Section: C Library Functions (3)
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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 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 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 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:


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 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.


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 number 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_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.  


The function regexec() is called to match a compiled pattern preg against a given string, which is terminated by a zero byte, subject to the options in eflags. These can be:


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


The PCRE_NOTEOL option is set when calling the underlying PCRE matching function. 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 3QG, England. Last updated: 16 January 2006
Copyright (c) 1997-2006 University of Cambridge.




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