_exec() to
       backtrack and try other alternatives. Ultimately, when it runs  out  of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


OBTAINING AN ESTIMATE OF STACK USAGE

       Matching  certain  patterns  using pcre_exec() can use a lot of process
       stack, which in certain environments can be  rather  limited  in  size.
       Some  users  find it helpful to have an estimate of the amount of stack
       that is used by pcre_exec(), to help  them  set  recursion  limits,  as
       described  in  the pcrestack documentation. The estimate that is output
       by pcretest when called with the -m and -C options is obtained by call-
       ing  pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its
       first five arguments.

       Normally, if  its  first  argument  is  NULL,  pcre_exec()  immediately
       returns  the negative error code PCRE_ERROR_NULL, but with this special
       combination of arguments, it returns instead a  negative  number  whose
       absolute  value  is the approximate stack frame size in bytes. (A nega-
       tive number is used so that it is clear that no  match  has  happened.)
       The  value  is  approximate  because  in some cases, recursive calls to
       pcre_exec() occur when there are one or two additional variables on the
       stack.

       If  PCRE  has  been  compiled  to use the heap instead of the stack for
       recursion, the value returned  is  the  size  of  each  block  that  is
       obtained from the heap.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       The  function  pcre_dfa_exec()  is  called  to  match  a subject string
       against a compiled pattern, using a matching algorithm that  scans  the
       subject  string  just  once, and does not backtrack. This has different
       characteristics to the normal algorithm, and  is  not  compatible  with
       Perl.  Some  of the features of PCRE patterns are not supported. Never-
       theless, there are times when this kind of matching can be useful.  For
       a  discussion  of  the  two matching algorithms, and a list of features
       that pcre_dfa_exec() does not support, see the pcrematching  documenta-
       tion.

       The  arguments  for  the  pcre_dfa_exec()  function are the same as for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent  way,  and  this is described below. The other common arguments are
       used in the same way as for pcre_exec(), so their  description  is  not
       repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace will be needed for patterns and subjects where  there  are  a
       lot of potential matches.

       Here is an example of a simple call to pcre_dfa_exec():

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The  unused  bits  of  the options argument for pcre_dfa_exec() must be
       zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
       LINE_xxx,        PCRE_NOTBOL,        PCRE_NOTEOL,        PCRE_NOTEMPTY,
       PCRE_NOTEMPTY_ATSTART,      PCRE_NO_UTF8_CHECK,       PCRE_BSR_ANYCRLF,
       PCRE_BSR_UNICODE,  PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
       TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART.  All but  the  last
       four  of  these  are  exactly  the  same  as  for pcre_exec(), so their
       description is not repeated here.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These have the same general effect as they do for pcre_exec(), but  the
       details  are  slightly  different.  When  PCRE_PARTIAL_HARD  is set for
       pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of  the  sub-
       ject  is  reached  and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
       code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
       of  the  subject  is  reached, there have been no complete matches, but
       there is still at least one matching possibility. The  portion  of  the
       string  that  was inspected when the longest partial match was found is
       set as the first matching string  in  both  cases.   There  is  a  more
       detailed  discussion  of partial and multi-segment matching, with exam-
       ples, in the pcrepartial documentation.

         PCRE_DFA_SHORTEST

       Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When pcre_dfa_exec() returns a partial match, it is possible to call it
       again, with additional subject characters, and have  it  continue  with
       the  same match. The PCRE_DFA_RESTART option requests this action; when
       it is set, the workspace and wscount options must  reference  the  same
       vector  as  before  because data about the match so far is left in them
       after a partial match. There is more discussion of this facility in the
       pcrepartial documentation.

   Successful returns from pcre_dfa_exec()

       When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of  the  function  start  at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For  example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On  success,  the  yield of the function is a number greater than zero,
       which is the number of matched substrings.  The  substrings  themselves
       are  returned  in  ovector. Each string uses two elements; the first is
       the offset to the start, and the second is the offset to  the  end.  In
       fact,  all  the  strings  have the same start offset. (Space could have
       been saved by giving this only once, but it was decided to retain  some
       compatibility  with  the  way pcre_exec() returns data, even though the
       meaning of the strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est  matching  string is given first. If there were too many matches to
       fit into ovector, the yield of the function is zero, and the vector  is
       filled  with  the  longest matches. Unlike pcre_exec(), pcre_dfa_exec()
       can use the entire ovector for returning matched strings.

   Error returns from pcre_dfa_exec()

       The pcre_dfa_exec() function returns a negative number when  it  fails.
       Many  of  the  errors  are  the  same as for pcre_exec(), and these are
       described above.  There are in addition the following errors  that  are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This  return is given if pcre_dfa_exec() encounters an item in the pat-
       tern that it does not support, for instance, the use of \C  or  a  back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This  return  is  given  if pcre_dfa_exec() encounters a condition item
       that uses a back reference for the condition, or a test  for  recursion
       in a specific group. These are not supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This  return  is given if pcre_dfa_exec() is called with an extra block
       that contains a setting of  the  match_limit  or  match_limit_recursion
       fields.  This  is  not  supported (these fields are meaningless for DFA
       matching).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When  a  recursive subpattern is processed, the matching function calls
       itself recursively, using private vectors for  ovector  and  workspace.
       This  error  is  given  if  the output vector is not large enough. This
       should be extremely rare, as a vector of size 1000 is used.


SEE ALSO

       pcre16(3),  pcrebuild(3),  pcrecallout(3),  pcrecpp(3)(3),   pcrematch-
       ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
       pcrestack(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout (pcre16_callout for
       the  16-bit  library).  By  default, this variable contains NULL, which
       disables all calling out.

       Within a regular expression, (?C) indicates the  points  at  which  the
       external  function  is  to  be  called. Different callout points can be
       identified by putting a number less than 256 after the  letter  C.  The
       default  value  is  zero.   For  example,  this pattern has two callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT option bit is set when a pattern is  compiled,
       PCRE  automatically  inserts callouts, all with number 255, before each
       item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
       pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice  that  there  is a callout before and after each parenthesis and
       alternation bar. Automatic  callouts  can  be  used  for  tracking  the
       progress  of  pattern matching. The pcretest command has an option that
       sets automatic callouts; when it is used, the output indicates how  the
       pattern  is  matched. This is useful information when you are trying to
       optimize the performance of a particular pattern.

       The use of callouts in a pattern makes it ineligible  for  optimization
       by  the  just-in-time  compiler.  Studying  such  a  pattern  with  the
       PCRE_STUDY_JIT_COMPILE option always fails.


MISSING CALLOUTS

       You should be aware that, because of  optimizations  in  the  way  PCRE
       matches  patterns  by  default,  callouts  sometimes do not happen. For
       example, if the pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject  string  is "abyz", the lack of "d" means that matching doesn't
       ever start, and the callout is never  reached.  However,  with  "abyd",
       though the result is still no match, the callout is obeyed.

       If  the pattern is studied, PCRE knows the minimum length of a matching
       string, and will immediately give a "no match" return without  actually
       running  a  match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the  PCRE_NO_START_OPTI-
       MIZE  option  to the matching function, or by starting the pattern with
       (*NO_START_OPT). This slows down the matching process, but does  ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During  matching, when PCRE reaches a callout point, the external func-
       tion defined by pcre_callout or pcre16_callout  is  called  (if  it  is
       set).   This applies to both normal and DFA matching. The only argument
       to the callout function is a pointer to a pcre_callout or  pcre16_call-
       out block.  These structures contains the following fields:

         int           version;
         int           callout_number;
         int          *offset_vector;
         const char   *subject;           (8-bit version)
         PCRE_SPTR16   subject;           (16-bit version)
         int           subject_length;
         int           start_match;
         int           current_position;
         int           capture_top;
         int           capture_last;
         void         *callout_data;
         int           pattern_position;
         int           next_item_length;
         const unsigned char *mark;       (8-bit version)
         const PCRE_UCHAR16  *mark;       (16-bit version)

       The  version  field  is an integer containing the version number of the
       block format. The initial version was 0; the current version is 2.  The
       version  number  will  change  again in future if additional fields are
       added, but the intention is never to remove any of the existing fields.

       The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that  was
       passed  by  the  caller  to  the matching function. When pcre_exec() or
       pcre16_exec() is used, the contents  can  be  inspected,  in  order  to
       extract  substrings  that  have been matched so far, in the same way as
       for extracting substrings after a match  has  completed.  For  the  DFA
       matching functions, this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The start_match field normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       When the pcre_exec() or pcre16_exec() is used,  the  capture_top  field
       contains one more than the number of the highest numbered captured sub-
       string so far. If no substrings have been captured, the value  of  cap-
       ture_top  is  one.  This  is always the case when the DFA functions are
       used, because they do not support captured substrings.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case for the DFA matching functions.

       The callout_data field contains a value that is passed  to  a  matching
       function  specifically so that it can be passed back in callouts. It is
       passed in the callout_data field of a pcre_extra or  pcre16_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       callout block is NULL. There is a description of the pcre_extra  struc-
       ture in the pcreapi documentation.

       The  pattern_position  field  is  present from version 1 of the callout
       structure. It contains the offset to the next item to be matched in the
       pattern string.

       The  next_item_length  field  is  present from version 1 of the callout
       structure. It contains the length of the next item to be matched in the
       pattern  string.  When  the callout immediately precedes an alternation
       bar, a closing parenthesis, or the end of the pattern,  the  length  is
       zero.  When  the callout precedes an opening parenthesis, the length is
       that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.

       The mark field is present from version 2 of the callout  structure.  In
       callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
       zero-terminated name of the most recently passed (*MARK), (*PRUNE),  or
       (*THEN)  item  in the match, or NULL if no such items have been passed.
       Instances of (*PRUNE) or (*THEN) without a name  do  not  obliterate  a
       previous  (*MARK).  In  callouts  from  the DFA matching functions this
       field always contains NULL.


RETURN VALUES

       The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  the
       matching function returns the negative value.

       Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 Janurary 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECOMPAT(3)                                                    PCRECOMPAT(3)


NAME
       PCRE - Perl-compatible regular expressions


DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl versions 5.10 and above.

       1. PCRE has only a subset of Perl's Unicode support. Details of what it
       does have are given in the pcreunicode page.

       2. PCRE allows repeat quantifiers only on parenthesized assertions, but
       they  do  not mean what you might think. For example, (?!a){3} does not
       assert that the next three characters are not "a". It just asserts that
       the next character is not "a" three times (in principle: PCRE optimizes
       this to run the assertion just once). Perl allows repeat quantifiers on
       other assertions such as \b, but these do not seem to have any use.

       3.  Capturing  subpatterns  that occur inside negative lookahead asser-
       tions are counted, but their entries in the offsets  vector  are  never
       set.  Perl sets its numerical variables from any such patterns that are
       matched before the assertion fails to match something (thereby succeed-
       ing),  but  only  if the negative lookahead assertion contains just one
       branch.

       4. Though binary zero characters are supported in the  subject  string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5.  The  following Perl escape sequences are not supported: \l, \u, \L,
       \U, and \N when followed by a character name or Unicode value.  (\N  on
       its own, matching a non-newline character, is supported.) In fact these
       are implemented by Perl's general string-handling and are not  part  of
       its  pattern  matching engine. If any of these are encountered by PCRE,
       an error is generated by default. However, if the  PCRE_JAVASCRIPT_COM-
       PAT  option  is set, \U and \u are interpreted as JavaScript interprets
       them.

       6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
       is  built  with Unicode character property support. The properties that
       can be tested with \p and \P are limited to the general category  prop-
       erties  such  as  Lu and Nd, script names such as Greek or Han, and the
       derived properties Any and L&. PCRE does  support  the  Cs  (surrogate)
       property,  which  Perl  does  not; the Perl documentation says "Because
       Perl hides the need for the user to understand the internal representa-
       tion  of Unicode characters, there is no need to implement the somewhat
       messy concept of surrogates."

       7. PCRE implements a simpler version of \X than Perl, which changed  to
       make  \X  match what Unicode calls an "extended grapheme cluster". This
       is more complicated than an extended Unicode sequence,  which  is  what
       PCRE matches.

       8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters in between are treated as literals.  This  is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions.  However,  there is support for recursive patterns. This
       is not available in Perl 5.8, but it is in Perl 5.10.  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat-
       tern matching. See the pcrecallout documentation for details.

       10. Subpatterns that are called as subroutines (whether or  not  recur-
       sively)  are  always  treated  as  atomic  groups in PCRE. This is like
       Python, but unlike Perl.  Captured values that are set outside  a  sub-
       routine  call  can  be  reference from inside in PCRE, but not in Perl.
       There is a discussion that explains these differences in more detail in
       the section on recursion differences from Perl in the pcrepattern page.

       11.  If  (*THEN)  is present in a group that is called as a subroutine,
       its action is limited to that group, even if the group does not contain
       any | characters.

       12.  There are some differences that are concerned with the settings of
       captured strings when part of  a  pattern  is  repeated.  For  example,
       matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
       unset, but in PCRE it is set to "b".

       13. PCRE's handling of duplicate subpattern numbers and duplicate  sub-
       pattern names is not as general as Perl's. This is a consequence of the
       fact the PCRE works internally just with numbers, using an external ta-
       ble  to  translate  between numbers and names. In particular, a pattern
       such as (?|(?<a>A)|(?<b)B), where the two  capturing  parentheses  have
       the  same  number  but different names, is not supported, and causes an
       error at compile time. If it were allowed, it would not be possible  to
       distinguish  which  parentheses matched, because both names map to cap-
       turing subpattern number 1. To avoid this confusing situation, an error
       is given at compile time.

       14.  Perl  recognizes  comments  in some places that PCRE does not, for
       example, between the ( and ? at the start of a subpattern.  If  the  /x
       modifier  is set, Perl allows whitespace between ( and ? but PCRE never
       does, even if the PCRE_EXTENDED option is set.

       15. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  includes new features that are not in earlier ver-
       sions of Perl, some of which (such as named parentheses) have  been  in
       PCRE for some time. This list is with respect to Perl 5.10:

       (a)  Although  lookbehind  assertions  in  PCRE must match fixed length
       strings, each alternative branch of a lookbehind assertion can match  a
       different  length  of  string.  Perl requires them all to have the same
       length.

       (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.  (Perl can be made to issue a warning.)

       (d)  If  PCRE_UNGREEDY is set, the greediness of the repetition quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
       and  PCRE_NO_AUTO_CAPTURE  options for pcre_exec() have no Perl equiva-
       lents.

       (g) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.  However,  this
       does not apply to optimized data created by the just-in-time compiler.

       (k)   The   alternative   matching   functions   (pcre_dfa_exec()   and
       pcre16_dfa_exec()) match in a different way and are  not  Perl-compati-
       ble.

       (l)  PCRE  recognizes some special sequences such as (*CR) at the start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 Januray 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there  is  now also support for UTF-8 strings in the original
       library, and a second library that supports 16-bit and UTF-16 character
       strings. To use these features, PCRE must be built to include appropri-
       ate support. When using UTF strings you must either call the  compiling
       function  with  the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
       start with one of these special sequences:

         (*UTF8)
         (*UTF16)

       Starting a pattern with such a sequence is equivalent  to  setting  the
       relevant option. This feature is not Perl-compatible. How setting a UTF
       mode affects pattern matching is mentioned  in  several  places  below.
       There is also a summary of features in the pcreunicode page.

       Another  special  sequence that may appear at the start of a pattern or
       in combination with (*UTF8) or (*UTF16) is:

         (*UCP)

       This has the same effect as setting  the  PCRE_UCP  option:  it  causes
       sequences  such  as  \d  and  \w to use Unicode properties to determine
       character types, instead of recognizing only characters with codes less
       than 128 via a lookup table.

       If  a  pattern  starts  with (*NO_START_OPT), it has the same effect as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
       time. There are also some more of these special sequences that are con-
       cerned with the handling of newlines; they are described below.

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE  when  one  its  main  matching functions, pcre_exec()
       (8-bit) or pcre16_exec() (16-bit), is used. PCRE also  has  alternative
       matching  functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
       using a different algorithm that is not Perl-compatible.  Some  of  the
       features  discussed  below are not available when DFA matching is used.
       The advantages and disadvantages of the alternative functions, and  how
       they  differ from the normal functions, are discussed in the pcrematch-
       ing page.


NEWLINE CONVENTIONS

       PCRE supports five different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or  any Unicode newline sequence. The pcreapi page has further
       discussion about newlines, and shows how to set the newline  convention
       in the options arguments for the compiling and matching functions.

       It  is also possible to specify a newline convention by starting a pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to the compiling func-
       tion.  For  example,  on  a Unix system where LF is the default newline
       sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no  longer  a  newline. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start  of  a  pattern,
       and  that  they  must  be  in  upper  case. If more than one of them is
       present, the last one is used.

       The newline convention affects the interpretation of the dot  metachar-
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How-
       ever, it does not affect  what  the  \R  escape  sequence  matches.  By
       default,  this is any Unicode newline sequence, for Perl compatibility.
       However, this can be changed; see the description of \R in the  section
       entitled  "Newline sequences" below. A change of \R setting can be com-
       bined with a change of newline convention.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched  independently  of case. In a UTF mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher val-
       ues, the concept of case is supported if PCRE is compiled with  Unicode
       property  support,  but  not  otherwise.   If  you want to use caseless
       matching for characters 128 and above, you must  ensure  that  PCRE  is
       compiled with Unicode property support as well as with UTF support.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

       In a UTF mode, only ASCII numbers and letters have any special  meaning
       after  a  backslash.  All  other characters (in particular, those whose
       codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap-
       ing  backslash  can  be  used to include a whitespace or # character as
       part of the pattern.

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.   An  isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the pattern, the literal  interpretation
       continues  to  the  end  of  the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class,  this  causes  an
       error, because the character class is not terminated.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a pattern  is  being  prepared  by  text
       editing,  it  is  often  easier  to  use  one  of  the following escape
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or back reference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
         \uhhhh    character with hex code hhhh (JavaScript mode only)

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),  while  \c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This  locks
       out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
       mode, all byte values are valid. A lower case letter  is  converted  to
       upper case, and then the 0xc0 bits are flipped.)

       By  default,  after  \x,  from  zero to two hexadecimal digits are read
       (letters can be in upper or lower case). Any number of hexadecimal dig-
       its may appear between \x{ and }, but the character code is constrained
       as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint

       Invalid Unicode codepoints are the range  0xd800  to  0xdfff  (the  so-
       called "surrogate" codepoints).

       If  characters  other than hexadecimal digits appear between \x{ and },
       or if there is no terminating }, this form of escape is not recognized.
       Instead,  the  initial  \x  will  be interpreted as a basic hexadecimal
       escape, with no following digits, giving a  character  whose  value  is
       zero.

       If  the  PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
       is as just described only when it is followed by two  hexadecimal  dig-
       its.   Otherwise,  it  matches  a  literal "x" character. In JavaScript
       mode, support for code points greater than 256 is provided by \u, which
       must  be  followed  by  four hexadecimal digits; otherwise it matches a
       literal "u" character.

       Characters whose value is less than 256 can be defined by either of the
       two  syntaxes for \x (or by \u in JavaScript mode). There is no differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as \x{dc} (or \u00dc in JavaScript mode).

       After  \0  up  to two further octal digits are read. If there are fewer
       than two digits, just  those  that  are  present  are  used.  Thus  the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code value 7). Make sure you supply two digits after the initial  zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its  as  a  decimal  number. If the number is less than 10, or if there
       have been at least that many previous capturing left parentheses in the
       expression,  the  entire  sequence  is  taken  as  a  back reference. A
       description of how this works is given later, following the  discussion
       of parenthesized subpatterns.

       Inside  a  character  class, or if the decimal number is greater than 9
       and there have not been that many capturing subpatterns, PCRE  re-reads
       up to three octal digits following the backslash, and uses them to gen-
       erate a data character. Any subsequent digits stand for themselves. The
       value  of  the  character  is constrained in the same way as characters
       specified in hexadecimal.  For example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the value 255 (decimal)
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

       Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

       \N  is not allowed in a character class. \B, \R, and \X are not special
       inside a character class. Like  other  unrecognized  escape  sequences,
       they  are  treated  as  the  literal  characters  "B",  "R", and "X" by
       default, but cause an error if the PCRE_EXTRA option is set. Outside  a
       character class, these sequences have different meanings.

   Unsupported escape sequences

       In  Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used  to  modify  the  case  of  following  characters.  By
       default,  PCRE does not support these escape sequences. However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U"  character,  and
       \u can be used to define a character by code point, as described in the
       previous section.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal whitespace character
         \H     any character that is not a horizontal whitespace character
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \v     any vertical whitespace character
         \V     any character that is not a vertical whitespace character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline char-
       acter.   This  is the same as the "." metacharacter when PCRE_DOTALL is
       not set. Perl also uses \N to match characters by name; PCRE  does  not
       support this.

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two disjoint  sets.  Any  given  character
       matches  one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate  type.  If the current matching point is at the end of
       the subject string, all of them fail, because there is no character  to
       match.

       For  compatibility  with Perl, \s does not match the VT character (code
       11).  This makes it different from the the POSIX "space" class. The  \s
       characters  are  HT  (9), LF (10), FF (12), CR (13), and space (32). If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.

       A  "word"  character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters  and  digits  is  con-
       trolled  by PCRE's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the  pcreapi
       page).  For  example,  in  a French locale such as "fr_FR" in Unix-like
       systems, or "french" in Windows, some character codes greater than  128
       are  used  for  accented letters, and these are then matched by \w. The
       use of locales with Unicode is discouraged.

       By default, in a UTF mode, characters  with  values  greater  than  128
       never  match  \d,  \s,  or  \w,  and always match \D, \S, and \W. These
       sequences retain their original meanings from before  UTF  support  was
       available,  mainly for efficiency reasons. However, if PCRE is compiled
       with Unicode property support, and the PCRE_UCP option is set, the  be-
       haviour  is  changed  so  that Unicode properties are used to determine
       character types, as follows:

         \d  any character that \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, plus underscore

       The upper case escapes match the inverse sets of characters. Note  that
       \d  matches  only decimal digits, whereas \w matches any Unicode digit,
       as well as any Unicode letter, and underscore. Note also that  PCRE_UCP
       affects  \b,  and  \B  because  they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when PCRE_UCP is set.

       The sequences \h, \H, \v, and \V are features that were added  to  Perl
       at  release  5.10. In contrast to the other sequences, which match only
       ASCII characters by default, these  always  match  certain  high-valued
       codepoints,  whether or not PCRE_UCP is set. The horizontal space char-
       acters are:

         U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is  equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In other modes, two additional characters whose codepoints are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode character property support is not  needed  for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when  PCRE  is  built;  if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
       specify  these  settings  by  starting a pattern string with one of the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion,  but  they  can  themselves  be  overridden by options given to a
       matching function. Note that these  special  settings,  which  are  not
       Perl-compatible,  are  recognized  only at the very start of a pattern,
       and that they must be in upper case.  If  more  than  one  of  them  is
       present,  the  last  one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
       sequences.  Inside  a character class, \R is treated as an unrecognized
       escape sequence, and so matches the letter "R" by default,  but  causes
       an error if PCRE_EXTRA is set.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional escape sequences that match characters with specific  properties
       are  available.   When  in 8-bit non-UTF-8 mode, these sequences are of
       course limited to testing characters whose  codepoints  are  less  than
       256, but they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character   (including  newline),  and  some  special  PCRE  properties
       (described in the next section).  Other Perl properties such as  "InMu-
       sicalSymbols"  are  not  currently supported by PCRE. Note that \P{Any}
       does not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

       Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
       Coptic,   Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyp-
       tian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic,   Gothic,   Greek,
       Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe-
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
       Latin,  Lepcha,  Limbu,  Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
       Phags_Pa,  Phoenician,  Rejang,  Runic, Samaritan, Saurashtra, Shavian,
       Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
       Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
       Ugaritic, Vai, Yi.

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings  and
       so  cannot  be  tested  by  PCRE, unless UTF validity checking has been
       turned   off   (see   the   discussion   of   PCRE_NO_UTF8_CHECK    and
       PCRE_NO_UTF16_CHECK  in the pcreapi page). Perl does not support the Cs
       property.

       The long synonyms for  property  names  that  Perl  supports  (such  as
       \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying caseless matching does not affect  these  escape  sequences.
       For example, \p{Lu} always matches only upper case letters.

       The  \X  escape  matches  any number of Unicode characters that form an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That is, it matches a character without the "mark"  property,  followed
       by  zero  or  more  characters with the "mark" property, and treats the
       sequence as an atomic group (see below).  Characters  with  the  "mark"
       property  are  typically  accents  that affect the preceding character.
       None of them have codepoints less than 256, so in 8-bit non-UTF-8  mode
       \X matches any one character.

       Note that recent versions of Perl have changed \X to match what Unicode
       calls an "extended grapheme cluster", which has a more complicated def-
       inition.

       Matching  characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE by  default,  though  you  can
       make  them do so by setting the PCRE_UCP option or by starting the pat-
       tern with (*UCP).

   PCRE's additional properties

       As well as the standard Unicode properties described  in  the  previous
       section,  PCRE supports four more that make it possible to convert tra-
       ditional escape sequences such as \w and \s and POSIX character classes
       to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
       erties internally when PCRE_UCP is set. They are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       formfeed, or carriage return, and any other character that  has  the  Z
       (separator) property.  Xsp is the same as Xps, except that vertical tab
       is excluded. Xwd matches the same characters as Xan, plus underscore.

   Resetting the match start

       The escape sequence \K causes any previously matched characters not  to
       be included in the final matched sequence. For example, the pattern:

         foo\Kbar

       matches  "foobar",  but reports that it has matched "bar". This feature
       is similar to a lookbehind assertion (described  below).   However,  in
       this  case, the part of the subject before the real match does not have
       to be of fixed length, as lookbehind assertions do. The use of \K  does
       not  interfere  with  the setting of captured substrings.  For example,
       when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl documents that the use  of  \K  within  assertions  is  "not  well
       defined".  In  PCRE,  \K  is  acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions.

   Simple assertions

       The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of subpatterns for more complicated assertions is described below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning;  it  matches  the
       backspace  character.  If  any  other  of these assertions appears in a
       character class, by default it matches the corresponding literal  char-
       acter  (for  example,  \B  matches  the  letter  B).  However,  if  the
       PCRE_EXTRA option is set, an "invalid escape sequence" error is  gener-
       ated instead.

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w,  respectively.  In  a
       UTF  mode,  the  meanings  of  \w  and \W can be changed by setting the
       PCRE_UCP option. When this is done, it also affects \b and \B.  Neither
       PCRE  nor  Perl has a separate "start of word" or "end of word" metase-
       quence. However, whatever follows \b normally determines which  it  is.
       For example, the fragment \ba matches "a" at the start of a word.

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect only the behaviour of the circumflex and dollar  metacharacters.
       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
       cating that matching is to start at a point other than the beginning of
       the  subject,  \A  can never match. The difference between \Z and \z is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the match, as specified by the startoffset  argument
       of  pcre_exec().  It  differs  from \A when the value of startoffset is
       non-zero. By calling pcre_exec() multiple times with appropriate  argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note, however, that PCRE's interpretation of \G, as the  start  of  the
       current match, is subtly different from Perl's, which defines it as the
       end of the previous match. In Perl, these can  be  different  when  the
       previously  matched  string was empty. Because PCRE does just one match
       at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character  is  an  assertion  that is true only if the current matching
       point is at the start of the subject string. If the  startoffset  argu-
       ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
       PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
       has an entirely different meaning (see below).

       Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       A  dollar  character  is  an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before a newline at the end of the string (by default). Dollar need not
       be the last character of the pattern if a number  of  alternatives  are
       involved,  but  it  should  be  the last item in any branch in which it
       appears. Dollar has no special meaning in a character class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex
       matches immediately after internal newlines as well as at the start  of
       the  subject  string.  It  does not match after a newline that ends the
       string. A dollar matches before any newlines in the string, as well  as
       at  the very end, when PCRE_MULTILINE is set. When newline is specified
       as the two-character sequence CRLF, isolated CR and  LF  characters  do
       not indicate newlines.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if
       PCRE_MULTILINE is set.

       Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter  in  the subject string except (by default) a character that signi-
       fies the end of a line.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like  a  dot,  except  that  it  is  not
       affected  by  the  PCRE_DOTALL  option.  In other words, it matches any
       character except one that signifies the end of a line. Perl  also  uses
       \N to match characters by name; PCRE does not support this.


MATCHING A SINGLE DATA UNIT

       Outside  a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one  data
       unit  is  one byte; in the 16-bit library it is a 16-bit unit. Unlike a
       dot, \C always matches line-ending characters. The feature is  provided
       in  Perl  in  order  to match individual bytes in UTF-8 mode, but it is
       unclear how it can usefully be used. Because \C  breaks  up  characters
       into  individual  data  units,  matching one unit with \C in a UTF mode
       means that the rest of the string may start with a malformed UTF  char-
       acter.  This  has  undefined  results,  because PCRE assumes that it is
       dealing with valid UTF strings (and by default it checks  this  at  the
       start of processing unless the PCRE_NO_UTF8_CHECK option is used).

       PCRE  does  not  allow \C to appear in lookbehind assertions (described
       below) in a UTF mode, because this would make it impossible  to  calcu-
       late the length of the lookbehind.

       In general, the \C escape sequence is best avoided. However, one way of
       using it that avoids the problem of malformed UTF characters is to  use
       a  lookahead to check the length of the next character, as in this pat-
       tern, which could be used with a UTF-8 string (ignore white  space  and
       line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A  group  that starts with (?| resets the capturing parentheses numbers
       in each alternative (see "Duplicate  Subpattern  Numbers"  below).  The
       assertions  at  the start of each branch check the next UTF-8 character
       for values whose encoding uses 1, 2, 3, or 4 bytes,  respectively.  The
       character's  individual bytes are then captured by the appropriate num-
       ber of groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is set,
       a lone closing square bracket causes a compile-time error. If a closing
       square bracket is required as a member of the class, it should  be  the
       first  data  character  in  the  class (after an initial circumflex, if
       present) or escaped with a backslash.

       A character class matches a single character in the subject. In  a  UTF
       mode,  the  character  may  be  more than one data unit long. A matched
       character must be in the set of characters defined by the class, unless
       the  first  character in the class definition is a circumflex, in which
       case the subject character must not be in the set defined by the class.
       If  a  circumflex is actually required as a member of the class, ensure
       it is not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case  vowel,
       while  [^aeiou]  matches  any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion; it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       In UTF-8  (UTF-16)  mode,  characters  with  values  greater  than  255
       (0xffff)  can be included in a class as a literal string of data units,
       or by using the \x{ escaping mechanism.

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In a UTF mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching in a UTF mode for characters 128 and above, you  must
       ensure  that  PCRE is compiled with Unicode property support as well as
       with UTF support.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. Ranges can include any characters that are valid  for  the
       current mode.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to  [][\\^_`wxyzabc],  matched  caselessly,  and  in a non-UTF mode, if
       character tables for a French locale are in  use,  [\xc8-\xcb]  matches
       accented  E  characters  in both cases. In UTF modes, PCRE supports the
       concept of case for characters with values greater than 128  only  when
       it is compiled with Unicode property support.

       The  character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
       \w, and \W may appear in a character class, and add the characters that
       they  match to the class. For example, [\dABCDEF] matches any hexadeci-
       mal digit. In UTF modes, the PCRE_UCP option affects  the  meanings  of
       \d,  \s,  \w  and  their upper case partners, just as it does when they
       appear outside a character class, as described in the section  entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning inside a character class; it matches the  backspace  character.
       The  sequences  \B,  \N,  \R, and \X are not special inside a character
       class. Like any other unrecognized escape sequences, they  are  treated
       as  the literal characters "B", "N", "R", and "X" by default, but cause
       an error if the PCRE_EXTRA option is set.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, in UTF modes, characters with values greater  than  128  do
       not  match any of the POSIX character classes. However, if the PCRE_UCP
       option is passed to pcre_compile(), some of the classes are changed  so
       that Unicode character properties are used. This is achieved by replac-
       ing the POSIX classes by other sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of  \p.  The  other
       POSIX classes are unchanged, and match only characters with code points
       less than 128.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the subpattern.


INTERNAL OPTION SETTING

       The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
       PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
       within the pattern by  a  sequence  of  Perl  option  letters  enclosed
       between "(?" and ")".  The option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
       LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
       is  also  permitted.  If  a  letter  appears  both before and after the
       hyphen, the option is unset.

       The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
       can  be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.

       When one of these option changes occurs at  top  level  (that  is,  not
       inside  subpattern parentheses), the change applies to the remainder of
       the pattern that follows. If the change is placed right at the start of
       a pattern, PCRE extracts it into the global options (and it will there-
       fore show up in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern (see below for  a  description  of
       subpatterns)  affects only that part of the subpattern that follows it,
       so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).   By  this means, options can be made to have different settings
       in different parts of the pattern. Any changes made in one  alternative
       do  carry  on  into subsequent branches within the same subpattern. For
       example,

         (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There  would  be
       some very weird behaviour otherwise.

       Note:  There  are  other  PCRE-specific  options that can be set by the
       application when the compiling or matching  functions  are  called.  In
       some  cases  the  pattern can contain special leading sequences such as
       (*CRLF) to override what the application  has  set  or  what  has  been
       defaulted.   Details   are  given  in  the  section  entitled  "Newline
       sequences" above. There are also  the  (*UTF8),  (*UTF16),  and  (*UCP)
       leading  sequences  that  can  be  used to set UTF and Unicode property
       modes; they are equivalent to setting the  PCRE_UTF8,  PCRE_UTF16,  and
       the PCRE_UCP options, respectively.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat". Without  the  parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
       that, when the whole pattern  matches,  that  portion  of  the  subject
       string that matched the subpattern is passed back to the caller via the
       ovector argument of the matching function. (This applies  only  to  the
       traditional  matching functions; the DFA matching functions do not sup-
       port capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain  numbers  for  the  capturing  subpatterns.  For example, if the
       string "the red king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when a grouping subpattern is  required
       without  a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any  captur-
       ing,  and  is  not  counted when computing the number of any subsequent
       capturing subpatterns. For example, if the string "the white queen"  is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required  at  the
       start  of  a  non-capturing  subpattern,  the option letters may appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does  affect
       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
       "Saturday".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses  the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For  example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because  the two alternatives are inside a (?| group, both sets of cap-
       turing parentheses are numbered one. Thus, when  the  pattern  matches,
       you  can  look  at captured substring number one, whichever alternative
       matched. This construct is useful when you want to  capture  part,  but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses are numbered as usual, but the number is reset at the  start  of
       each  branch.  The numbers of any capturing parentheses that follow the
       subpattern start after the highest number used in any branch. The  fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A back reference to a numbered subpattern uses the  most  recent  value
       that  is  set  for that number by any subpattern. The following pattern
       matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a numbered subpattern  always  refers
       to  the  first  one in the pattern with the given number. The following
       pattern matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       If a condition test for a subpattern's having matched refers to a  non-
       unique  number, the test is true if any of the subpatterns of that num-
       ber have matched.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had the feature earlier, and PCRE introduced it at release  4.0,  using
       the  Python syntax. PCRE now supports both the Perl and the Python syn-
       tax. Perl allows identically numbered  subpatterns  to  have  different
       names, but PCRE does not.

       In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
       or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
       to  capturing parentheses from other parts of the pattern, such as back
       references, recursion, and conditions, can be made by name as  well  as
       by number.

       Names  consist  of  up  to  32 alphanumeric characters and underscores.
       Named capturing parentheses are still  allocated  numbers  as  well  as
       names,  exactly as if the names were not present. The PCRE API provides
       function calls for extracting the name-to-number translation table from
       a compiled pattern. There is also a convenience function for extracting
       a captured substring by name.

       By default, a name must be unique within a pattern, but it is  possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time. (Duplicate names are also always permitted for  subpatterns  with
       the  same  number, set up as described in the previous section.) Dupli-
       cate names can be useful for patterns where only one  instance  of  the
       named  parentheses  can  match. Suppose you want to match the name of a
       weekday, either as a 3-letter abbreviation or as the full name, and  in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There are five capturing substrings, but only one is ever set  after  a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The convenience function for extracting the data by  name  returns  the
       substring  for  the first (and in this example, the only) subpattern of
       that name that matched. This saves searching  to  find  which  numbered
       subpattern it was.

       If  you  make  a  back  reference to a non-unique named subpattern from
       elsewhere in the pattern, the one that corresponds to the first  occur-
       rence of the name is used. In the absence of duplicate numbers (see the
       previous section) this is the one with the lowest number. If you use  a
       named  reference  in a condition test (see the section about conditions
       below), either to check whether a subpattern has matched, or  to  check
       for  recursion,  all  subpatterns with the same name are tested. If the
       condition is true for any one of them, the overall condition  is  true.
       This is the same behaviour as testing by number. For further details of
       the interfaces for handling named subpatterns, see the pcreapi documen-
       tation.

       Warning: You cannot use different names to distinguish between two sub-
       patterns with the same number because PCRE uses only the  numbers  when
       matching. For this reason, an error is given at compile time if differ-
       ent names are given to subpatterns with the same number.  However,  you
       can  give  the same name to subpatterns with the same number, even when
       PCRE_DUPNAMES is not set.


REPETITION

       Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by  a comma. The numbers must be less than 65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches exactly 8 digits. An opening curly bracket that  appears  in  a
       position  where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In UTF modes, quantifiers apply to characters rather than to individual
       data units. Thus, for example, \x{100}{2} matches two characters,  each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended sequences,  each  of  which
       may be several data units long (and they may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful  for  subpatterns that are referenced as subroutines from elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for  use  by  reference only" below). Items other than subpatterns that
       have a {0} quantifier are omitted from the compiled pattern.

       For convenience, the three most common quantifiers have  single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It  is  possible  to construct infinite loops by following a subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for such patterns. However, because there are cases where this  can  be
       useful,  such  patterns  are now accepted, but if any repetition of the
       subpattern does in fact match no characters, the loop is forcibly  bro-
       ken.

       By  default,  the quantifiers are "greedy", that is, they match as much
       as possible (up to the maximum  number  of  permitted  times),  without
       causing  the  rest of the pattern to fail. The classic example of where
       this gives problems is in trying to match comments in C programs. These
       appear  between  /*  and  */ and within the comment, individual * and /
       characters may appear. An attempt to match C comments by  applying  the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails,  because it matches the entire string owing to the greediness of
       the .*  item.

       However, if a quantifier is followed by a question mark, it  ceases  to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does the right thing with the C comments. The meaning  of  the  various
       quantifiers  is  not  otherwise  changed,  just the preferred number of
       matches.  Do not confuse this use of question mark with its  use  as  a
       quantifier  in its own right. Because it has two uses, it can sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If  the PCRE_UNGREEDY option is set (an option that is not available in
       Perl), the quantifiers are not greedy by default, but  individual  ones
       can  be  made  greedy  by following them with a question mark. In other
       words, it inverts the default behaviour.

       When a parenthesized subpattern is quantified  with  a  minimum  repeat
       count  that is greater than 1 or with a limited maximum, more memory is
       required for the compiled pattern, in proportion to  the  size  of  the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent to Perl's /s) is set, thus allowing the dot  to  match  newlines,
       the  pattern  is  implicitly anchored, because whatever follows will be
       tried against every character position in the subject string, so  there
       is  no  point  in  retrying the overall match at any position after the
       first. PCRE normally treats such a pattern as though it  were  preceded
       by \A.

       In  cases  where  it  is known that the subject string contains no new-
       lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However,  there is one situation where the optimization cannot be used.
       When .*  is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

         (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth  charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is  "tweedledee".  However,  if there are nested capturing subpatterns,
       the corresponding captured values may have been set in previous  itera-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the  repeated  item
       to  be  re-evaluated to see if a different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to  prevent  this,
       either  to  change the nature of the match, or to cause it fail earlier
       than it otherwise might, when the author of the pattern knows there  is
       no point in carrying on.

       Consider,  for  example, the pattern \d+foo when applied to the subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action  of  the matcher is to try again with only 5 digits matching the
       \d+ item, and then with  4,  and  so  on,  before  ultimately  failing.
       "Atomic  grouping"  (a  term taken from Jeffrey Friedl's book) provides
       the means for specifying that once a subpattern has matched, it is  not
       to be re-evaluated in this way.

       If  we  use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo" the first time.  The  notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of  this  type  matches
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive   quantifiers   are   always  greedy;  the  setting  of  the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler  forms  of atomic group. However, there is no difference in the
       meaning of a possessive quantifier and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built  Sun's Java package, and PCRE copied it from there. It ultimately
       found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain sim-
       ple  pattern  constructs.  For  example, the sequence A+B is treated as
       A++B because there is no point in backtracking into a sequence  of  A's
       when B must follow.

       When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE and Perl have an optimization that allows  for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern earlier (that is, to its left) in the pattern,  provided  there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it is always taken as a back reference, and causes  an  error  only  if
       there  are  not that many capturing left parentheses in the entire pat-
       tern. In other words, the parentheses that are referenced need  not  be
       to  the left of the reference for numbers less than 10. A "forward back
       reference" of this type can make sense when a  repetition  is  involved
       and  the  subpattern to the right has participated in an earlier itera-
       tion.

       It is not possible to have a numerical "forward back  reference"  to  a
       subpattern  whose  number  is  10  or  more using this syntax because a
       sequence such as \50 is interpreted as a character  defined  in  octal.
       See the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash.  There  is  no
       such  problem  when named parentheses are used. A back reference to any
       subpattern is possible using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in  the  use  of  digits
       following  a  backslash  is  to use the \g escape sequence. This escape
       must be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An  unsigned number specifies an absolute reference without the ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing subpattern before \g, that is, is it equivalent to \2 in this exam-
       ple.  Similarly, \g{-2} would be equivalent to \1. The use of  relative
       references  can  be helpful in long patterns, and also in patterns that
       are created by  joining  together  fragments  that  contain  references
       within themselves.

       A  back  reference matches whatever actually matched the capturing sub-
       pattern in the current subject string, rather  than  anything  matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not  "sense and responsibility". If caseful matching is in force at the
       time of the back reference, the case of letters is relevant. For  exam-
       ple,

         ((?i)rah)\s+\1

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
       original capturing subpattern is matched caselessly.

       There are several different ways of writing back  references  to  named
       subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
       \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and named references, is also supported. We  could  rewrite  the  above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A  subpattern  that  is  referenced  by  name may appear in the pattern
       before or after the reference.

       There may be more than one back reference to the same subpattern. If  a
       subpattern  has  not actually been used in a particular match, any back
       references to it always fail by default. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than  "bc".  However,  if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
       ence to an unset value matches an empty string.

       Because there may be many capturing parentheses in a pattern, all  dig-
       its  following a backslash are taken as part of a potential back refer-
       ence number.  If the pattern continues with  a  digit  character,  some
       delimiter  must  be  used  to  terminate  the  back  reference.  If the
       PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
       syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       A  back reference that occurs inside the parentheses to which it refers
       fails when the subpattern is first used, so, for example,  (a\1)  never
       matches.   However,  such references can be useful inside repeated sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  of  the  subpattern,  the  back  reference matches the character
       string corresponding to the previous iteration. In order  for  this  to
       work,  the  pattern must be such that the first iteration does not need
       to match the back reference. This can be done using alternation, as  in
       the example above, or by a quantifier with a minimum of zero.

       Back  references of this type cause the group that they reference to be
       treated as an atomic group.  Once the whole group has been  matched,  a
       subsequent  matching  failure cannot cause backtracking into the middle
       of the group.


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current  matching  point that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
       described above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,  and  those  that  look  behind  it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching position to be changed.

       Assertion  subpatterns are not capturing subpatterns. If such an asser-
       tion contains capturing subpatterns within it, these  are  counted  for
       the  purposes  of numbering the capturing subpatterns in the whole pat-
       tern. However, substring capturing is carried  out  only  for  positive
       assertions, because it does not make sense for negative assertions.

       For  compatibility  with  Perl,  assertion subpatterns may be repeated;
       though it makes no sense to assert the same thing  several  times,  the
       side  effect  of  capturing  parentheses may occasionally be useful. In
       practice, there only three cases:

       (1) If the quantifier is {0}, the  assertion  is  never  obeyed  during
       matching.   However,  it  may  contain internal capturing parenthesized
       groups that are called from elsewhere via the subroutine mechanism.

       (2) If quantifier is {0,n} where n is greater than zero, it is  treated
       as  if  it  were  {0,1}.  At run time, the rest of the pattern match is
       tried with and without the assertion, the order depending on the greed-
       iness of the quantifier.

       (3)  If  the minimum repetition is greater than zero, the quantifier is
       ignored.  The assertion is obeyed just  once  when  encountered  during
       matching.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches a word followed by a semicolon, but does not include the  semi-
       colon in the match, and

         foo(?!bar)

       matches  any  occurrence  of  "foo" that is not followed by "bar". Note
       that the apparently similar pattern

         (?!foo)bar

       does not find an occurrence of "bar"  that  is  preceded  by  something
       other  than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is  with  (?!)  because  an  empty  string
       always  matches, so an assertion that requires there not to be an empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind  assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does find an occurrence of "bar" that is not  preceded  by  "foo".  The
       contents  of  a  lookbehind  assertion are restricted such that all the
       strings it matches must have a fixed length. However, if there are sev-
       eral  top-level  alternatives,  they  do  not all have to have the same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes an error at compile time. Branches that match  different  length
       strings  are permitted only at the top level of a lookbehind assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

         (?<=ab(c|de))

       is  not  permitted,  because  its single top-level branch can match two
       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

         (?<=abc|abde)

       In  some  cases, the escape sequence \K (see above) can be used instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The implementation of lookbehind assertions is, for  each  alternative,
       to  temporarily  move the current position back by the fixed length and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       In  a UTF mode, PCRE does not allow the \C escape (which matches a sin-
       gle data unit even in a UTF mode) to appear in  lookbehind  assertions,
       because  it  makes it impossible to calculate the length of the lookbe-
       hind. The \X and \R escapes, which can match different numbers of  data
       units, are also not permitted.

       "Subroutine"  calls  (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the subpattern matches a  fixed-length  string.
       Recursion, however, is not supported.

       Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

         abcd$

       when  applied  to  a  long string that does not match. Because matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and  then  see  if what follows matches the rest of the pattern. If the
       pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this  fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       again  the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item; it can  match  only  the
       entire  string.  The subsequent lookbehind assertion does a single test
       on the last four characters. If it fails, the match fails  immediately.
       For  long  strings, this approach makes a significant difference to the
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This  time  the  first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern con-
       ditionally or to choose between two alternative subpatterns,  depending
       on  the result of an assertion, or whether a specific capturing subpat-
       tern has already been matched. The two possible  forms  of  conditional
       subpattern are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more  than  two  alterna-
       tives  in  the subpattern, a compile-time error occurs. Each of the two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing  conditional  subpatterns;  the  restriction  to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There  are  four  kinds of condition: references to subpatterns, refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the condition is true if a capturing subpattern of that number has pre-
       viously matched. If there is more than one  capturing  subpattern  with
       the  same  number  (see  the earlier section about duplicate subpattern
       numbers), the condition is true if any of them have matched. An  alter-
       native  notation is to precede the digits with a plus or minus sign. In
       this case, the subpattern number is relative rather than absolute.  The
       most  recently opened parentheses can be referenced by (?(-1), the next
       most recent by (?(-2), and so on. Inside loops it can also  make  sense
       to refer to subsequent groups. The next parentheses to be opened can be
       referenced as (?(+1), and so on. (The value zero in any of these  forms
       is not used; it provokes a compile-time error.)

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional subpattern that tests whether  or  not  the
       first  set  of  parentheses  matched.  If they did, that is, if subject
       started with an opening parenthesis, the condition is true, and so  the
       yes-pattern  is  executed and a closing parenthesis is required. Other-
       wise, since no-pattern is not present, the subpattern matches  nothing.
       In  other  words,  this  pattern matches a sequence of non-parentheses,
       optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  subpattern  by  name.  For compatibility with earlier versions of
       PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
       also  recognized. However, there is a possible ambiguity with this syn-
       tax, because subpattern names may  consist  entirely  of  digits.  PCRE
       looks  first for a named subpattern; if it cannot find one and the name
       consists entirely of digits, PCRE looks for a subpattern of  that  num-
       ber,  which must be greater than zero. Using subpattern names that con-
       sist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate,  the  test
       is  applied to all subpatterns of the same name, and is true if any one
       of them has matched.

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name  R, the condition is true if a recursive call to the whole pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion  stack.  If  the  name  used in a condition of this kind is a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At  "top  level",  all  these recursion test conditions are false.  The
       syntax for recursive patterns is described below.

   Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and  there  is  no  subpattern
       with  the  name  DEFINE,  the  condition is always false. In this case,
       there may be only one alternative  in  the  subpattern.  It  is  always
       skipped  if  control  reaches  this  point  in the pattern; the idea of
       DEFINE is that it can be used to define subroutines that can be  refer-
       enced  from elsewhere. (The use of subroutines is described below.) For
       example, a pattern to match an IPv4 address  such  as  "192.168.23.245"
       could be written like this (ignore whitespace and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The  first part of the pattern is a DEFINE group inside which a another
       group named "byte" is defined. This matches an individual component  of
       an  IPv4  address  (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts  like  a  false
       condition.  The  rest of the pattern uses references to the named group
       to match the four dot-separated components of an IPv4 address,  insist-
       ing on a word boundary at each end.

   Assertion conditions

       If  the  condition  is  not  in any of the above formats, it must be an
       assertion.  This may be a positive or negative lookahead or  lookbehind
       assertion.  Consider  this  pattern,  again  containing non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition  is  a  positive  lookahead  assertion  that  matches  an
       optional  sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject.  If  a
       letter  is found, the subject is matched against the first alternative;
       otherwise it is  matched  against  the  second.  This  pattern  matches
       strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE. In both cases, the start of the comment must not be in a char-
       acter class, nor in the middle of any other sequence of related charac-
       ters  such  as  (?: or a subpattern name or number. The characters that
       make up a comment play no part in the pattern matching.

       The sequence (?# marks the start of a comment that continues up to  the
       next  closing parenthesis. Nested parentheses are not permitted. If the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment,  which  in  this  case continues to immediately after the next
       newline character or character sequence in the pattern.  Which  charac-
       ters are interpreted as newlines is controlled by the options passed to
       a compiling function or by a special sequence at the start of the  pat-
       tern, as described in the section entitled "Newline conventions" above.
       Note that the end of this type of comment is a literal newline sequence
       in  the pattern; escape sequences that happen to represent a newline do
       not count. For example, consider this  pattern  when  PCRE_EXTENDED  is
       set, and the default newline convention is in force:

         abc #comment \n still comment

       On  encountering  the  # character, pcre_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it supports special syntax for recursion of  the  entire  pattern,  and
       also  for  individual  subpattern  recursion. After its introduction in
       PCRE and Python, this kind of  recursion  was  subsequently  introduced
       into Perl at release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine  call  of  the
       subpattern  of  the  given  number, provided that it occurs inside that
       subpattern. (If not, it is a non-recursive subroutine  call,  which  is
       described  in  the  next  section.)  The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       This PCRE pattern solves the nested  parentheses  problem  (assume  the
       PCRE_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a  sequence  of  non-parentheses,  or  a
       recursive  match  of the pattern itself (that is, a correctly parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       of a possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       If this were part of a larger pattern, you would not  want  to  recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We  have  put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
       tricky.  This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most  recently  opened  parentheses  preceding  the recursion. In other
       words, a negative number counts capturing  parentheses  leftwards  from
       the point at which it is encountered.

       It  is  also  possible  to refer to subsequently opened parentheses, by
       writing references such as (?+2). However, these  cannot  be  recursive
       because  the  reference  is  not inside the parentheses that are refer-
       enced. They are always non-recursive subroutine calls, as described  in
       the next section.

       An  alternative  approach is to use named parentheses instead. The Perl
       syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
       supported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If  there  is more than one subpattern with the same name, the earliest
       one is used.

       This particular example pattern that we have been looking  at  contains
       nested unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying the pat-
       tern  to  strings  that do not match. For example, when this pattern is
       applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a  possessive  quantifier  is
       not  used, the match runs for a very long time indeed because there are
       so many different ways the + and * repeats can carve  up  the  subject,
       and all have to be tested before failure can be reported.

       At  the  end  of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values,  a
       callout  function can be used (see below and the pcrecallout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the value for the inner capturing parentheses  (numbered  2)  is  "ef",
       which  is the last value taken on at the top level. If a capturing sub-
       pattern is not matched at the top level, its final  captured  value  is
       unset,  even  if  it was (temporarily) set at a deeper level during the
       matching process.

       If there are more than 15 capturing parentheses in a pattern, PCRE  has
       to  obtain extra memory to store data during a recursion, which it does
       by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
       can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE and Perl

       Recursion  processing  in PCRE differs from Perl in two important ways.
       In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives  and  there  is a subsequent matching failure. This can be
       illustrated by the following pattern, which purports to match a  palin-
       dromic  string  that contains an odd number of characters (for example,
       "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters  surrounding  a sub-palindrome. In Perl, this pattern works;
       in PCRE it does not if the pattern is  longer  than  three  characters.
       Consider the subject string "abcba":

       At  the  top level, the first character is matched, but as it is not at
       the end of the string, the first alternative fails; the second alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern 1 successfully matches the next character ("b").  (Note  that  the
       beginning and end of line tests are not part of the recursion).

       Back  at  the top level, the next character ("c") is compared with what
       subpattern 2 matched, which was "a". This fails. Because the  recursion
       is  treated  as  an atomic group, there are now no backtracking points,
       and so the entire match fails. (Perl is able, at  this  point,  to  re-
       enter  the  recursion  and try the second alternative.) However, if the
       pattern is written with the alternatives in the other order, things are
       different:

         ^((.)(?1)\2|.)$

       This  time,  the recursing alternative is tried first, and continues to
       recurse until it runs out of characters, at which point  the  recursion
       fails.  But  this  time  we  do  have another alternative to try at the
       higher level. That is the big difference:  in  the  previous  case  the
       remaining alternative is at a deeper recursion level, which PCRE cannot
       use.

       To change the pattern so that it matches all palindromic  strings,  not
       just  those  with an odd number of characters, it is tempting to change
       the pattern to this:

         ^((.)(?1)\2|.?)$

       Again, this works in Perl, but not in PCRE, and for  the  same  reason.
       When  a  deeper  recursion has matched a single character, it cannot be
       entered again in order to match an empty string.  The  solution  is  to
       separate  the two cases, and write out the odd and even cases as alter-
       natives at the higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If you want to match typical palindromic phrases, the  pattern  has  to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl.  Note the use of the possessive quantifier *+ to avoid backtrack-
       ing into sequences of non-word characters. Without this, PCRE  takes  a
       great  deal  longer  (ten  times or more) to match typical phrases, and
       Perl takes so long that you think it has gone into a loop.

       WARNING: The palindrome-matching patterns above work only if  the  sub-
       ject  string  does not start with a palindrome that is shorter than the
       entire string.  For example, although "abcba" is correctly matched,  if
       the  subject  is "ababa", PCRE finds the palindrome "aba" at the start,
       then fails at top level because the end of the string does not  follow.
       Once  again, it cannot jump back into the recursion to try other alter-
       natives, so the entire match fails.

       The second way in which PCRE and Perl differ in  their  recursion  pro-
       cessing  is in the handling of captured values. In Perl, when a subpat-
       tern is called recursively or as a subpattern (see the  next  section),
       it  has  no  access to any values that were captured outside the recur-
       sion, whereas in PCRE these values can  be  referenced.  Consider  this
       pattern:

         ^(.)(\1|a(?2))

       In  PCRE,  this  pattern matches "bab". The first capturing parentheses
       match "b", then in the second group, when the back reference  \1  fails
       to  match "b", the second alternative matches "a" and then recurses. In
       the recursion, \1 does now match "b" and so the whole  match  succeeds.
       In  Perl,  the pattern fails to match because inside the recursive call
       \1 cannot access the externally set value.


SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern call (either by number  or  by
       name)  is  used outside the parentheses to which it refers, it operates
       like a subroutine in a programming language. The called subpattern  may
       be  defined  before or after the reference. A numbered reference can be
       absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Another example is  given  in  the  discussion  of  DEFINE
       above.

       All  subroutine  calls, whether recursive or not, are always treated as
       atomic groups. That is, once a subroutine has matched some of the  sub-
       ject string, it is never re-entered, even if it contains untried alter-
       natives and there is  a  subsequent  matching  failure.  Any  capturing
       parentheses  that  are  set  during the subroutine call revert to their
       previous values afterwards.

       Processing options such as case-independence are fixed when  a  subpat-
       tern  is defined, so if it is used as a subroutine, such options cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the  change  of
       processing option does not affect the called subpattern.


ONIGURUMA SUBROUTINE SYNTAX

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax  for  referencing a subpattern as a subroutine,
       possibly recursively. Here are two of the examples used above,  rewrit-
       ten using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE  supports  an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are  not
       synonymous.  The former is a back reference; the latter is a subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl  code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an external function by putting its entry point in the global  variable
       pcre_callout  (8-bit  library)  or  pcre16_callout (16-bit library). By
       default, this variable contains NULL, which disables all calling out.

       Within a regular expression, (?C) indicates the  points  at  which  the
       external  function  is  to be called. If you want to identify different
       callout points, you can put a number less than 256 after the letter  C.
       The  default  value is zero.  For example, this pattern has two callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to a compiling function,  call-
       outs  are automatically installed before each item in the pattern. They
       are all numbered 255.

       During matching, when PCRE reaches a callout point, the external  func-
       tion  is  called.  It  is  provided with the number of the callout, the
       position in the pattern, and, optionally, one item of  data  originally
       supplied  by  the caller of the matching function. The callout function
       may cause matching to proceed, to backtrack, or to fail  altogether.  A
       complete  description of the interface to the callout function is given
       in the pcrecallout documentation.


BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
       which are described in the Perl documentation as "experimental and sub-
       ject to change or removal in a future version of Perl". It goes  on  to
       say:  "Their usage in production code should be noted to avoid problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

       Since  these  verbs  are  specifically related to backtracking, most of
       them can be used only when the pattern is to be matched  using  one  of
       the traditional matching functions, which use a backtracking algorithm.
       With the exception of (*FAIL), which behaves like  a  failing  negative
       assertion,  they  cause an error if encountered by a DFA matching func-
       tion.

       If any of these verbs are used in an assertion or in a subpattern  that
       is called as a subroutine (whether or not recursively), their effect is
       confined to that subpattern; it does not extend to the surrounding pat-
       tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
       that is encountered in a successful positive assertion is  passed  back
       when  a  match  succeeds (compare capturing parentheses in assertions).
       Note that such subpatterns are processed as anchored at the point where
       they are tested. Note also that Perl's treatment of subroutines is dif-
       ferent in some cases.

       The new verbs make use of what was previously invalid syntax: an  open-
       ing parenthesis followed by an asterisk. They are generally of the form
       (*VERB) or (*VERB:NAME). Some may take either form, with differing  be-
       haviour,  depending on whether or not an argument is present. A name is
       any sequence of characters that does not include a closing parenthesis.
       If  the  name is empty, that is, if the closing parenthesis immediately
       follows the colon, the effect is as if the colon were  not  there.  Any
       number of these verbs may occur in a pattern.

       PCRE  contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may  know  the minimum length of matching subject, or that a particular
       character must be present. When one of these  optimizations  suppresses
       the  running  of  a match, any included backtracking verbs will not, of
       course, be processed. You can suppress the start-of-match optimizations
       by  setting  the  PCRE_NO_START_OPTIMIZE  option when calling pcre_com-
       pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).

       Experiments with Perl suggest that it too  has  similar  optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately

       The  following  verbs act as soon as they are encountered. They may not
       be followed by a name.

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. However, when it is inside a subpattern that is called
       as a subroutine, only that subpattern is ended  successfully.  Matching
       then  continues  at  the  outer level. If (*ACCEPT) is inside capturing
       parentheses, the data so far is captured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B"  is  cap-
       tured by the outer parentheses.

         (*FAIL) or (*F)

       This  verb causes a matching failure, forcing backtracking to occur. It
       is equivalent to (?!) but easier to read. The Perl documentation  notes
       that  it  is  probably  useful only when combined with (?{}) or (??{}).
       Those are, of course, Perl features that are not present in  PCRE.  The
       nearest  equivalent is the callout feature, as for example in this pat-
       tern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There  is  one  verb  whose  main  purpose  is to track how a match was
       arrived at, though it also has a  secondary  use  in  conjunction  with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A  name  is  always  required  with  this  verb.  There  may be as many
       instances of (*MARK) as you like in a pattern, and their names  do  not
       have to be unique.

       When  a match succeeds, the name of the last-encountered (*MARK) on the
       matching path is passed back to the caller as described in the  section
       entitled  "Extra  data  for  pcre_exec()" in the pcreapi documentation.
       Here is an example of pcretest output, where the /K  modifier  requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple it indicates which of the two alternatives matched. This is a  more
       efficient  way of obtaining this information than putting each alterna-
       tive in its own capturing parentheses.

       If (*MARK) is encountered in a positive assertion, its name is recorded
       and passed back if it is the last-encountered. This does not happen for
       negative assertions.

       After a partial match or a failed match, the name of the  last  encoun-
       tered (*MARK) in the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XP
         No match, mark = B

       Note  that  in  this  unanchored  example the mark is retained from the
       match attempt that started at the letter "X". Subsequent match attempts
       starting  at "P" and then with an empty string do not get as far as the
       (*MARK) item, but nevertheless do not reset it.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues  with what follows, but if there is no subsequent match, causing
       a backtrack to the verb, a failure is  forced.  That  is,  backtracking
       cannot  pass  to the left of the verb. However, when one of these verbs
       appears inside an atomic group, its effect is confined to  that  group,
       because  once the group has been matched, there is never any backtrack-
       ing into it. In this situation, backtracking can  "jump  back"  to  the
       left  of the entire atomic group. (Remember also, as stated above, that
       this localization also applies in subroutine calls and assertions.)

       These verbs differ in exactly what kind of failure  occurs  when  back-
       tracking reaches them.

         (*COMMIT)

       This  verb, which may not be followed by a name, causes the whole match
       to fail outright if the rest of the pattern does not match. Even if the
       pattern is unanchored, no further attempts to find a match by advancing
       the  starting  point  take  place.  Once  (*COMMIT)  has  been  passed,
       pcre_exec()  is  committed  to  finding a match at the current starting
       point, or not at all. For example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as  a  kind
       of dynamic anchor, or "I've started, so I must finish." The name of the
       most recently passed (*MARK) in the path is passed back when  (*COMMIT)
       forces a match failure.

       Note  that  (*COMMIT)  at  the start of a pattern is not the same as an
       anchor, unless PCRE's start-of-match optimizations are turned  off,  as
       shown in this pcretest example:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         xyzabc\Y
         No match

       PCRE  knows  that  any  match  must start with "a", so the optimization
       skips along the subject to "a" before running the first match  attempt,
       which  succeeds.  When the optimization is disabled by the \Y escape in
       the second subject, the match starts at "x" and so the (*COMMIT) causes
       it to fail without trying any other starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This  verb causes the match to fail at the current starting position in
       the subject if the rest of the pattern does not match. If  the  pattern
       is  unanchored,  the  normal  "bumpalong"  advance to the next starting
       character then happens. Backtracking can occur as usual to the left  of
       (*PRUNE),  before  it  is  reached,  or  when  matching to the right of
       (*PRUNE), but if there is no match to the  right,  backtracking  cannot
       cross  (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
       native to an atomic group or possessive quantifier, but there are  some
       uses of (*PRUNE) that cannot be expressed in any other way.  The behav-
       iour of (*PRUNE:NAME)  is  the  same  as  (*MARK:NAME)(*PRUNE).  In  an
       anchored pattern (*PRUNE) has the same effect as (*COMMIT).

         (*SKIP)

       This  verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered. (*SKIP) signifies that whatever text was matched leading  up  to
       it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

         (*SKIP:NAME)

       When  (*SKIP) has an associated name, its behaviour is modified. If the
       following pattern fails to match, the previous path through the pattern
       is  searched for the most recent (*MARK) that has the same name. If one
       is found, the "bumpalong" advance is to the subject position that  cor-
       responds  to  that (*MARK) instead of to where (*SKIP) was encountered.
       If no (*MARK) with a matching name is found, the (*SKIP) is ignored.

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative if  the  rest
       of  the  pattern does not match. That is, it cancels pending backtrack-
       ing, but only within the current alternative. Its name comes  from  the
       observation that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on  failure,  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. The behaviour  of  (*THEN:NAME)  is  exactly  the  same  as
       (*MARK:NAME)(*THEN).   If (*THEN) is not inside an alternation, it acts
       like (*PRUNE).

       Note that a subpattern that does not contain a | character  is  just  a
       part  of the enclosing alternative; it is not a nested alternation with
       only one alternative. The effect of (*THEN) extends beyond such a  sub-
       pattern  to  the enclosing alternative. Consider this pattern, where A,
       B, etc. are complex pattern fragments that do not contain any | charac-
       ters at this level:

         A (B(*THEN)C) | D

       If  A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However,  if the subpattern containing (*THEN) is given an alternative,
       it behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner subpattern. After  a
       failure in C, matching moves to (*FAIL), which causes the whole subpat-
       tern to fail because there are no more alternatives  to  try.  In  this
       case, matching does now backtrack into A.

       Note also that a conditional subpattern is not considered as having two
       alternatives, because only one is ever used.  In  other  words,  the  |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not  match.  Because  .*?  is
       ungreedy,  it  initially  matches  zero characters. The condition (?=a)
       then fails, the character "b" is matched,  but  "c"  is  not.  At  this
       point,  matching does not backtrack to .*? as might perhaps be expected
       from the presence of the | character.  The  conditional  subpattern  is
       part of the single alternative that comprises the whole pattern, and so
       the match fails. (If there was a backtrack into  .*?,  allowing  it  to
       match "b", the match would succeed.)

       The  verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match  at  the next alternative. (*PRUNE) comes next, failing the match
       at the current starting position, but allowing an advance to  the  next
       character  (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

       If more than one such verb is present in a pattern, the "strongest" one
       wins.  For example, consider this pattern, where A, B, etc. are complex
       pattern fragments:

         (A(*COMMIT)B(*THEN)C|D)

       Once  A  has  matched,  PCRE is committed to this match, at the current
       starting position. If subsequently B matches, but C does not, the  nor-
       mal (*THEN) action of trying the next alternative (that is, D) does not
       happen because (*COMMIT) overrides.


SEE ALSO

       pcreapi(3), pcrecallout(3),  pcrematching(3),  pcresyntax(3),  pcre(3),
       pcre16(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRESYNTAX(3)                                                    PCRESYNTAX(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE are described in  the  pcrepattern  documentation.  This
       document contains a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


CHARACTERS

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any ASCII character
         \e         escape (hex 1B)
         \f         formfeed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \ddd       character with octal code ddd, or backreference
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one data unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal whitespace character
         \H         a character that is not a horizontal whitespace character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a whitespace character
         \S         a character that is not a whitespace character
         \v         a vertical whitespace character
         \V         a character that is not a vertical whitespace character
         \w         a "word" character
         \W         a "non-word" character
         \X         an extended Unicode sequence

       In  PCRE,  by  default, \d, \D, \s, \S, \w, and \W recognize only ASCII
       characters, even in a UTF mode. However, this can be changed by setting
       the PCRE_UCP option.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, FF, CR
         Xwd        Perl word: property Xan or underscore


SCRIPT NAMES FOR \p AND \P

       Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
       Coptic,   Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyp-
       tian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic,   Gothic,   Greek,
       Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe-
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
       Latin,  Lepcha,  Limbu,  Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
       Phags_Pa,  Phoenician,  Rejang,  Runic, Samaritan, Saurashtra, Shavian,
       Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
       Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
       Ugaritic, Vai, Yi.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       whitespace
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE, POSIX character set names recognize only ASCII  characters  by
       default,  but  some  of them use Unicode properties if PCRE_UCP is set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                      also after internal newline in multiline mode
         \A          start of subject
         $           end of subject
                      also before newline at end of subject
                      also before internal newline in multiline mode
         \Z          end of subject
                      also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


MATCH POINT RESET

         \K          reset start of match


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capturing group
         (?<name>...)    named capturing group (Perl)
         (?'name'...)    named capturing group (Perl)
         (?P<name>...)   named capturing group (Python)
         (?:...)         non-capturing group
         (?|...)         non-capturing group; reset group numbers for
                          capturing groups in each alternative


ATOMIC GROUPS

         (?>...)         atomic, non-capturing group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING

         (?i)            caseless
         (?J)            allow duplicate names
         (?m)            multiline
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            extended (ignore white space)
         (?-...)         unset option(s)

       The following are recognized only at the start of a  pattern  or  after
       one of the newline-setting options with similar syntax:

         (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
         (*UTF8)         set UTF-8 mode: 8-bit library (PCRE_UTF8)
         (*UTF16)        set UTF-16 mode: 16-bit library (PCRE_UTF16)
         (*UCP)          set PCRE_UCP (use Unicode properties for \d etc)


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)         positive look ahead
         (?!...)         negative look ahead
         (?<=...)        positive look behind
         (?<!...)        negative look behind

       Each top-level branch of a look behind must be of a fixed length.


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subpattern by absolute number
         (?+n)           call subpattern by relative number
         (?-n)           call subpattern by relative number
         (?&name)        call subpattern by name (Perl)
         (?P>name)       call subpattern by name (Python)
         \g<name>        call subpattern by name (Oniguruma)
         \g'name'        call subpattern by name (Oniguruma)
         \g<n>           call subpattern by absolute number (Oniguruma)
         \g'n'           call subpattern by absolute number (Oniguruma)
         \g<+n>          call subpattern by relative number (PCRE extension)
         \g'+n'          call subpattern by relative number (PCRE extension)
         \g<-n>          call subpattern by relative number (PCRE extension)
         \g'-n'          call subpattern by relative number (PCRE extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)...        absolute reference condition
         (?(+n)...       relative reference condition
         (?(-n)...       relative reference condition
         (?(<name>)...   named reference condition (Perl)
         (?('name')...   named reference condition (Perl)
         (?(name)...     named reference condition (PCRE)
         (?(R)...        overall recursion condition
         (?(Rn)...       specific group recursion condition
         (?(R&name)...   specific recursion condition
         (?(DEFINE)...   define subpattern for reference
         (?(assert)...   assertion condition


BACKTRACKING CONTROL

       The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The  following  act only when a subsequent match failure causes a back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*PRUNE:NAME)   equivalent to (*MARK:NAME)(*PRUNE)
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation
         (*THEN:NAME)    equivalent to (*MARK:NAME)(*THEN)


NEWLINE CONVENTIONS

       These are recognized only at the very start of the pattern or  after  a
       (*BSR_...), (*UTF8), (*UTF16) or (*UCP) option.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence


WHAT \R MATCHES

       These  are  recognized only at the very start of the pattern or after a
       (*...) option that sets the newline convention or a UTF or UCP mode.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


CALLOUTS

         (?C)      callout
         (?Cn)     callout with data n


SEE ALSO

       pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREUNICODE(3)                                                  PCREUNICODE(3)


NAME
       PCRE - Perl-compatible regular expressions


UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT

       From Release 8.30, in addition to its previous UTF-8 support, PCRE also
       supports UTF-16 by means of a separate  16-bit  library.  This  can  be
       built as well as, or instead of, the 8-bit library.


UTF-8 SUPPORT

       In  order  process  UTF-8  strings, you must build PCRE's 8-bit library
       with UTF support, and, in addition, you must call  pcre_compile()  with
       the  PCRE_UTF8 option flag, or the pattern must start with the sequence
       (*UTF8). When either of these is the case, both  the  pattern  and  any
       subject  strings  that  are  matched  against  it  are treated as UTF-8
       strings instead of strings of 1-byte characters.


UTF-16 SUPPORT

       In order process UTF-16 strings, you must build PCRE's  16-bit  library
       with UTF support, and, in addition, you must call pcre16_compile() with
       the PCRE_UTF16 option flag, or the pattern must start with the sequence
       (*UTF16).  When  either  of these is the case, both the pattern and any
       subject strings that are matched  against  it  are  treated  as  UTF-16
       strings instead of strings of 16-bit characters.


UTF SUPPORT OVERHEAD

       If  you  compile  PCRE with UTF support, but do not use it at run time,
       the library will be a bit bigger, but the additional run time  overhead
       is limited to testing the PCRE_UTF8/16 flag occasionally, so should not
       be very big.


UNICODE PROPERTY SUPPORT

       If PCRE is built with Unicode character property support (which implies
       UTF  support), the escape sequences \p{..}, \P{..}, and \X can be used.
       The available properties that can be tested are limited to the  general
       category  properties  such  as  Lu for an upper case letter or Nd for a
       decimal number, the Unicode script names such as Arabic or Han, and the
       derived  properties Any and L&. A full list is given in the pcrepattern
       documentation. Only the short names for properties are  supported.  For
       example,  \p{L}  matches a letter. Its Perl synonym, \p{Letter}, is not
       supported.  Furthermore, in Perl, many  properties  may  optionally  be
       prefixed  by  "Is", for compatibility with Perl 5.6. PCRE does not sup-
       port this.

   Validity of UTF-8 strings

       When you set the PCRE_UTF8 flag, the byte strings  passed  as  patterns
       and subjects are (by default) checked for validity on entry to the rel-
       evant functions. From release 7.3 of PCRE, the check is  according  the
       rules of RFC 3629, which are themselves derived from the Unicode speci-
       fication. Earlier releases of PCRE followed  the  rules  of  RFC  2279,
       which  allows  the  full  range of 31-bit values (0 to 0x7FFFFFFF). The
       current check allows only values in the range U+0 to U+10FFFF,  exclud-
       ing U+D800 to U+DFFF.

       The  excluded code points are the "Surrogate Area" of Unicode. They are
       reserved for use by UTF-16, where they are  used  in  pairs  to  encode
       codepoints  with  values  greater than 0xFFFF. The code points that are
       encoded by UTF-16 pairs are available independently in the UTF-8 encod-
       ing.  (In  other words, the whole surrogate thing is a fudge for UTF-16
       which unfortunately messes up UTF-8.)

       If an invalid UTF-8 string is passed to PCRE, an error return is given.
       At  compile  time, the only additional information is the offset to the
       first byte of the failing character. The runtime functions  pcre_exec()
       and  pcre_dfa_exec() also pass back this information, as well as a more
       detailed reason code if the caller has provided memory in which  to  do
       this.

       In  some  situations, you may already know that your strings are valid,
       and therefore want to skip these checks in  order  to  improve  perfor-
       mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run
       time, PCRE assumes that the pattern or subject  it  is  given  (respec-
       tively)  contains  only  valid  UTF-8  codes. In this case, it does not
       diagnose an invalid UTF-8 string.

       If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
       what  happens  depends on why the string is invalid. If the string con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string  of  characters  in the range 0 to 0x7FFFFFFF by pcre_dfa_exec()
       and the interpreted version of pcre_exec(). In other words, apart  from
       the  initial validity test, these functions (when in UTF-8 mode) handle
       strings according to the more liberal rules of RFC 2279.  However,  the
       just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629.
       If you are using JIT optimization, or if the string does not even  con-
       form to RFC 2279, the result is undefined. Your program may crash.

       If  you  want  to  process  strings  of  values  in the full range 0 to
       0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this situation, you will have to apply your  own  validity  check,  and
       avoid the use of JIT optimization.

   Validity of UTF-16 strings

       When you set the PCRE_UTF16 flag, the strings of 16-bit data units that
       are passed as patterns and subjects are (by default) checked for valid-
       ity  on entry to the relevant functions. Values other than those in the
       surrogate range U+D800 to U+DFFF are independent code points. Values in
       the surrogate range must be used in pairs in the correct manner.

       If  an  invalid  UTF-16  string  is  passed to PCRE, an error return is
       given. At compile time, the only additional information is  the  offset
       to  the first data unit of the failing character. The runtime functions
       pcre16_exec() and pcre16_dfa_exec() also pass back this information, as
       well  as  a more detailed reason code if the caller has provided memory
       in which to do this.

       In some situations, you may already know that your strings  are  valid,
       and  therefore  want  to  skip these checks in order to improve perfor-
       mance. If you set the PCRE_NO_UTF16_CHECK flag at compile  time  or  at
       run time, PCRE assumes that the pattern or subject it is given (respec-
       tively) contains only valid UTF-16 sequences. In this case, it does not
       diagnose an invalid UTF-16 string.

   General comments about UTF modes

       1.  Codepoints  less  than  256  can  be  specified by either braced or
       unbraced hexadecimal escape sequences (for example,  \x{b3}  or  \xb3).
       Larger values have to use braced sequences.

       2.  Octal  numbers  up  to \777 are recognized, and in UTF-8 mode, they
       match two-byte characters for values greater than \177.

       3. Repeat quantifiers apply to complete UTF characters, not to individ-
       ual data units, for example: \x{100}{3}.

       4.  The dot metacharacter matches one UTF character instead of a single
       data unit.

       5. The escape sequence \C can be used to match a single byte  in  UTF-8
       mode, or a single 16-bit data unit in UTF-16 mode, but its use can lead
       to some strange effects because it breaks up multi-unit characters (see
       the  description of \C in the pcrepattern documentation). The use of \C
       is   not   supported   in    the    alternative    matching    function
       pcre[16]_dfa_exec(),  nor  is it supported in UTF mode by the JIT opti-
       mization of pcre[16]_exec(). If JIT optimization is requested for a UTF
       pattern that contains \C, it will not succeed, and so the matching will
       be carried out by the normal interpretive function.

       6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
       test characters of any code value, but, by default, the characters that
       PCRE recognizes as digits, spaces, or word characters remain  the  same
       set  as  in  non-UTF  mode, all with values less than 256. This remains
       true even when PCRE is  built  to  include  Unicode  property  support,
       because to do otherwise would slow down PCRE in many common cases. Note
       in particular that this applies to \b and \B, because they are  defined
       in terms of \w and \W. If you really want to test for a wider sense of,
       say, "digit", you can use  explicit  Unicode  property  tests  such  as
       \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the
       character escapes work is changed so that Unicode properties  are  used
       to determine which characters match. There are more details in the sec-
       tion on generic character types in the pcrepattern documentation.

       7. Similarly, characters that match the POSIX named  character  classes
       are all low-valued characters, unless the PCRE_UCP option is set.

       8.  However,  the  horizontal  and vertical whitespace matching escapes
       (\h, \H, \v, and \V) do match all the appropriate  Unicode  characters,
       whether or not PCRE_UCP is set.

       9.  Case-insensitive  matching  applies only to characters whose values
       are less than 128, unless PCRE is built with Unicode property  support.
       Even  when  Unicode  property support is available, PCRE still uses its
       own character tables when checking the case of  low-valued  characters,
       so  as not to degrade performance.  The Unicode property information is
       used only for characters with higher values. Furthermore, PCRE supports
       case-insensitive  matching  only  when  there  is  a one-to-one mapping
       between a letter's cases. There are a small number of many-to-one  map-
       pings in Unicode; these are not supported by PCRE.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 13 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREJIT(3)                                                          PCREJIT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing before the match is performed. Therefore, it is of most benefit
       when the same pattern is going to be matched many times. This does  not
       necessarily  mean  many calls of a matching function; if the pattern is
       not anchored, matching attempts may take place many  times  at  various
       positions  in  the  subject, even for a single call.  Therefore, if the
       subject string is very long, it may still pay to use  JIT  for  one-off
       matches.

       JIT  support  applies  only to the traditional Perl-compatible matching
       function.  It does not apply when the DFA matching  function  is  being
       used. The code for this support was written by Zoltan Herczeg.


8-BIT and 16-BIT SUPPORT

       JIT  support is available for both the 8-bit and 16-bit PCRE libraries.
       To  keep  this  documentation  simple,  only  the  8-bit  interface  is
       described in what follows. If you are using the 16-bit library, substi-
       tute  the  16-bit  functions  and  16-bit  structures   (for   example,
       pcre16_jit_stack instead of pcre_jit_stack).


AVAILABILITY OF JIT SUPPORT

       JIT  support  is  an  optional  feature of PCRE. The "configure" option
       --enable-jit (or equivalent CMake option) must  be  set  when  PCRE  is
       built  if  you want to use JIT. The support is limited to the following
       hardware platforms:

         ARM v5, v7, and Thumb2
         Intel x86 32-bit and 64-bit
         MIPS 32-bit
         Power PC 32-bit and 64-bit

       The Power PC support is designated as experimental because it  has  not
       been  fully  tested. If --enable-jit is set on an unsupported platform,
       compilation fails.

       A program that is linked with PCRE 8.20 or later can tell if  JIT  sup-
       port  is  available  by  calling pcre_config() with the PCRE_CONFIG_JIT
       option. The result is 1 when JIT is available, and  0  otherwise.  How-
       ever, a simple program does not need to check this in order to use JIT.
       The API is implemented in a way that falls back to  the  ordinary  PCRE
       code if JIT is not available.

       If  your program may sometimes be linked with versions of PCRE that are
       older than 8.20, but you want to use JIT when it is available, you  can
       test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
       macro such as PCRE_CONFIG_JIT, for compile-time control of your code.


SIMPLE USE OF JIT

       You have to do two things to make use of the JIT support  in  the  sim-
       plest way:

         (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
             each compiled pattern, and pass the resulting pcre_extra block to
             pcre_exec().

         (2) Use pcre_free_study() to free the pcre_extra block when it is
             no longer needed instead of just freeing it yourself. This
             ensures that any JIT data is also freed.

       For  a  program  that may be linked with pre-8.20 versions of PCRE, you
       can insert

         #ifndef PCRE_STUDY_JIT_COMPILE
         #define PCRE_STUDY_JIT_COMPILE 0
         #endif

       so that no option is passed to pcre_study(),  and  then  use  something
       like this to free the study data:

         #ifdef PCRE_CONFIG_JIT
             pcre_free_study(study_ptr);
         #else
             pcre_free(study_ptr);
         #endif

       In  some circumstances you may need to call additional functions. These
       are described in the  section  entitled  "Controlling  the  JIT  stack"
       below.

       If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and
       no JIT data is set up. Otherwise, the compiled pattern is passed to the
       JIT  compiler,  which  turns  it  into  machine code that executes much
       faster than the normal interpretive code. When pcre_exec() is passed  a
       pcre_extra  block  containing  a  pointer  to  JIT  code, it obeys that
       instead of the normal code. The result is identical, but the code  runs
       much faster.

       There  are some pcre_exec() options that are not supported for JIT exe-
       cution. There are also some  pattern  items  that  JIT  cannot  handle.
       Details  are  given below. In both cases, execution automatically falls
       back to the interpretive code.

       If the JIT compiler finds an unsupported item, no JIT  data  is  gener-
       ated.  You  can find out if JIT execution is available after studying a
       pattern by calling pcre_fullinfo() with  the  PCRE_INFO_JIT  option.  A
       result  of  1  means that JIT compilation was successful. A result of 0
       means that JIT support is not available, or the pattern was not studied
       with PCRE_STUDY_JIT_COMPILE, or the JIT compiler was not able to handle
       the pattern.

       Once a pattern has been studied, with or without JIT, it can be used as
       many times as you like for matching different subject strings.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The  only  pcre_exec() options that are supported for JIT execution are
       PCRE_NO_UTF8_CHECK,  PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   and
       PCRE_NOTEMPTY_ATSTART.  Note in particular that partial matching is not
       supported.

       The unsupported pattern items are:

         \C             match a single byte; not supported in UTF-8 mode
         (?Cn)          callouts
         (*COMMIT)      )
         (*MARK)        )
         (*PRUNE)       ) the backtracking control verbs
         (*SKIP)        )
         (*THEN)        )

       Support for some of these may be added in future.


RETURN VALUES FROM JIT EXECUTION

       When a pattern is matched using JIT execution, the  return  values  are
       the  same as those given by the interpretive pcre_exec() code, with the
       addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT.  This  means
       that  the memory used for the JIT stack was insufficient. See "Control-
       ling the JIT stack" below for a discussion of JIT stack usage. For com-
       patibility  with  the  interpretive pcre_exec() code, no more than two-
       thirds of the ovector argument is used for passing back  captured  sub-
       strings.

       The  error  code  PCRE_ERROR_MATCHLIMIT  is returned by the JIT code if
       searching a very large pattern tree goes on for too long, as it  is  in
       the  same circumstance when JIT is not used, but the details of exactly
       what is counted are not the same. The  PCRE_ERROR_RECURSIONLIMIT  error
       code is never returned by JIT execution.


SAVING AND RESTORING COMPILED PATTERNS

       The  code  that  is  generated by the JIT compiler is architecture-spe-
       cific, and is also position dependent. For those reasons it  cannot  be
       saved  (in a file or database) and restored later like the bytecode and
       other data of a compiled pattern. Saving and  restoring  compiled  pat-
       terns  is not something many people do. More detail about this facility
       is given in the pcreprecompile documentation. It should be possible  to
       run  pcre_study() on a saved and restored pattern, and thereby recreate
       the JIT data, but because JIT compilation uses  significant  resources,
       it  is  probably  not worth doing this; you might as well recompile the
       original pattern.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.   By  default,  it  uses 32K on the machine stack. However, some
       large  or  complicated  patterns  need  more  than  this.   The   error
       PCRE_ERROR_JIT_STACKLIMIT  is  given  when  there  is not enough stack.
       Three functions are provided for managing blocks of memory for  use  as
       JIT  stacks. There is further discussion about the use of JIT stacks in
       the section entitled "JIT stack FAQ" below.

       The pcre_jit_stack_alloc() function creates a JIT stack. Its  arguments
       are  a starting size and a maximum size, and it returns a pointer to an
       opaque structure of type pcre_jit_stack, or NULL if there is an  error.
       The  pcre_jit_stack_free() function can be used to free a stack that is
       no longer needed. (For the technically minded:  the  address  space  is
       allocated by mmap or VirtualAlloc.)

       JIT  uses far less memory for recursion than the interpretive code, and
       a maximum stack size of 512K to 1M should be more than enough  for  any
       pattern.

       The  pcre_assign_jit_stack()  function  specifies  which stack JIT code
       should use. Its arguments are as follows:

         pcre_extra         *extra
         pcre_jit_callback  callback
         void               *data

       The extra argument must be  the  result  of  studying  a  pattern  with
       PCRE_STUDY_JIT_COMPILE.  There  are  three  cases for the values of the
       other two options:

         (1) If callback is NULL and data is NULL, an internal 32K block
             on the machine stack is used.

         (2) If callback is NULL and data is not NULL, data must be
             a valid JIT stack, the result of calling pcre_jit_stack_alloc().

         (3) If callback not NULL, it must point to a function that is called
             with data as an argument at the start of matching, in order to
             set up a JIT stack. If the result is NULL, the internal 32K stack
             is used; otherwise the return value must be a valid JIT stack,
             the result of calling pcre_jit_stack_alloc().

       You may safely assign the same JIT stack to more than one  pattern,  as
       long as they are all matched sequentially in the same thread. In a mul-
       tithread application, each thread must use its own JIT stack.

       Strictly speaking, even more is allowed. You can assign the same  stack
       to  any number of patterns as long as they are not used for matching by
       multiple threads at the same time. For example, you can assign the same
       stack  to all compiled patterns, and use a global mutex in the callback
       to wait until the stack is available for use. However, this is an inef-
       ficient solution, and not recommended.

       This  is  a  suggestion  for  how a typical multithreaded program might
       operate:

         During thread initalization
           thread_local_var = pcre_jit_stack_alloc(...)

         During thread exit
           pcre_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All the functions described in this section do nothing if  JIT  is  not
       available,  and  pcre_assign_jit_stack()  does nothing unless the extra
       argument is non-NULL and points to  a  pcre_extra  block  that  is  the
       result of a successful study with PCRE_STUDY_JIT_COMPILE.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE  (and JIT) is a recursive, depth-first engine, so it needs a stack
       where the local data of the current node is pushed before checking  its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend  the  stack  on  PowerPC.  Although it is possible, its updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern operating systems have a  nice  feature:  they  can  reserve  an
       address space instead of allocating memory. We can safely allocate mem-
       ory pages inside this address space, so the stack  could  grow  without
       moving memory data (this is important because of pointers). Thus we can
       allocate 1M address space, and use only a single memory  page  (usually
       4K)  if  that is enough. However, we can still grow up to 1M anytime if
       needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or  anything else. The user program must ensure that if a stack is used
       by pcre_exec(), (that is, it is assigned to the pattern currently  run-
       ning), that stack must not be used by any other threads (to avoid over-
       writing the same memory area). The best practice for multithreaded pro-
       grams  is  to  allocate  a stack for each thread, and return this stack
       through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre_exec()  again.  When  you  assign  the  stack to a pattern, only a
       pointer is set. There is no reference counting or any other magic.  You
       can  free  the  patterns  and stacks in any order, anytime. Just do not
       call pcre_exec() with a pattern pointing to an already freed stack,  as
       that  will cause SEGFAULT. (Also, do not free a stack currently used by
       pcre_exec() in another thread). You can also replace the  stack  for  a
       pattern  at  any  time.  You  can  even  free the previous stack before
       assigning a replacement.

       (5) Should I allocate/free a  stack  every  time  before/after  calling
       pcre_exec()?

       No,  because  this  is  too  costly in terms of resources. However, you
       could implement some clever idea which release the stack if it  is  not
       used in let's say two minutes. The JIT callback can help to achive this
       without keeping a list of the currently JIT studied patterns.

       (6) OK, the stack is for long term memory allocation. But what  happens
       if  a pattern causes stack overflow with a stack of 1M? Is that 1M kept
       until the stack is freed?

       Especially on embedded sytems, it might be a good idea to release  mem-
       ory  sometimes  without  freeing the stack. There is no API for this at
       the moment. Probably a function call which returns with  the  currently
       allocated  memory for any stack and another which allows releasing mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No,  thanks to Windows. If POSIX threads were used everywhere, we could
       throw out this complicated API.


EXAMPLE CODE

       This is a single-threaded example that specifies a  JIT  stack  without
       using a callback.

         int rc;
         int ovector[30];
         pcre *re;
         pcre_extra *extra;
         pcre_jit_stack *jit_stack;

         re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
         /* Check for errors */
         extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
         jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
         /* Check for error (NULL) */
         pcre_assign_jit_stack(extra, NULL, jit_stack);
         rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
         /* Check results */
         pcre_free(re);
         pcre_free_study(extra);
         pcre_jit_stack_free(jit_stack);


SEE ALSO

       pcreapi(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPARTIAL(3)                                                  PCREPARTIAL(3)


NAME
       PCRE - Perl-compatible regular expressions


PARTIAL MATCHING IN PCRE

       In normal use of PCRE, if the subject string that is passed to a match-
       ing function matches as far as it goes, but is too short to  match  the
       entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances
       where it might be helpful to distinguish this case from other cases  in
       which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error  as  soon  as  a  mistake  is made, by beeping and not
       reflecting the character that has been typed, for example. This immedi-
       ate  feedback is likely to be a better user interface than a check that
       is delayed until the entire string has been entered.  Partial  matching
       can  also be useful when the subject string is very long and is not all
       available at once.

       PCRE supports partial matching by means of  the  PCRE_PARTIAL_SOFT  and
       PCRE_PARTIAL_HARD  options,  which  can  be set when calling any of the
       matching functions. For backwards compatibility, PCRE_PARTIAL is a syn-
       onym  for  PCRE_PARTIAL_SOFT.  The essential difference between the two
       options is whether or not a partial match is preferred to  an  alterna-
       tive complete match, though the details differ between the two types of
       matching function. If both options  are  set,  PCRE_PARTIAL_HARD  takes
       precedence.

       Setting  a partial matching option disables the use of any just-in-time
       code that was  set  up  by  studying  the  compiled  pattern  with  the
       PCRE_STUDY_JIT_COMPILE  option. It also disables two of PCRE's standard
       optimizations. PCRE remembers the last literal data unit in a  pattern,
       and  abandons  matching immediately if it is not present in the subject
       string. This optimization cannot be used  for  a  subject  string  that
       might  match only partially. If the pattern was studied, PCRE knows the
       minimum length of a matching string, and does not  bother  to  run  the
       matching  function  on  shorter strings. This optimization is also dis-
       abled for partial matching.


PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()

       A partial match occurs during a call to  pcre_exec()  or  pcre16_exec()
       when  the end of the subject string is reached successfully, but match-
       ing cannot continue because more characters  are  needed.  However,  at
       least one character in the subject must have been inspected. This char-
       acter need not form part of the final matched string; lookbehind asser-
       tions  and the \K escape sequence provide ways of inspecting characters
       before the start of a matched substring. The requirement for inspecting
       at  least  one  character  exists because an empty string can always be
       matched; without such a restriction there would  always  be  a  partial
       match of an empty string at the end of the subject.

       If  there  are  at least two slots in the offsets vector when a partial
       match is returned, the first slot is set to the offset of the  earliest
       character that was inspected. For convenience, the second offset points
       to the end of the subject so that a substring can easily be identified.

       For the majority of patterns, the first offset identifies the start  of
       the  partially matched string. However, for patterns that contain look-
       behind assertions, or \K, or begin with \b or  \B,  earlier  characters
       have been inspected while carrying out the match. For example:

         /(?<=abc)123/

       This pattern matches "123", but only if it is preceded by "abc". If the
       subject string is "xyzabc12", the offsets after a partial match are for
       the  substring  "abc12",  because  all  these  characters are needed if
       another match is tried with extra characters added to the subject.

       What happens when a partial match is identified depends on which of the
       two partial matching options are set.

   PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()

       If  PCRE_PARTIAL_SOFT  is set when pcre_exec() or pcre16_exec() identi-
       fies a partial match, the partial match  is  remembered,  but  matching
       continues  as  normal, and other alternatives in the pattern are tried.
       If no complete match  can  be  found,  PCRE_ERROR_PARTIAL  is  returned
       instead of PCRE_ERROR_NOMATCH.

       This  option  is "soft" because it prefers a complete match over a par-
       tial match.  All the various matching items in a pattern behave  as  if
       the  subject string is potentially complete. For example, \z, \Z, and $
       match at the end of the subject, as normal, and for \b and \B  the  end
       of the subject is treated as a non-alphanumeric.

       If  there  is more than one partial match, the first one that was found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If this is matched against the subject string "abc123dog", both  alter-
       natives  fail  to  match,  but the end of the subject is reached during
       matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set  to  3
       and  9, identifying "123dog" as the first partial match that was found.
       (In this example, there are two partial matches, because "dog"  on  its
       own partially matches the second alternative.)

   PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()

       If   PCRE_PARTIAL_HARD   is   set  for  pcre_exec()  or  pcre16_exec(),
       PCRE_ERROR_PARTIAL is returned as soon as a  partial  match  is  found,
       without continuing to search for possible complete matches. This option
       is "hard" because it prefers an earlier partial match over a later com-
       plete  match.  For  this reason, the assumption is made that the end of
       the supplied subject string may not be the true end  of  the  available
       data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the
       subject, the result is PCRE_ERROR_PARTIAL, provided that at  least  one
       character in the subject has been inspected.

       Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject
       strings are checked for validity. Normally, an invalid sequence  causes
       the  error  PCRE_ERROR_BADUTF8  or PCRE_ERROR_BADUTF16. However, in the
       special case of a truncated  character  at  the  end  of  the  subject,
       PCRE_ERROR_SHORTUTF8   or   PCRE_ERROR_SHORTUTF16   is   returned  when
       PCRE_PARTIAL_HARD is set.

   Comparing hard and soft partial matching

       The difference between the two partial matching options can  be  illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This  matches either "dog" or "dogsbody", greedily (that is, it prefers
       the longer string if possible). If it is  matched  against  the  string
       "dog"  with  PCRE_PARTIAL_SOFT,  it  yields a complete match for "dog".
       However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
       On  the  other hand, if the pattern is made ungreedy the result is dif-
       ferent:

         /dog(sbody)??/

       In this case the result is always a  complete  match  because  that  is
       found  first,  and  matching  never  continues after finding a complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The  second pattern will never match "dogsbody", because it will always
       find the shorter match first.


PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()

       The DFA functions move along the subject string character by character,
       without  backtracking,  searching  for  all possible matches simultane-
       ously. If the end of the subject is reached before the end of the  pat-
       tern,  there is the possibility of a partial match, again provided that
       at least one character has been inspected.

       When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned  only  if
       there  have  been  no complete matches. Otherwise, the complete matches
       are returned.  However, if PCRE_PARTIAL_HARD is set,  a  partial  match
       takes  precedence  over any complete matches. The portion of the string
       that was inspected when the longest partial match was found is  set  as
       the first matching string, provided there are at least two slots in the
       offsets vector.

       Because the DFA functions always search for all possible  matches,  and
       there  is  no  difference between greedy and ungreedy repetition, their
       behaviour is different  from  the  standard  functions  when  PCRE_PAR-
       TIAL_HARD  is  set.  Consider  the  string  "dog"  matched  against the
       ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas the standard functions stop as soon as they find  the  complete
       match  for  "dog",  the  DFA  functions also find the partial match for
       "dogsbody", and so return that when PCRE_PARTIAL_HARD is set.


PARTIAL MATCHING AND WORD BOUNDARIES

       If a pattern ends with one of sequences \b or \B, which test  for  word
       boundaries,  partial  matching with PCRE_PARTIAL_SOFT can give counter-
       intuitive results. Consider this pattern:

         /\bcat\b/

       This matches "cat", provided there is a word boundary at either end. If
       the subject string is "the cat", the comparison of the final "t" with a
       following character cannot take place, so a  partial  match  is  found.
       However,  normal  matching carries on, and \b matches at the end of the
       subject when the last character is a letter, so  a  complete  match  is
       found.   The   result,  therefore,  is  not  PCRE_ERROR_PARTIAL.  Using
       PCRE_PARTIAL_HARD in this case does yield  PCRE_ERROR_PARTIAL,  because
       then the partial match takes precedence.


FORMERLY RESTRICTED PATTERNS

       For releases of PCRE prior to 8.00, because of the way certain internal
       optimizations  were  implemented  in  the  pcre_exec()  function,   the
       PCRE_PARTIAL  option  (predecessor  of  PCRE_PARTIAL_SOFT) could not be
       used with all patterns. From release 8.00 onwards, the restrictions  no
       longer  apply,  and partial matching with can be requested for any pat-
       tern.

       Items that were formerly restricted were repeated single characters and
       repeated  metasequences. If PCRE_PARTIAL was set for a pattern that did
       not conform to the restrictions, pcre_exec() returned  the  error  code
       PCRE_ERROR_BADPARTIAL  (-13).  This error code is no longer in use. The
       PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if  a  compiled
       pattern can be used for partial matching now always returns 1.


EXAMPLE OF PARTIAL MATCHING USING PCRETEST

       If  the  escape  sequence  \P  is  present in a pcretest data line, the
       PCRE_PARTIAL_SOFT option is used for  the  match.  Here  is  a  run  of
       pcretest that uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\P
          0: 25jun04
          1: jun
         data> 25dec3\P
         Partial match: 23dec3
         data> 3ju\P
         Partial match: 3ju
         data> 3juj\P
         No match
         data> j\P
         No match

       The  first  data  string  is  matched completely, so pcretest shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete pattern, but the first two are partial matches. Similar output is
       obtained if DFA matching is used.

       If the escape sequence \P is present more than once in a pcretest  data
       line, the PCRE_PARTIAL_HARD option is set for the match.


MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()

       When  a  partial match has been found using a DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE_DFA_RESTART option. You must pass  the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. Here  is  an  example  using  pcretest,
       using  the  \R  escape  sequence to set the PCRE_DFA_RESTART option (\D
       specifies the use of the DFA matching function):

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\P\D
         Partial match: 23ja
         data> n05\R\D
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE  does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.

       You  can  set  the  PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
       PCRE_DFA_RESTART to continue partial matching over  multiple  segments.
       This  facility can be used to pass very long subject strings to the DFA
       matching functions.


MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()

       From release 8.00, the standard matching functions can also be used  to
       do multi-segment matching. Unlike the DFA functions, it is not possible
       to restart the previous match with a new segment of data. Instead,  new
       data must be added to the previous subject string, and the entire match
       re-run, starting from the point where the partial match occurred.  Ear-
       lier data can be discarded.

       It  is best to use PCRE_PARTIAL_HARD in this situation, because it does
       not treat the end of a segment as the end of the subject when  matching
       \z,  \Z,  \b,  \B,  and  $. Consider an unanchored pattern that matches
       dates:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> The date is 23ja\P\P
         Partial match: 23ja

       At this stage, an application could discard the text preceding  "23ja",
       add  on  text  from  the  next  segment, and call the matching function
       again. Unlike the DFA matching functions  the  entire  matching  string
       must  always be available, and the complete matching process occurs for
       each call, so more memory and more processing time is needed.

       Note: If the pattern contains lookbehind assertions, or \K,  or  starts
       with \b or \B, the string that is returned for a partial match includes
       characters that precede the partially matched  string  itself,  because
       these  must be retained when adding on more characters for a subsequent
       matching attempt.


ISSUES WITH MULTI-SEGMENT MATCHING

       Certain types of pattern may give problems with multi-segment matching,
       whichever matching function is used.

       1. If the pattern contains a test for the beginning of a line, you need
       to pass the PCRE_NOTBOL option when the subject  string  for  any  call
       does  start  at  the  beginning  of a line. There is also a PCRE_NOTEOL
       option, but in practice when doing multi-segment matching you should be
       using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.

       2.  Lookbehind  assertions at the start of a pattern are catered for in
       the offsets that are returned for a partial match. However, in  theory,
       a  lookbehind assertion later in the pattern could require even earlier
       characters to be inspected, and it might not have been reached  when  a
       partial  match occurs. This is probably an extremely unlikely case; you
       could guard against it to a certain extent by  always  including  extra
       characters at the start.

       3.  Matching  a subject string that is split into multiple segments may
       not always produce exactly the same result as matching over one  single
       long  string,  especially  when  PCRE_PARTIAL_SOFT is used. The section
       "Partial Matching and Word Boundaries" above describes  an  issue  that
       arises  if  the  pattern ends with \b or \B. Another kind of difference
       may occur when there are multiple matching possibilities, because  (for
       PCRE_PARTIAL_SOFT)  a partial match result is given only when there are
       no completed matches. This means that as soon as the shortest match has
       been  found,  continuation to a new subject segment is no longer possi-
       ble. Consider again this pcretest example:

           re> /dog(sbody)?/
         data> dogsb\P
          0: dog
         data> do\P\D
         Partial match: do
         data> gsb\R\P\D
          0: g
         data> dogsbody\D
          0: dogsbody
          1: dog

       The first data line passes the string "dogsb" to  a  standard  matching
       function,  setting the PCRE_PARTIAL_SOFT option. Although the string is
       a partial match for "dogsbody", the result is  not  PCRE_ERROR_PARTIAL,
       because  the  shorter string "dog" is a complete match. Similarly, when
       the subject is presented to a DFA matching function  in  several  parts
       ("do"  and  "gsb"  being  the first two) the match stops when "dog" has
       been found, and it is not possible to continue.  On the other hand,  if
       "dogsbody"  is  presented  as  a single string, a DFA matching function
       finds both matches.

       Because of these problems, it is best  to  use  PCRE_PARTIAL_HARD  when
       matching  multi-segment  data.  The  example above then behaves differ-
       ently:

           re> /dog(sbody)?/
         data> dogsb\P\P
         Partial match: dogsb
         data> do\P\D
         Partial match: do
         data> gsb\R\P\P\D
         Partial match: gsb

       4. Patterns that contain alternatives at the top level which do not all
       start  with  the  same  pattern  item  may  not  work  as expected when
       PCRE_DFA_RESTART is used. For example, consider this pattern:

         1234|3789

       If the first part of the subject is "ABC123", a partial  match  of  the
       first  alternative  is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at  one  point in the subject are remembered. The problem arises
       because the start of the second alternative matches  within  the  first
       alternative.  There  is  no  problem with anchored patterns or patterns
       such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.  This  is
       not  a  problem  if  a  standard matching function is used, because the
       entire match has to be rerun each time:

           re> /1234|3789/
         data> ABC123\P\P
         Partial match: 123
         data> 1237890
          0: 3789

       Of course, instead of using PCRE_DFA_RESTART, the same technique of re-
       running  the  entire match can also be used with the DFA matching func-
       tions. Another possibility is to work with two buffers.  If  a  partial
       match  at  offset  n in the first buffer is followed by "no match" when
       PCRE_DFA_RESTART is used on the second buffer, you can then try  a  new
       match starting at offset n+1 in the first buffer.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPRECOMPILE(3)                                            PCREPRECOMPILE(3)


NAME
       PCRE - Perl-compatible regular expressions


SAVING AND RE-USING PRECOMPILED PCRE PATTERNS

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit  more  complicated.
       However,  if you are using the just-in-time optimization feature, it is
       not possible to save and reload the JIT data.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent host and run them there. If the two hosts have different endianness
       (byte order), you should run the  pcre[16]_pattern_to_host_byte_order()
       function on the new host before trying to match the pattern. The match-
       ing functions return PCRE_ERROR_BADENDIANNESS if they detect a  pattern
       with the wrong endianness.

       Compiling  regular  expressions with one version of PCRE for use with a
       different version is not guaranteed to work and may cause crashes,  and
       saving  and  restoring  a  compiled  pattern loses any JIT optimization
       data.


SAVING A COMPILED PATTERN

       The value returned by pcre[16]_compile() points to a  single  block  of
       memory  that  holds  the  compiled pattern and associated data. You can
       find the length of this block in bytes by  calling  pcre[16]_fullinfo()
       with  an  argument of PCRE_INFO_SIZE. You can then save the data in any
       appropriate manner. Here is sample code for the 8-bit library that com-
       piles  a  pattern and writes it to a file. It assumes that the variable
       fd refers to a file that is open for output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If the pattern has been studied, it is also possible to save the normal
       study data in a similar way to the compiled pattern itself. However, if
       the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
       ated cannot be saved because it is too dependent on the  current  envi-
       ronment.    When    studying    generates    additional    information,
       pcre[16]_study() returns a pointer to a pcre[16]_extra data block.  Its
       format  is  defined in the section on matching a pattern in the pcreapi
       documentation. The study_data field points to the  binary  study  data,
       and  this  is what you must save (not the pcre[16]_extra block itself).
       The  length  of  the  study   data   can   be   obtained   by   calling
       pcre[16]_fullinfo()  with  an argument of PCRE_INFO_STUDYSIZE. Remember
       to check that pcre[16]_study() did return a non-NULL value before  try-
       ing to save the study data.


RE-USING A PRECOMPILED PATTERN

       Re-using  a  precompiled pattern is straightforward. Having reloaded it
       into main memory, called pcre[16]_pattern_to_host_byte_order() if  nec-
       essary,  you pass its pointer to pcre[16]_exec() or pcre[16]_dfa_exec()
       in the usual way.

       However, if you passed a pointer to custom character  tables  when  the
       pattern was compiled (the tableptr argument of pcre[16]_compile()), you
       must   now   pass   a   similar   pointer   to    pcre[16]_exec()    or
       pcre[16]_dfa_exec(),  because the value saved with the compiled pattern
       will obviously be nonsense. A field in a pcre[16]_extra() block is used
       to pass this data, as described in the section on matching a pattern in
       the pcreapi documentation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled, the pointer in the compiled pattern is NULL, which causes the
       matching functions to use PCRE's internal tables. Thus, you do not need
       to take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre[16]_extra data block and  set  the  study_data  field  to
       point   to   the   reloaded   study   data.   You  must  also  set  the
       PCRE_EXTRA_STUDY_DATA bit in the flags field  to  indicate  that  study
       data  is  present.  Then  pass the pcre[16]_extra block to the matching
       function in the usual way. If the pattern was studied for  just-in-time
       optimization,  that  data  cannot  be  saved,  and  so  is  lost  by  a
       save/restore cycle.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES

       In general, it is safest to  recompile  all  saved  patterns  when  you
       update  to  a new PCRE release, though not all updates actually require
       this.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPERFORM(3)                                                  PCREPERFORM(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns  are compiled by PCRE into a reasonably efficient interpretive
       code, so that most simple patterns do not  use  much  memory.  However,
       there  is  one case where the memory usage of a compiled pattern can be
       unexpectedly large. If a parenthesized subpattern has a quantifier with
       a minimum greater than 1 and/or a limited maximum, the whole subpattern
       is repeated in the compiled code. For example, the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled using the 8-bit library. When PCRE is com-
       piled  with  its  default  internal pointer size of two bytes, the size
       limit on a compiled pattern is 64K data units, and this is reached with
       the  above  pattern  if  the outer repetition is increased from 3 to 4.
       PCRE can be compiled to use larger internal pointers  and  thus  handle
       larger  compiled patterns, but it is better to try to rewrite your pat-
       tern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to  make  use
       of PCRE's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to 18K, and indeed it remains under 20K
       even with the outer repetition increased to 100. However, this  pattern
       is  not  exactly equivalent, because the "subroutine" calls are treated
       as atomic groups into which there can be no backtracking if there is  a
       subsequent  matching  failure.  Therefore,  PCRE cannot do this kind of
       rewriting automatically.  Furthermore, there is a  noticeable  loss  of
       speed  when executing the modified pattern. Nevertheless, if the atomic
       grouping is not a problem and the loss of  speed  is  acceptable,  this
       kind  of  rewriting will allow you to process patterns that PCRE cannot
       otherwise handle.


STACK USAGE AT RUN TIME

       When pcre_exec() or pcre16_exec() is used for matching,  certain  kinds
       of  pattern  can cause it to use large amounts of the process stack. In
       some environments the default process stack is quite small, and  if  it
       runs  out  the result is often SIGSEGV. This issue is probably the most
       frequently raised problem with PCRE. Rewriting your pattern  can  often
       help. The pcrestack documentation discusses this issue in detail.


PROCESSING TIME

       Certain  items  in regular expression patterns are processed more effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion  about  optimizing  regular
       expressions  for  efficient  performance.  This document contains a few
       observations about PCRE.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a  character's  property.
       If  you  can  find  an  alternative pattern that does not use character
       properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s,  and  \w,  and  the  POSIX
       character  classes  such  as  [:alpha:]  do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you can set PCRE_UCP if you want Unicode character properties
       to be used. This can double the matching time for  items  such  as  \d,
       when matched with a traditional matching function; the performance loss
       is less with a DFA matching function, and in both cases  there  is  not
       much difference for \b.

       When  a  pattern  begins  with .* not in parentheses, or in parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is  set, the pattern is implicitly anchored by PCRE, since it can match
       only at the start of a subject string. However, if PCRE_DOTALL  is  not
       set,  PCRE  cannot  make this optimization, because the . metacharacter
       does not then match a newline, and if the subject string contains  new-
       lines,  the  pattern may match from the character immediately following
       one of them instead of from the very start. For example, the pattern

         .*second

       matches the subject "first\nand second" (where \n stands for a  newline
       character),  with the match starting at the seventh character. In order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

       If  you  are using such a pattern with subject strings that do not con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or  starting  the pattern with ^.* or ^.*? to indicate explicit anchor-
       ing. That saves PCRE from having to scan along the subject looking  for
       a newline to restart at.

       Beware  of  patterns  that contain nested indefinite repeats. These can
       take a long time to run when applied to a string that does  not  match.
       Consider the pattern fragment

         ^(a+)*

       This  can  match "aaaa" in 16 different ways, and this number increases
       very rapidly as the string gets longer. (The * repeat can match  0,  1,
       2,  3, or 4 times, and for each of those cases other than 0 or 4, the +
       repeats can match different numbers of times.) When  the  remainder  of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle to try  every  possible  variation,  and  this  can  take  an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where  a  literal  character  follows. Before embarking on the standard
       matching procedure, PCRE checks that there is a "b" later in  the  sub-
       ject  string, and if there is not, it fails the match immediately. How-
       ever, when there is no following literal this  optimization  cannot  be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with  the  pattern  above.  The former gives a failure almost instantly
       when applied to a whole line of  "a"  characters,  whereas  the  latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREPOSIX(3)                                                      PCREPOSIX(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION

       This  set  of functions provides a POSIX-style API for the PCRE regular
       expression 8-bit library. See the pcreapi documentation for a  descrip-
       tion  of  PCRE's native API, which contains much additional functional-
       ity. There is no POSIX-style wrapper for PCRE's 16-bit library.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h header file, and on Unix  systems  the  library  itself  is
       called  pcreposix.a,  so  can  be accessed by adding -lpcreposix to the
       command for linking an application that uses them.  Because  the  POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I  have implemented only those POSIX option bits that can be reasonably
       mapped to PCRE native options. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot  in PCRE as a replacement library. Other POSIX options
       are not even defined.

       There are also some other options that are not defined by POSIX.  These
       have been added at the request of users who want to make use of certain
       PCRE-specific features via the POSIX calling interface.

       When PCRE is called via these functions, it is only  the  API  that  is
       POSIX-like  in  style.  The syntax and semantics of the regular expres-
       sions themselves are still those of Perl, subject  to  the  setting  of
       various  PCRE  options, as described below. "POSIX-like in style" means
       that the API approximates to the POSIX  definition;  it  is  not  fully
       POSIX-compatible,  and  in  multi-byte  encoding domains it is probably
       even less compatible.

       The header for these functions is supplied as pcreposix.h to avoid  any
       potential  clash  with  other  POSIX  libraries.  It can, of course, be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two  structure  types,  regex_t  for  compiled internal forms, and reg-
       match_t for returning captured substrings. It also  defines  some  con-
       stants  whose  names  start  with  "REG_";  these  are used for setting
       options and identifying error codes.


COMPILING A PATTERN

       The function regcomp() is called to compile a pattern into an  internal
       form.  The  pattern  is  a C string terminated by a binary zero, and is
       passed in the argument pattern. The preg argument is  a  pointer  to  a
       regex_t  structure that is used as a base for storing information about
       the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE_DOTALL option is set when the regular expression is passed for
       compilation to the native function. Note that REG_DOTALL is not part of
       the POSIX standard.

         REG_ICASE

       The  PCRE_CASELESS  option is set when the regular expression is passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE_MULTILINE option is set when the regular expression is  passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSUB

       The  PCRE_NO_AUTO_CAPTURE  option is set when the regular expression is
       passed for compilation to the native function. In addition, when a pat-
       tern  that is compiled with this flag is passed to regexec() for match-
       ing, the nmatch and pmatch  arguments  are  ignored,  and  no  captured
       strings are returned.

         REG_UCP

       The  PCRE_UCP  option  is set when the regular expression is passed for
       compilation to the native function. This causes  PCRE  to  use  Unicode
       properties  when  matchine  \d,  \w,  etc., instead of just recognizing
       ASCII values. Note that REG_UTF8 is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE_UNGREEDY option is set when the regular expression  is  passed
       for  compilation  to the native function. Note that REG_UNGREEDY is not
       part of the POSIX standard.

         REG_UTF8

       The PCRE_UTF8 option is set when the regular expression is  passed  for
       compilation  to the native function. This causes the pattern itself and
       all data strings used for matching it to be treated as  UTF-8  strings.
       Note that REG_UTF8 is not part of the POSIX standard.

       In  the  absence  of  these  flags, no options are passed to the native
       function.  This means the the  regex  is  compiled  with  PCRE  default
       semantics.  In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they are not)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.

       NOTE: If the yield of regcomp() is non-zero, you must  not  attempt  to
       use the contents of the preg structure. If, for example, you pass it to
       regexec(), the result is undefined and your program is likely to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.   It  is  not possible to get PCRE to obey POSIX semantics, but
       then PCRE was never intended to be a POSIX engine. The following  table
       lists  the  different  possibilities for matching newline characters in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent  for  PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
       no way to stop newline from matching [^a].

       The  default  POSIX  newline  handling  can  be  obtained  by   setting
       PCRE_DOTALL  and  PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN

       The function regexec() is called  to  match  a  compiled  pattern  preg
       against  a  given string, which is by default terminated by a zero byte
       (but see REG_STARTEND below), subject to the options in  eflags.  These
       can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEMPTY

       The PCRE_NOTEMPTY option is set when calling the underlying PCRE match-
       ing function. Note that REG_NOTEMPTY is not part of the POSIX standard.
       However, setting this option can give more POSIX-like behaviour in some
       situations.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

         REG_STARTEND

       The string is considered to start at string +  pmatch[0].rm_so  and  to
       have  a terminating NUL located at string + pmatch[0].rm_eo (there need
       not actually be a NUL at that location), regardless  of  the  value  of
       nmatch.  This  is a BSD extension, compatible with but not specified by
       IEEE Standard 1003.2 (POSIX.2), and should  be  used  with  caution  in
       software intended to be portable to other systems. Note that a non-zero
       rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
       of the string, not how it is matched.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

       If the value of nmatch is zero, or if the value pmatch is NULL, no data
       about any matched strings is returned.

       Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an array of nmatch structures of type regmatch_t, containing  the  mem-
       bers  rm_so  and rm_eo. These contain the offset to the first character
       of each substring and the offset to the first character after  the  end
       of  each substring, respectively. The 0th element of the vector relates
       to the entire portion of string that was matched;  subsequent  elements
       relate  to  the capturing subpatterns of the regular expression. Unused
       entries in the array have both structure members set to -1.

       A successful match yields  a  zero  return;  various  error  codes  are
       defined  in  the  header  file,  of which REG_NOMATCH is the "expected"
       failure code.


ERROR MESSAGES

       The regerror() function maps a non-zero errorcode from either regcomp()
       or  regexec()  to  a  printable message. If preg is not NULL, the error
       should have arisen from the use of that structure. A message terminated
       by  a  binary  zero  is  placed  in  errbuf. The length of the message,
       including the zero, is limited to errbuf_size. The yield of  the  func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE

       Compiling  a regular expression causes memory to be allocated and asso-
       ciated with the preg structure. The function regfree() frees  all  such
       memory,  after  which  preg may no longer be used as a compiled expres-
       sion.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECPP(3)                                                          PCRECPP(3)


NAME
       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF C++ WRAPPER

       #include <pcrecpp.h>


DESCRIPTION

       The  C++  wrapper  for PCRE was provided by Google Inc. Some additional
       functionality was added by Giuseppe Maxia. This brief man page was con-
       structed  from  the  notes  in the pcrecpp.h file, which should be con-
       sulted for further details. Note that the C++ wrapper supports only the
       original 8-bit PCRE library. There is no 16-bit support at present.


MATCHING INTERFACE

       The  "FullMatch" operation checks that supplied text matches a supplied
       pattern exactly. If pointer arguments are supplied, it  copies  matched
       sub-strings that match sub-patterns into them.

         Example: successful match
            pcrecpp::RE re("h.*o");
            re.FullMatch("hello");

         Example: unsuccessful match (requires full match):
            pcrecpp::RE re("e");
            !re.FullMatch("hello");

         Example: creating a temporary RE object:
            pcrecpp::RE("h.*o").FullMatch("hello");

       You  can pass in a "const char*" or a "string" for "text". The examples
       below tend to use a const char*. You can, as in the different  examples
       above,  store the RE object explicitly in a variable or use a temporary
       RE object. The examples below use one mode or  the  other  arbitrarily.
       Either could correctly be used for any of these examples.

       You must supply extra pointer arguments to extract matched subpieces.

         Example: extracts "ruby" into "s" and 1234 into "i"
            int i;
            string s;
            pcrecpp::RE re("(\\w+):(\\d+)");
            re.FullMatch("ruby:1234", &s, &i);

         Example: does not try to extract any extra sub-patterns
            re.FullMatch("ruby:1234", &s);

         Example: does not try to extract into NULL
            re.FullMatch("ruby:1234", NULL, &i);

         Example: integer overflow causes failure
            !re.FullMatch("ruby:1234567891234", NULL, &i);

         Example: fails because there aren't enough sub-patterns:
            !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);

         Example: fails because string cannot be stored in integer
            !pcrecpp::RE("(.*)").FullMatch("ruby", &i);

       The  provided  pointer  arguments can be pointers to any scalar numeric
       type, or one of:

          string        (matched piece is copied to string)
          StringPiece   (StringPiece is mutated to point to matched piece)
          T             (where "bool T::ParseFrom(const char*, int)" exists)
          NULL          (the corresponding matched sub-pattern is not copied)

       The function returns true iff all of the following conditions are  sat-
       isfied:

         a. "text" matches "pattern" exactly;

         b. The number of matched sub-patterns is >= number of supplied
            pointers;

         c. The "i"th argument has a suitable type for holding the
            string captured as the "i"th sub-pattern. If you pass in
            void * NULL for the "i"th argument, or a non-void * NULL
            of the correct type, or pass fewer arguments than the
            number of sub-patterns, "i"th captured sub-pattern is
            ignored.

       CAVEAT:  An  optional  sub-pattern  that  does not exist in the matched
       string is assigned the empty  string.  Therefore,  the  following  will
       return false (because the empty string is not a valid number):

          int number;
          pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);

       The  matching interface supports at most 16 arguments per call.  If you
       need   more,   consider    using    the    more    general    interface
       pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.

       NOTE:  Do not use no_arg, which is used internally to mark the end of a
       list of optional arguments, as a placeholder for missing arguments,  as
       this can lead to segfaults.


QUOTING METACHARACTERS

       You  can use the "QuoteMeta" operation to insert backslashes before all
       potentially meaningful characters in a  string.  The  returned  string,
       used as a regular expression, will exactly match the original string.

         Example:
            string quoted = RE::QuoteMeta(unquoted);

       Note  that  it's  legal to escape a character even if it has no special
       meaning in a regular expression -- so this function  does  that.  (This
       also  makes  it  identical  to  the perl function of the same name; see
       "perldoc   -f   quotemeta".)    For   example,    "1.5-2.0?"    becomes
       "1\.5\-2\.0\?".


PARTIAL MATCHES

       You  can  use the "PartialMatch" operation when you want the pattern to
       match any substring of the text.

         Example: simple search for a string:
            pcrecpp::RE("ell").PartialMatch("hello");

         Example: find first number in a string:
            int number;
            pcrecpp::RE re("(\\d+)");
            re.PartialMatch("x*100 + 20", &number);
            assert(number == 100);


UTF-8 AND THE MATCHING INTERFACE

       By default, pattern and text are plain text, one  byte  per  character.
       The  UTF8  flag,  passed  to  the  constructor, causes both pattern and
       string to be treated as UTF-8 text, still a byte stream but potentially
       multiple  bytes  per character. In practice, the text is likelier to be
       UTF-8 than the pattern, but the match returned may depend on  the  UTF8
       flag,  so  always use it when matching UTF8 text. For example, "." will
       match one byte normally but with UTF8 set may match up to  three  bytes
       of a multi-byte character.

         Example:
            pcrecpp::RE_Options options;
            options.set_utf8();
            pcrecpp::RE re(utf8_pattern, options);
            re.FullMatch(utf8_string);

         Example: using the convenience function UTF8():
            pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
            re.FullMatch(utf8_string);

       NOTE: The UTF8 flag is ignored if pcre was not configured with the
             --enable-utf8 flag.


PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE

       PCRE  defines  some  modifiers  to  change  the behavior of the regular
       expression  engine.  The  C++  wrapper  defines  an  auxiliary   class,
       RE_Options,  as  a  vehicle  to pass such modifiers to a RE class. Cur-
       rently, the following modifiers are supported:

          modifier              description               Perl corresponding

          PCRE_CASELESS         case insensitive match      /i
          PCRE_MULTILINE        multiple lines match        /m
          PCRE_DOTALL           dot matches newlines        /s
          PCRE_DOLLAR_ENDONLY   $ matches only at end       N/A
          PCRE_EXTRA            strict escape parsing       N/A
          PCRE_EXTENDED         ignore whitespaces          /x
          PCRE_UTF8             handles UTF8 chars          built-in
          PCRE_UNGREEDY         reverses * and *?           N/A
          PCRE_NO_AUTO_CAPTURE  disables capturing parens   N/A (*)

       (*) Both Perl and PCRE allow non capturing parentheses by means of  the
       "?:"  modifier  within the pattern itself. e.g. (?:ab|cd) does not cap-
       ture, while (ab|cd) does.

       For a full account on how each modifier works, please  check  the  PCRE
       API reference page.

       For  each  modifier,  there are two member functions whose name is made
       out of the modifier in  lowercase,  without  the  "PCRE_"  prefix.  For
       instance, PCRE_CASELESS is handled by

         bool caseless()

       which returns true if the modifier is set, and

         RE_Options & set_caseless(bool)

       which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
       be accessed through  the  set_match_limit()  and  match_limit()  member
       functions.  Setting match_limit to a non-zero value will limit the exe-
       cution of pcre to keep it from doing bad things like blowing the  stack
       or  taking  an  eternity  to  return  a result. A value of 5000 is good
       enough to stop stack blowup in a 2MB thread stack. Setting  match_limit
       to   zero   disables   match  limiting.  Alternatively,  you  can  call
       match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION  to
       limit  how  much  PCRE  recurses.  match_limit()  limits  the number of
       matches PCRE does; match_limit_recursion() limits the depth of internal
       recursion, and therefore the amount of stack that is used.

       Normally,  to  pass  one or more modifiers to a RE class, you declare a
       RE_Options object, set the appropriate options, and pass this object to
       a RE constructor. Example:

          RE_Options opt;
          opt.set_caseless(true);
          if (RE("HELLO", opt).PartialMatch("hello world")) ...

       RE_options has two constructors. The default constructor takes no argu-
       ments and creates a set of flags that are off by default. The  optional
       parameter  option_flags is to facilitate transfer of legacy code from C
       programs.  This lets you do

          RE(pattern,
            RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);

       However, new code is better off doing

          RE(pattern,
            RE_Options().set_caseless(true).set_multiline(true))
              .PartialMatch(str);

       If you are going to pass one of the most used modifiers, there are some
       convenience functions that return a RE_Options class with the appropri-
       ate modifier already set: CASELESS(),  UTF8(),  MULTILINE(),  DOTALL(),
       and EXTENDED().

       If  you  need  to set several options at once, and you don't want to go
       through the pains of declaring a RE_Options object and setting  several
       options,  there  is a parallel method that give you such ability on the
       fly. You can concatenate several set_xxxxx()  member  functions,  since
       each  of  them returns a reference to its class object. For example, to
       pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with  one
       statement, you may write:

          RE(" ^ xyz \\s+ .* blah$",
            RE_Options()
              .set_caseless(true)
              .set_extended(true)
              .set_multiline(true)).PartialMatch(sometext);


SCANNING TEXT INCREMENTALLY

       The  "Consume"  operation may be useful if you want to repeatedly match
       regular expressions at the front of a string and skip over them as they
       match.  This requires use of the "StringPiece" type, which represents a
       sub-range of a real string. Like RE,  StringPiece  is  defined  in  the
       pcrecpp namespace.

         Example: read lines of the form "var = value" from a string.
            string contents = ...;                 // Fill string somehow
            pcrecpp::StringPiece input(contents);  // Wrap in a StringPiece

            string var;
            int value;
            pcrecpp::RE re("(\\w+) = (\\d+)\n");
            while (re.Consume(&input, &var, &value)) {
              ...;
            }

       Each  successful  call  to  "Consume"  will  set  "var/value", and also
       advance "input" so it points past the matched text.

       The "FindAndConsume" operation is similar to  "Consume"  but  does  not
       anchor  your  match  at  the  beginning of the string. For example, you
       could extract all words from a string by repeatedly calling

         pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)


PARSING HEX/OCTAL/C-RADIX NUMBERS

       By default, if you pass a pointer to a numeric value, the corresponding
       text  is  interpreted  as  a  base-10  number. You can instead wrap the
       pointer with a call to one of the operators Hex(), Octal(), or CRadix()
       to  interpret  the text in another base. The CRadix operator interprets
       C-style "0" (base-8) and  "0x"  (base-16)  prefixes,  but  defaults  to
       base-10.

         Example:
           int a, b, c, d;
           pcrecpp::RE re("(.*) (.*) (.*) (.*)");
           re.FullMatch("100 40 0100 0x40",
                        pcrecpp::Octal(&a), pcrecpp::Hex(&b),
                        pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));

       will leave 64 in a, b, c, and d.


REPLACING PARTS OF STRINGS

       You  can  replace the first match of "pattern" in "str" with "rewrite".
       Within "rewrite", backslash-escaped digits (\1 to \9) can  be  used  to
       insert  text  matching  corresponding parenthesized group from the pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

       will leave "s" containing "yada dabba doo". The result is true  if  the
       pattern matches and a replacement occurs, false otherwise.

       GlobalReplace  is  like Replace except that it replaces all occurrences
       of the pattern in the string with the  rewrite.  Replacements  are  not
       subject to re-matching. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

       will  leave  "s"  containing  "yada dada doo". It returns the number of
       replacements made.

       Extract is like Replace, except that if the pattern matches,  "rewrite"
       is  copied into "out" (an additional argument) with substitutions.  The
       non-matching portions of "text" are ignored. Returns true iff  a  match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.


AUTHOR

       The C++ wrapper was contributed by Google Inc.
       Copyright (c) 2007 Google Inc.


REVISION

       Last updated: 08 January 2012
------------------------------------------------------------------------------


PCRESAMPLE(3)                                                    PCRESAMPLE(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE SAMPLE PROGRAM

       A simple, complete demonstration program, to get you started with using
       PCRE, is supplied in the file pcredemo.c in the  PCRE  distribution.  A
       listing  of this program is given in the pcredemo documentation. If you
       do not have a copy of the PCRE distribution, you can save this  listing
       to re-create pcredemo.c.

       The  demonstration program, which uses the original PCRE 8-bit library,
       compiles the regular expression that is its first argument, and matches
       it  against  the subject string in its second argument. No PCRE options
       are set, and default character tables are used. If  matching  succeeds,
       the  program  outputs the portion of the subject that matched, together
       with the contents of any captured substrings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       If PCRE is installed in the standard include  and  library  directories
       for your operating system, you should be able to compile the demonstra-
       tion program using this command:

         gcc -o pcredemo pcredemo.c -lpcre

       If PCRE is installed elsewhere, you may need to add additional  options
       to  the  command line. For example, on a Unix-like system that has PCRE
       installed in /usr/local, you  can  compile  the  demonstration  program
       using a command like this:

         gcc -o pcredemo -I/usr/local/include pcredemo.c \
             -L/usr/local/lib -lpcre

       In  a  Windows  environment, if you want to statically link the program
       against a non-dll pcre.a file, you must uncomment the line that defines
       PCRE_STATIC  before  including  pcre.h, because otherwise the pcre_mal-
       loc()   and   pcre_free()   exported   functions   will   be   declared
       __declspec(dllimport), with unwanted results.

       Once  you  have  compiled and linked the demonstration program, you can
       run simple tests like this:

         ./pcredemo 'cat|dog' 'the cat sat on the mat'
         ./pcredemo -g 'cat|dog' 'the dog sat on the cat'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcretest,  which  supports  many  more  facilities  for testing regular
       expressions and both PCRE libraries. The pcredemo program  is  provided
       as a simple coding example.

       If  you  try to run pcredemo when PCRE is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1:  a.out:  fatal:  libpcre.so.0:  open failed: No such file or
       directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------
PCRELIMITS(3)                                                    PCRELIMITS(3)


NAME
       PCRE - Perl-compatible regular expressions


SIZE AND OTHER LIMITATIONS

       There  are some size limitations in PCRE but it is hoped that they will
       never in practice be relevant.

       The maximum length of a compiled  pattern  is  approximately  64K  data
       units  (bytes  for  the  8-bit  library,  16-bit  units  for the 16-bit
       library) if PCRE is compiled with the default internal linkage size  of
       2  bytes.  If  you  want  to process regular expressions that are truly
       enormous, you can compile PCRE with an internal linkage size of 3 or  4
       (when  building  the  16-bit  library,  3  is rounded up to 4). See the
       README file in the source distribution and the pcrebuild  documentation
       for  details.  In  these cases the limit is substantially larger.  How-
       ever, the speed of execution is slower.

       All values in repeating quantifiers must be less than 65536.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

       There is a limit to the number of forward references to subsequent sub-
       patterns of around 200,000.  Repeated  forward  references  with  fixed
       upper  limits,  for example, (?2){0,100} when subpattern number 2 is to
       the right, are included in the count. There is no limit to  the  number
       of backward references.

       The maximum length of name for a named subpattern is 32 characters, and
       the maximum number of named subpatterns is 10000.

       The maximum length of a subject string is the largest  positive  number
       that  an integer variable can hold. However, when using the traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite  repetition.  This means that the available stack space may limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRESTACK(3)                                                      PCRESTACK(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE DISCUSSION OF STACK USAGE

       When  you  call  pcre[16]_exec(),  it makes use of an internal function
       called match(). This calls itself recursively at branch points  in  the
       pattern,  in  order  to  remember the state of the match so that it can
       back up and try a different alternative if  the  first  one  fails.  As
       matching proceeds deeper and deeper into the tree of possibilities, the
       recursion depth increases. The match() function is also called in other
       circumstances,  for  example,  whenever  a parenthesized sub-pattern is
       entered, and in certain cases of repetition.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The  above  comments  apply  when  pcre[16]_exec() is run in its normal
       interpretive  manner.   If   the   pattern   was   studied   with   the
       PCRE_STUDY_JIT_COMPILE  option, and just-in-time compiling was success-
       ful, and the options passed to pcre[16]_exec() were  not  incompatible,
       the  matching process uses the JIT-compiled code instead of the match()
       function. In this case, the memory requirements  are  handled  entirely
       differently. See the pcrejit documentation for details.

       The pcre[16]_dfa_exec() function operates in an entirely different way,
       and uses recursion only when there is a regular expression recursion or
       subroutine  call in the pattern. This includes the processing of asser-
       tion and "once-only" subpatterns, which  are  handled  like  subroutine
       calls.  Normally,  these are never very deep, and the limit on the com-
       plexity of pcre[16]_dfa_exec() is controlled by the amount of workspace
       it  is  given.   However, it is possible to write patterns with runaway
       infinite recursions; such patterns will  cause  pcre[16]_dfa_exec()  to
       run out of stack. At present, there is no protection against this.

       The  comments that follow do NOT apply to pcre[16]_dfa_exec(); they are
       relevant only for pcre[16]_exec() without the JIT optimization.

   Reducing pcre[16]_exec()'s stack usage

       Each time that match() is actually called recursively, it  uses  memory
       from  the  process  stack.  For certain kinds of pattern and data, very
       large amounts of stack may be needed, despite the recognition of  "tail
       recursion".   You  can often reduce the amount of recursion, and there-
       fore the amount of stack used, by modifying the pattern that  is  being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       It  matches  from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that  might  be  used  when
       processing an XML file. Each iteration of the outer parentheses matches
       either one character that is not "<" or a "<" that is not  followed  by
       "inet".  However,  each  time  a  parenthesis is processed, a recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter.  For  a long string, a lot of stack is required. Consider now this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))+

       This uses very much less stack, because runs of characters that do  not
       contain  "<" are "swallowed" in one item inside the parentheses. Recur-
       sion happens only when a "<" character that is not followed  by  "inet"
       is  encountered  (and  we assume this is relatively rare). A possessive
       quantifier is used to stop any backtracking into the  runs  of  non-"<"
       characters, but that is not related to stack usage.

       This  example shows that one way of avoiding stack problems when match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

   Compiling PCRE to use heap instead of stack for pcre[16]_exec()

       In  environments  where  stack memory is constrained, you might want to
       compile PCRE to use heap memory instead of stack for remembering  back-
       up points when pcre[16]_exec() is running. This makes it run a lot more
       slowly, however.  Details of how to do this are given in the  pcrebuild
       documentation. When built in this way, instead of using the stack, PCRE
       obtains and frees memory by calling the functions that are  pointed  to
       by  the  pcre[16]_stack_malloc  and  pcre[16]_stack_free  variables. By
       default, these point to malloc() and free(), but you  can  replace  the
       pointers to cause PCRE to use your own functions. Since the block sizes
       are always the same, and are always freed in reverse order, it  may  be
       possible  to  implement  customized memory handlers that are more effi-
       cient than the standard functions.

   Limiting pcre[16]_exec()'s stack usage

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively.  If  a  limit is exceeded, pcre[16]_exec()
       returns an error code. Setting suitable limits should prevent  it  from
       running  out of stack. The default values of the limits are very large,
       and unlikely ever to operate. They can be changed when PCRE  is  built,
       and they can also be set when pcre[16]_exec() is called. For details of
       these interfaces, see the pcrebuild documentation and  the  section  on
       extra data for pcre[16]_exec() in the pcreapi documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
       should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
       hand, can support around 128000 recursions.

       In Unix-like environments, the pcretest test program has a command line
       option (-S) that can be used to increase the size of its stack. As long
       as the stack is large enough, another option (-M) can be used  to  find
       the  smallest  limits  that allow a particular pattern to match a given
       subject string. This is done by calling pcre[16]_exec() repeatedly with
       different limits.

   Obtaining an estimate of stack usage

       The  actual  amount  of  stack used per recursion can vary quite a lot,
       depending on the compiler that was used to build PCRE and the optimiza-
       tion or debugging options that were set for it. The rule of thumb value
       of 500 bytes mentioned above may be larger  or  smaller  than  what  is
       actually needed. A better approximation can be obtained by running this
       command:

         pcretest -m -C

       The -C option causes pcretest to output information about  the  options
       with which PCRE was compiled. When -m is also given (before -C), infor-
       mation about stack use is given in a line like this:

         Match recursion uses stack: approximate frame size = 640 bytes

       The value is approximate because some recursions need a bit more (up to
       perhaps 16 more bytes).

       If  the  above  command  is given when PCRE is compiled to use the heap
       instead of the stack for recursion, the value that  is  output  is  the
       size of each block that is obtained from the heap.

   Changing stack size in Unix-like systems

       In  Unix-like environments, there is not often a problem with the stack
       unless very long strings are involved,  though  the  default  limit  on
       stack  size  varies  from system to system. Values from 8Mb to 64Mb are
       common. You can find your default limit by running the command:

         ulimit -s

       Unfortunately, the effect of running out of  stack  is  often  SIGSEGV,
       though  sometimes  a more explicit error message is given. You can nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using  getrlimit(),  then
       attempts  to  increase  the  soft limit to 100Mb using setrlimit(). You
       must do this before calling pcre[16]_exec().

   Changing stack size in Mac OS X

       Using setrlimit(), as described above, should also work on Mac OS X. It
       is also possible to set a stack size when linking a program. There is a
       discussion  about  stack  sizes  in  Mac  OS  X  at  this   web   site:
       http://developer.apple.com/qa/qa2005/qa1419.html.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


