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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 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 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

								1

PCREPOSIX(3)					     PCREPOSIX(3)

       regmatch_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_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.

								2

PCREPOSIX(3)					     PCREPOSIX(3)

       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

								3

PCREPOSIX(3)					     PCREPOSIX(3)

       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.

								4

PCREPOSIX(3)					     PCREPOSIX(3)

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: 02 September 2009
       Copyright (c) 1997-2009 University of Cambridge.

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