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### POSIX Compatible C API's

| Note
this is an abridged reference to the POSIX API functions, these are provided for compatibility with other libraries, rather than as an API to be used in new code (unless you need access from a language other than C++). This version of these functions should also happily coexist with other versions, as the names used are macros that expand to the actual function names.

    #include <boost/cregex.hpp>

or:

    #include <boost/regex.h>

The following functions are available for users who need a POSIX compatible C library, they are available in both Unicode and narrow character versions, the standard POSIX API names are macros that expand to one version or the other depending upon whether UNICODE is defined or not.

| Important
Note that all the symbols defined here are enclosed inside namespace `boost` when used in C++ programs, unless you use `#include <boost/regex.h>` instead - in which case the symbols are still defined in namespace boost, but are made available in the global namespace as well.

The functions are defined as:

    extern "C" {

    struct regex_tA;
    struct regex_tW;

    int regcompA(regex_tA*, const char*, int);
    unsigned int regerrorA(int, const regex_tA*, char*, unsigned int);
    int regexecA(const regex_tA*, const char*, unsigned int, regmatch_t*, int);
    void regfreeA(regex_tA*);

    int regcompW(regex_tW*, const wchar_t*, int);
    unsigned int regerrorW(int, const regex_tW*, wchar_t*, unsigned int);
    int regexecW(const regex_tW*, const wchar_t*, unsigned int, regmatch_t*, int);
    void regfreeW(regex_tW*);

    #ifdef UNICODE
    #define regcomp regcompW
    #define regerror regerrorW
    #define regexec regexecW
    #define regfree regfreeW
    #define regex_t regex_tW
    #else
    #define regcomp regcompA
    #define regerror regerrorA
    #define regexec regexecA
    #define regfree regfreeA
    #define regex_t regex_tA
    #endif
    }

All the functions operate on structure regex_t, which exposes two public members:

Member | Meaning
--- | ---
`unsigned int re_nsub` | This is filled in by `regcomp` and indicates the number of sub-expressions contained in the regular expression.
`const TCHAR* re_endp` | Points to the end of the expression to compile when the flag REG_PEND is set.

| Note
`regex_t` is actually a `#define` - it is either `regex_tA` or `regex_tW` depending upon whether `UNICODE` is defined or not, `TCHAR` is either `char` or `wchar_t` again depending upon the macro `UNICODE`.

##### regcomp

`regcomp` takes a pointer to a `regex_t`, a pointer to the expression to compile and a flags parameter which can be a combination of:

Flag | Meaning
--- | ---
REG_EXTENDED | Compiles modern regular expressions. Equivalent to `regbase::char_classes | regbase::intervals | regbase::bk_refs`.
REG_BASIC | Compiles basic (obsolete) regular expression syntax. Equivalent to `regbase::char_classes | regbase::intervals | regbase::limited_ops | regbase::bk_braces | regbase::bk_parens | regbase::bk_refs`.
REG_NOSPEC | All characters are ordinary, the expression is a literal string.
REG_ICASE | Compiles for matching that ignores character case.
REG_NOSUB | Has no effect in this library.
REG_NEWLINE | When this flag is set a dot does not match the newline character.
REG_PEND | When this flag is set the re_endp parameter of the regex_t structure must point to the end of the regular expression to compile.
REG_NOCOLLATE | When this flag is set then locale dependent collation for character ranges is turned off.
REG_ESCAPE_IN_LISTS | When this flag is set, then escape sequences are permitted in bracket expressions (character sets).
REG_NEWLINE_ALT | When this flag is set then the newline character is equivalent to the alternation operator |.
REG_PERL | Compiles Perl like regular expressions.
REG_AWK | A shortcut for awk-like behavior: `REG_EXTENDED | REG_ESCAPE_IN_LISTS`
REG_GREP | A shortcut for grep like behavior: `REG_BASIC | REG_NEWLINE_ALT`
REG_EGREP | A shortcut for egrep like behavior: `REG_EXTENDED | REG_NEWLINE_ALT`

##### regerror

regerror takes the following parameters, it maps an error code to a human readable string:

Parameter | Meaning
--- | ---
int code | The error code.
const regex_t* e | The regular expression (can be null).
char* buf | The buffer to fill in with the error message.
unsigned int buf_size | The length of buf.

If the error code is OR'ed with REG_ITOA then the message that results is the printable name of the code rather than a message, for example "REG_BADPAT". If the code is REG_ATIO then e must not be null and e->re_pend must point to the printable name of an error code, the return value is then the value of the error code. For any other value of code, the return value is the number of characters in the error message, if the return value is greater than or equal to buf_size then regerror will have to be called again with a larger buffer.

##### regexec

regexec finds the first occurrence of expression e within string buf. If len is non-zero then *m is filled in with what matched the regular expression, m[0] contains what matched the whole string, m[1] the first sub-expression etc, see regmatch_t in the header file declaration for more details. The eflags parameter can be a combination of:

Flag | Meaning
--- | ---
REG_NOTBOL | Parameter buf does not represent the start of a line.
REG_NOTEOL | Parameter buf does not terminate at the end of a line.
REG_STARTEND | The string searched starts at buf + pmatch[0].rm_so and ends at buf + pmatch[0].rm_eo.

##### regfree

`regfree` frees all the memory that was allocated by regcomp.