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NAME
pcreposix - POSIX API for Perl-compatible regular expres-
sions.
SYNOPSIS
#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 pcre documentation for a
description of the native API, which contains additional
functionality.
The functions described here are just wrapper functions that
ultimately call the 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
which 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 rea-
sonably mapped to PCRE native options. In addition, the
options REG_EXTENDED and REG_NOSUB are defined with the
value zero. They have no effect, but since programs that are
written to the POSIX interface often use them, 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, sub-
ject to the setting of various PCRE options, as described
below.
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 terminated by a
binary zero, and is passed in the argument pattern. The preg
argument is a pointer to a regex_t structure which is used
as a base for storing information about the compiled expres-
sion.
The argument cflags is either zero, or contains one or more
of the bits defined by the following macros:
REG_ICASE
The PCRE_CASELESS option is set when the expression is
passed for compilation to the native function.
REG_NEWLINE
The PCRE_MULTILINE option is set when the expression is
passed for compilation to the native function.
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 a
negative class such as [^a] (they are).
The yield of regcomp() is zero on success, and non-zero oth-
erwise. The preg structure is filled in on success, and one
member of the structure is publicized: re_nsub contains the
number of capturing subpatterns in the regular expression.
Various error codes are defined in the header file.
MATCHING A PATTERN
The function regexec() is called to match a pre-compiled
pattern preg against a given string, which is terminated by
a zero byte, 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_NOTEOL
The PCRE_NOTEOL option is set when calling the underlying
PCRE matching function.
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.
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.
STORAGE
Compiling a regular expression causes memory to be allocated
and associated with the preg structure. The function reg-
free() frees all such memory, after which preg may no longer
be used as a compiled expression.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Copyright (c) 1997-2000 University of Cambridge.
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