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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 to the PCRE regular expression package. See the pcreapi docu-
       mentation for a description of PCRE's native API, which contains much additional functionality.

       The functions described here are just wrapper functions that ultimately call the PCRE native API. Their  proto-
       types 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 addi-
       tion, 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 seman-
       tics of the regular expressions 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 regmatch_t for returning captured substrings. It also
       defines  some constants 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  termi-
       nated  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 section).

         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 pattern that is compiled with this flag is passed to regexec() for matching, 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 com-
       piled 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 equivalent 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 matching function. Note that REG_NOTEMPTY is
       not part of the POSIX standard. However, setting this option can give more POSIX-like behaviour in some  situa-
       tions.

         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 captured substrings, are returned via the
       pmatch argument, which points to an array of nmatch structures of type regmatch_t, containing the members rm_so
       and  rm_eo. These contain the offset to the first character of each substring and the offset to the first char-
       acter after the end of each substring, respectively. The 0th element of the vector relates to the  entire  por-
       tion of string that was matched; subsequent elements relate to the capturing subpatterns of the regular expres-
       sion. 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 function is the size of buffer needed to hold the whole message.

MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and associated with the preg structure. The  func-
       tion regfree() frees all such memory, after which preg may no longer be used as a compiled expression.

AUTHOR

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

REVISION

       Last updated: 16 May 2010
       Copyright (c) 1997-2010 University of Cambridge.



                                                                  PCREPOSIX(3)