This file contains the PCRE man page that describes the regular expressions 
supported by PCRE version 4.4. Note that not all of the features are relevant 
in the context of Exim. In particular, the version of PCRE that is compiled
with Exim does not include UTF-8 support, there is no mechanism for changing
the options with which the PCRE functions are called, and features such as
callout are not accessible.
-----------------------------------------------------------------------------

NAME
     PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

     The syntax and semantics of  the  regular  expressions  sup-
     ported  by PCRE are described below. Regular expressions are
     also described in the Perl documentation and in a number  of
     other  books,  some  of which have copious examples. Jeffrey
     Friedl's  "Mastering  Regular  Expressions",  published   by
     O'Reilly,  covers them in great detail. The description here
     is intended as reference documentation.

     The basic operation of PCRE is on strings of bytes. However,
     there  is  also  support for UTF-8 character strings. To use
     this support you must build PCRE to include  UTF-8  support,
     and  then call pcre_compile() with the PCRE_UTF8 option. How
     this affects the pattern matching is  mentioned  in  several
     places  below.  There is also a summary of UTF-8 features in
     the section on UTF-8 support in the main pcre page.

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

     There are two different sets of meta-characters: those  that
     are  recognized anywhere in the pattern except within square
     brackets, and those that are recognized in square  brackets.
     Outside square brackets, the meta-characters 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 meta-
     characters 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
     meta-characters.


BACKSLASH

     The backslash character has several uses. Firstly, if it  is
     followed  by  a  non-alphameric character, 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 meta-character, so it is always safe to precede a non-
     alphameric with backslash to  specify  that  it  stands  for
     itself. In particular, if you want to match a backslash, you
     write \\.

     If a pattern is compiled with the PCRE_EXTENDED option, whi-
     tespace in the pattern (other than in a character class) and
     characters between a # outside a  character  class  and  the
     next  newline  character  are ignored. An escaping 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
     characters, you can do so by putting them between \Q and \E.
     This is different from Perl in that $ and @ are  handled  as
     literals  in  \Q...\E  sequences in PCRE, whereas in Perl, $
     and @ cause variable interpolation. Note the following exam-
     ples:

       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.

     A second use of backslash provides a way  of  encoding  non-
     printing  characters  in patterns in a visible manner. There
     is no restriction on the appearance of non-printing  charac-
     ters,  apart from the binary zero that terminates a pattern,
     but when a pattern is being prepared by text editing, it  is
     usually  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 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... (UTF-8 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, but \c{ becomes hex 3B, while \c; becomes hex 7B.

     After \x, from zero  to  two  hexadecimal  digits  are  read
     (letters  can be in upper or lower case). In UTF-8 mode, any
     number of hexadecimal digits may appear between \x{  and  },
     but  the value of the character code must be less than 2**31
     (that is, the maximum hexadecimal  value  is  7FFFFFFF).  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 inter-
     preted as a basic  hexadecimal  escape,  with  no  following
     digits, giving a byte whose value is zero.

     Characters whose value is less than 256 can  be  defined  by
     either  of  the  two  syntaxes  for \x when PCRE is in UTF-8
     mode. There is no difference in the way  they  are  handled.
     For example, \xdc is exactly the same as \x{dc}.

     After \0 up to two further octal digits are  read.  In  both
     cases,  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 character that follows is itself an octal digit.

     The handling of a backslash followed by a digit other than 0
     is  complicated.   Outside  a character class, PCRE reads it
     and any following digits 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  follow-
     ing  the  backslash,  and  generates  a single byte from the
     least significant 8 bits of the value. Any subsequent digits
     stand for themselves.  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 byte consisting entirely of 1 bits
       \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  intro-
     duced  by  a  leading zero, because no more than three octal
     digits are ever read.

     All the sequences that define a single byte value or a  sin-
     gle  UTF-8 character (in UTF-8 mode) can be used both inside
     and outside character classes. In addition, inside a charac-
     ter  class,  the sequence \b is interpreted as the backspace
     character (hex 08). Outside a character class it has a  dif-
     ferent meaning (see below).

     The third use of backslash is for specifying generic charac-
     ter types:

       \d     any decimal digit
       \D     any character that is not a decimal digit
       \s     any whitespace character
       \S     any character that is not a whitespace character
       \w     any "word" character
       W     any "non-word" character

     Each pair of escape sequences partitions the complete set of
     characters  into  two  disjoint  sets.  Any  given character
     matches one, and only one, of each pair.

     In UTF-8 mode, characters with values greater than 255 never
     match \d, \s, or \w, and always match \D, \S, and \W.

     For compatibility with Perl, \s does not match the VT  char-
     acter (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).

     A "word" character is any letter or digit or the  underscore
     character,  that  is,  any  character which can be part of a
     Perl "word". The definition of letters and  digits  is  con-
     trolled  by PCRE's character tables, and may vary if locale-
     specific matching is taking place (see "Locale  support"  in
     the pcreapi page). For example, in the "fr" (French) locale,
     some character codes greater than 128 are used for  accented
     letters, and these are matched by \w.

     These character type 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, since there
     is no character to match.

     The fourth use of backslash is  for  certain  simple  asser-
     tions. An assertion 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 start of subject
       \Z     matches at end of subject or before newline at end
       \z     matches at end of subject
       \G     matches at first matching position in subject

     These assertions may not appear in  character  classes  (but
     note  that  \b has a different meaning, namely the backspace
     character, inside a character class).

     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.
     The \A, \Z, and \z assertions differ  from  the  traditional
     circumflex  and  dollar  (described below) 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 mul-
     tiline mode.

     They are not affected  by  the  PCRE_NOTBOL  or  PCRE_NOTEOL
     options.  If the startoffset argument of pcre_exec() is non-
     zero, indicating 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  that is the last character of the string as well as
     at the end of the string, 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  arguments,  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  expres-
     sion.


CIRCUMFLEX AND DOLLAR

     Outside a character class, in the default matching mode, the
     circumflex  character  is an assertion which is true only if
     the current matching point is at the start  of  the  subject
     string.  If  the startoffset argument 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 alter-
     natives start with a circumflex, that is, if the pattern  is
     constrained to match only at the start of the subject, it is
     said to be an "anchored" pattern. (There are also other con-
     structs that can cause a pattern to be anchored.)

     A dollar character is an assertion which is true only if the
     current  matching point is at the end of the subject string,
     or immediately before a newline character that is  the  last
     character in 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,  they  match  immediately  after  and  immediately
     before an internal newline character, respectively, in addi-
     tion to matching at the start and end of the subject string.
     For  example, the pattern /^abc$/ matches the subject string
     "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-
     quently,  patterns  that  are  anchored  in single line mode
     because all branches start with ^ are not anchored in multi-
     line  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 PCRE_MULTILINE is set or not.


FULL STOP (PERIOD, DOT)

     Outside a character class, a dot in the pattern matches  any
     one character in the subject, including a non-printing char-
     acter, but not (by default) newline.  In UTF-8 mode,  a  dot
     matches  any  UTF-8  character, which might be more than one
     byte  long,  except  (by  default)  for  newline.   If   the
     PCRE_DOTALL  option is set, dots match newlines as well. The
     handling of dot is entirely independent of the  handling  of
     circumflex and dollar, the only relationship being that they
     both involve newline characters. Dot has no special  meaning
     in a character class.



MATCHING A SINGLE BYTE

     Outside a character class, the escape  sequence  \C  matches
     any  one  byte, both in and out of UTF-8 mode. Unlike a dot,
     it always matches a newline. The feature is provided in Perl
     in  order  to match individual bytes in UTF-8 mode.  Because
     it breaks up UTF-8 characters into  individual  bytes,  what
     remains  in  the string may be a malformed UTF-8 string. For
     this reason it is best avoided.

     PCRE does not allow \C to appear  in  lookbehind  assertions
     (see below), because in UTF-8 mode it makes it impossible to
     calculate the length of the lookbehind.


SQUARE BRACKETS

     An opening square bracket introduces a character class, ter-
     minated  by  a  closing  square  bracket.  A  closing square
     bracket on its own is  not  special.  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 cir-
     cumflex, if present) or escaped with a backslash.

     A character class matches a single character in the subject.
     In  UTF-8 mode, the character may occupy more than one byte.
     A matched character must be in the set of characters defined
     by the class, unless the first character in the class defin-
     ition 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  con-
     venient  notation for specifying the characters which are in
     the class by enumerating those that are not. It  is  not  an
     assertion:  it  still  consumes a character from the subject
     string, and fails if the current pointer is at  the  end  of
     the string.

     In UTF-8 mode, characters with values greater than  255  can
     be  included  in a class as a literal string of bytes, 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 case-
     ful version would. PCRE does not support the concept of case
     for characters with values greater than 255.
     The newline character is never treated in any special way in
     character  classes,  whatever the setting of the PCRE_DOTALL
     or PCRE_MULTILINE options is. A  class  such  as  [^a]  will
     always match a newline.

     The minus (hyphen) character can be used to specify a  range
     of  characters  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  inter-
     preted 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  character  of  a  range.  A  pattern such as [W-]46] is
     interpreted as a class of two characters ("W" and "-")  fol-
     lowed 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 single class containing a range followed by  two
     separate 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]. In UTF-8 mode,  ranges
     can  include  characters  whose values are greater than 255,
     for example [\x{100}-\x{2ff}].

     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 if character tables for the "fr" locale are
     in use, [\xc8-\xcb] matches accented E  characters  in  both
     cases.

     The character types \d, \D, \s, \S,  \w,  and  \W  may  also
     appear  in  a  character  class, and add the characters that
     they match to the class. For example, [\dABCDEF] matches any
     hexadecimal  digit.  A  circumflex  can conveniently be used
     with the upper case character types to specify a  more  res-
     tricted set of characters than the matching lower case type.
     For example, the class [^\W_] matches any letter  or  digit,
     but not underscore.

     All non-alphameric characters other than \,  -,  ^  (at  the
     start)  and  the  terminating ] are non-special in character
     classes, but it does no harm if they are escaped.


POSIX CHARACTER CLASSES

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

       [01[:alpha:]%]

     matches "0", "1", any alphabetic character, or "%". The sup-
     ported 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
       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" dif-
     ferent  to  \s, which does not include VT (for Perl compati-
     bility).

     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.

     In UTF-8 mode, characters with values greater  than  255  do
     not match any of the POSIX character classes.


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 alter-
     natives  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 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 possible to unset these options by preceding the letter
     with a hyphen, and a combined setting and unsetting such  as
     (?im-sx),  which sets PCRE_CASELESS 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.

     When an option change 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 therefore  show  up  in
     data extracted by the pcre_fullinfo() function).

     An option change within a subpattern affects only that  part
     of the current pattern 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 oth-
     erwise.

     The PCRE-specific options PCRE_UNGREEDY and  PCRE_EXTRA  can
     be changed in the same way as the Perl-compatible options by
     using the characters U and X  respectively.  The  (?X)  flag
     setting  is  special in that it must always occur earlier in
     the pattern than any of the additional features it turns on,
     even when it is at top level. It is best put at the start.


SUBPATTERNS

     Subpatterns are delimited by parentheses  (round  brackets),
     which can be nested.  Marking part of a pattern as a subpat-
     tern does two things:

     1. It localizes a set of alternatives. For example, the pat-
     tern

       cat(aract|erpillar|)

     matches one of the words "cat",  "cataract",  or  "caterpil-
     lar".  Without  the  parentheses, it would match "cataract",
     "erpillar" or the empty string.

     2. It sets up the subpattern as a capturing  subpattern  (as
     defined  above).   When the whole pattern matches, that por-
     tion of the subject string that matched  the  subpattern  is
     passed  back  to  the  caller  via  the  ovector argument of
     pcre_exec(). Opening parentheses are counted  from  left  to
     right (starting from 1) to obtain the numbers of the captur-
     ing 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 numbered 1, 2, and 3, respectively.

     The fact that plain parentheses fulfil two functions is  not
     always  helpful.  There are often times when a grouping sub-
     pattern 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 capturing, and is  not
     counted  when computing the number of any subsequent captur-
     ing 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 sub-
     patterns is 65535, and the maximum depth of nesting  of  all
     subpatterns, both capturing and non-capturing, is 200.

     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".


NAMED SUBPATTERNS

     Identifying capturing parentheses by number is  simple,  but
     it  can be very hard to keep track of the numbers in compli-
     cated regular expressions. Furthermore, if an expression  is
     modified,  the  numbers  may change. To help with the diffi-
     culty, PCRE supports the naming  of  subpatterns,  something
     that  Perl does not provide. The Python syntax (?P<name>...)
     is used. Names consist of alphanumeric characters and under-
     scores, and must be unique within a pattern.

     Named capturing parentheses are still allocated  numbers  as
     well  as  names.  The  PCRE  API provides function calls for
     extracting the name-to-number translation table from a  com-
     piled  pattern. For further details see the pcreapi documen-
     tation.


REPETITION

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

       a literal data character
       the . metacharacter
       the \C escape sequence
       escapes such as \d that match single characters
       a character class
       a back reference (see next section)
       a parenthesized subpattern (unless it is an assertion)

     The general repetition quantifier specifies  a  minimum  and
     maximum  number  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 example, {,6} is not a quantif-
     ier, but a literal string of four characters.

     In UTF-8 mode, quantifiers apply to UTF-8 characters  rather
     than  to  individual  bytes.  Thus,  for example, \x{100}{2}
     matches two UTF-8 characters, each of which  is  represented
     by a two-byte sequence.

     The quantifier {0} is permitted, causing the  expression  to
     behave  as  if the previous item and the quantifier were not
     present.

     For convenience (and  historical  compatibility)  the  three
     most common quantifiers have single-character 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 broken.

     By default, the quantifiers  are  "greedy",  that  is,  they
     match  as much as possible (up to the maximum number of per-
     mitted 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
     the  sequences /* and */ and within the sequence, individual
     * and / characters may appear. An attempt to  match  C  com-
     ments by applying the pattern

       /\*.*\*/

     to the string

       /* first command */  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 pre-
     ferred number of matches.  Do not confuse this use of  ques-
     tion  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 which  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 max-
     imum, more store 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 (equivalent to Perl's /s) is set, thus allowing the .
     to match  newlines,  the  pattern  is  implicitly  anchored,
     because whatever follows will be tried against every charac-
     ter 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  pre-
     ceded by \A.

     In cases where it is known that the subject string  contains
     no  newlines,  it  is  worth setting PCRE_DOTALL in order to
     obtain this optimization, or alternatively using ^ to  indi-
     cate anchoring explicitly.

     However, there is one situation where the optimization  can-
     not  be  used. When .*  is inside capturing parentheses that
     are the subject of a backreference elsewhere in the pattern,
     a match at the start may fail, and a later one succeed. Con-
     sider, for example:

       (.*)abc\1

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

     When a capturing subpattern is repeated, the value  captured
     is the substring that matched the final iteration. For exam-
     ple, after

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

     has matched "tweedledum tweedledee" the value  of  the  cap-
     tured  substring  is  "tweedledee".  However,  if  there are
     nested capturing  subpatterns,  the  corresponding  captured
     values  may  have been set in previous iterations. For exam-
     ple, after

       /(a|(b))+/

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


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

     With both maximizing and minimizing 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  specify-
     ing  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  would give 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+)bar

     This kind of parenthesis "locks up" the  part of the pattern
     it  contains once it has matched, and a failure further into
     the pattern is prevented from backtracking  into  it.  Back-
     tracking  past  it to previous items, however, works as nor-
     mal.

     An alternative description is that a subpattern of this type
     matches  the  string  of  characters that an identical stan-
     dalone 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 prepared 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++bar

     Possessive quantifiers are always greedy; the setting of the
     PCRE_UNGREEDY option is ignored. They are a convenient nota-
     tion for the simpler forms of atomic group.  However,  there
     is  no  difference in the meaning or processing of a posses-
     sive quantifier and the equivalent atomic group.

     The possessive quantifier syntax is an extension to the Perl
     syntax. It originates in Sun's Java package.

     When a pattern contains an unlimited repeat inside a subpat-
     tern  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  con-
     sist  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 two repeats in
     a large number of ways, and all have to be tried. (The exam-
     ple  used  [!?]  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 character that is  required  for  a
     match,  and  fail early if it is not present in the string.)
     If the pattern is changed to

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

     sequences of non-digits cannot be broken, and  failure  hap-
     pens 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 subpattern 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 pattern. In other words, the
     parentheses that are referenced need not be to the  left  of
     the  reference  for  numbers  less  than 10. See the section
     entitled "Backslash" above for further details of  the  han-
     dling of digits following a backslash.

     A back reference matches whatever actually matched the  cap-
     turing subpattern 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  responsi-
     bility",  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 example,

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

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

     Back references to named subpatterns use the  Python  syntax
     (?P=name). We could rewrite the above example as follows:

       (?<p1>(?i)rah)\s+(?P=p1)

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

       (a|(bc))\2

     always fails if it starts to match  "a"  rather  than  "bc".
     Because  there  may  be many capturing parentheses in a pat-
     tern, all digits following the backslash are taken  as  part
     of a potential back reference number. If the pattern contin-
     ues 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 an empty comment can
     be used.

     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 subpatterns. For example, the pat-
     tern

       (a|b\1)+

     matches any number of "a"s and also "aba", "ababbaa" etc. At
     each iteration of the subpattern, the back reference matches
     the character string corresponding to  the  previous  itera-
     tion.  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.


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 com-
     plicated 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. 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 semicolon 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 this 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.

     Lookbehind assertions start with (?<=  for  positive  asser-
     tions 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 several 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  dif-
     ferent length strings are permitted only at the top level of
     a lookbehind assertion. This is an extension  compared  with
     Perl  (at  least  for  5.8),  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 if rewrit-
     ten to use two top-level branches:

       (?<=abc|abde)

     The implementation of lookbehind  assertions  is,  for  each
     alternative,  to  temporarily move the current position back
     by the fixed width and then  try  to  match.  If  there  are
     insufficient  characters  before  the  current position, the
     match is deemed to fail.

     PCRE does not allow the \C escape (which  matches  a  single
     byte  in  UTF-8  mode)  to  appear in lookbehind assertions,
     because it makes it impossible to calculate  the  length  of
     the lookbehind.

     Atomic groups can be used  in  conjunction  with  lookbehind
     assertions  to  specify efficient matching at the end of the
     subject string. 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 back-
     tracks 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)

     or, equivalently,

       ^.*+(?<=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.

     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" preceded by six
     characters, the first of which are digits and the last three
     of  which  are  not  "999".  For  example,  it doesn't match
     "123abcfoo". 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 which matches  "foo"  preceded  by  three
     digits and any three characters that are not "999".

     Assertion subpatterns are not capturing subpatterns, and may
     not  be  repeated,  because  it makes no sense to assert the
     same thing several times. If any kind of assertion  contains
     capturing  subpatterns  within it, these are counted for the
     purposes of numbering the capturing subpatterns in the whole
     pattern.   However,  substring capturing is carried out only
     for positive assertions, because it does not make sense  for
     negative assertions.


CONDITIONAL SUBPATTERNS

     It is possible to cause the matching process to obey a  sub-
     pattern  conditionally  or to choose between two alternative
     subpatterns, depending on the result  of  an  assertion,  or
     whether  a previous capturing subpattern matched or not. 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; oth-
     erwise  the  no-pattern  (if  present) is used. If there are
     more than two alternatives in the subpattern, a compile-time
     error occurs.

     There are three kinds of condition. If the text between  the
     parentheses  consists of a sequence of digits, the condition
     is satisfied if the capturing subpattern of that number  has
     previously  matched.  The  number must be greater than zero.
     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 second part matches one  or  more  characters
     that  are  not  parentheses. The third part is a conditional
     subpattern that tests whether the first set  of  parentheses
     matched  or  not.  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. Otherwise, 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 the condition is the string (R), it  is  satisfied  if  a
     recursive  call  to the pattern or subpattern has been made.
     At "top level", the condition is  false.   This  is  a  PCRE
     extension.  Recursive  patterns  are  described  in the next
     section.

     If the condition is not a sequence of digits or (R), it must
     be  an assertion.  This may be a positive or negative looka-
     head 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

     The sequence (?# marks the start of a comment which  contin-
     ues  up  to the next closing parenthesis. Nested parentheses
     are not permitted. The characters that  make  up  a  comment
     play no part in the pattern matching at all.

     If the PCRE_EXTENDED option is set, an unescaped # character
     outside  a character class introduces a comment that contin-
     ues up to the next newline character in the pattern.


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. Perl has pro-
     vided  an  experimental 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
     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  some special syntax for
     recursion of the entire pattern,  and  also  for  individual
     subpattern recursion.

     The special item that consists of (? followed  by  a  number
     greater  than  zero and a closing parenthesis is a recursive
     call of the subpattern of the given number, provided that it
     occurs inside that subpattern. (If not, it is a "subroutine"
     call, which is described in the next section.)  The  special
     item  (?R) is a recursive call of the entire regular expres-
     sion.

     For example, 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  parenthesized  substring).  Finally
     there is a closing parenthesis.

     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.   It  may  be  more  convenient  to  use  named
     parentheses instead. For this, PCRE uses (?P>name), which is
     an  extension  to the Python syntax that PCRE uses for named
     parentheses (Perl does not provide  named  parentheses).  We
     could rewrite the above example as follows:

       (?<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )

     This particular example pattern  contains  nested  unlimited
     repeats,  and  so  the  use  of atomic grouping for matching
     strings of non-parentheses is important  when  applying  the
     pattern to strings that do not match. For example, when this
     pattern is applied to

       (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

     it yields "no match" quickly. However, if atomic grouping 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 set for any capturing sub-
     patterns are those from the outermost level of the recursion
     at which the subpattern value is set.  If you want to obtain
     intermediate  values,  a  callout  function can be used (see
     below and the pcrecallout  documentation).  If  the  pattern
     above is matched against

       (ab(cd)ef)

     the value for the capturing parentheses is  "ef",  which  is
     the  last  value  taken  on  at the top level. If additional
     parentheses are added, giving

       \( ( ( (?>[^()]+) | (?R) )* ) \)
          ^                        ^
          ^                        ^

     the string they capture is "ab(cd)ef", the contents  of  the
     top  level  parentheses. 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 brackets, allowing for arbitrary nesting. Only
     digits are allowed in nested brackets (that is, when recurs-
     ing), whereas any characters  are  permitted  at  the  outer
     level.

       < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

     In this pattern, (?(R) is the start of a conditional subpat-
     tern,  with two different alternatives for the recursive and
     non-recursive cases. The (?R) item is the  actual  recursive
     call.


SUBPATTERNS AS SUBROUTINES

     If the syntax for a recursive subpattern  reference  (either
     by  number  or  by  name) is used outside the parentheses to
     which it refers, it operates like a subroutine in a program-
     ming  language. An earlier example pointed out that the pat-
     tern

       (sens|respons)e and \1ibility

     matches "sense and sensibility" and "response and  responsi-
     bility",  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. Such references must, however, follow
     the subpattern to which they refer.


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 substrings 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.  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)9abc(?C2)def

     During matching, when PCRE  reaches  a  callout  point  (and
     pcre_callout is set), the external function is called. It is
     provided with the number of the  callout,  and,  optionally,
     one  item  of  data  originally  supplied  by  the caller of
     pcre_exec(). The callout  function  may  cause  matching  to
     backtrack,  or to fail altogether. A complete description of
     the interface to the callout function is given in the  pcre-
     callout documentation.

Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
