pcreposix
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 ulti-
mately 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 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 pro-
vides 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 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
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_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 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 struc-
ture is public: re_nsub contains the number of capturing subpatterns
in the regular expression. Various error codes are defined in the
header file.
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 charac-
ters 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 equiv-
alent 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 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 match-
ing function.
REG_NOTEOL
The PCRE_NOTEOL option is set when calling the underlying PCRE match-
ing 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; sub-
sequent 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 reg-
comp() 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 function 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: 06 March 2007
Copyright (c) 1997-2007 University of Cambridge.
PCREPOSIX(3)
