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X11R7.4 - man page for pcreposix (x11r4 section 3)

PCREPOSIX(3)			     Library Functions Manual			     PCREPOSIX(3)

       PCRE - Perl-compatible regular expressions.


       #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);


       This  set  of functions provides a POSIX-style API to the PCRE regular expression package.
       See the pcreapi documentation 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 prototypes are defined in the pcreposix.h header file, and on Unix  sys-
       tems 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.

       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 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.


       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 informa-
       tion about the compiled regular expression.

       The argument cflags is either zero, or contains one or more of the  bits  defined  by  the
       following macros:


       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.


       The PCRE_CASELESS option is set when the regular expression is passed for  compilation  to
       the native function.


       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).


       The  PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed for compila-
       tion 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.


       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 aren't) 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 num-
       ber of capturing subpatterns in the regular expression. Various error codes are defined in
       the header file.


       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_DOL-
       LAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop newline from  matching

       The  default  POSIX  newline handling can be obtained by setting PCRE_DOTALL and PCRE_DOL-
       LAR_ENDONLY, but there is no way to make  PCRE  behave  exactly	as  for  the  REG_NEWLINE


       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:


       The PCRE_NOTBOL option is set when calling the underlying PCRE matching function.


       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 situations.


       The PCRE_NOTEOL option is set when calling the underlying PCRE matching function.


       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  loca-
       tion), 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.

       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 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.


       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.


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


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


       Last updated: 11 March 2009
       Copyright (c) 1997-2009 University of Cambridge.


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