Manual Reference Pages - PCRE2POSIX (3)
PCRE2 - Perl-compatible regular expressions (revised API)
int regcomp(regex_t *preg, const char *pattern,
int regexec(const 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 for the PCRE2 regular
expression 8-bit library. See the
documentation for a description of PCRE2s native API, which contains much
additional functionality. There is no POSIX-style wrapper for PCRE2s 16-bit
and 32-bit libraries.
The functions described here are just wrapper functions that ultimately call
the PCRE2 native API. Their prototypes are defined in the pcre2posix.h
header file, and on Unix systems the library itself is called
libpcre2-posix.a, so can be accessed by adding -lpcre2-posix to the
command for linking an application that uses them. Because the POSIX functions
call the native ones, it is also necessary to add -lpcre2-8.
Those POSIX option bits that can reasonably be mapped to PCRE2 native options
have been implemented. 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 PCRE2 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
PCRE2-specific features via the POSIX calling interface.
When PCRE2 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 PCRE2 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-unit encoding
domains it is probably even less compatible.
The header for these functions is supplied as pcre2posix.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 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 PCRE2_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
The PCRE2_CASELESS option is set when the regular expression is passed for
compilation to the native function.
The PCRE2_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 PCRE2_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
The PCRE2_UCP option is set when the regular expression is passed for
compilation to the native function. This causes PCRE2 to use Unicode properties
when matchine \d, \w, etc., instead of just recognizing ASCII values. Note
that REG_UCP is not part of the POSIX standard.
The PCRE2_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
The PCRE2_UTF 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_UTF
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 PCRE2 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 PCRE2_MULTILINE has only
some of the effects specified for REG_NEWLINE. It does not affect the way
newlines are matched by the dot metacharacter (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 PCRE2 to obey POSIX semantics, but then PCRE2 was
never intended to be a POSIX engine. The following table lists the different
possibilities for matching newline characters in Perl and PCRE2:
Default Change with
. matches newline no PCRE2_DOTALL
newline matches [^a] yes not changeable
$ matches \n at end yes PCRE2_DOLLAR_ENDONLY
$ matches \n in middle no PCRE2_MULTILINE
^ matches \n in middle no PCRE2_MULTILINE
This is the equivalent table for a POSIX-compatible pattern matcher:
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
This behaviour is not what happens when PCRE2 is called via its POSIX
API. By default, PCRE2s behaviour is the same as Perls, except that there is
no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there
is no way to stop newline from matching [^a].
Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and
PCRE2_DOLLAR_ENDONLY when calling pcre2_compile() directly, but there is
no way to make PCRE2 behave exactly as for the REG_NEWLINE action. When using
the POSIX API, passing REG_NEWLINE to PCRE2s regcomp() function
causes PCRE2_MULTILINE to be passed to pcre2_compile(), and REG_DOTALL
passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
MATCHING A PATTERN
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
The PCRE2_NOTBOL option is set when calling the underlying PCRE2 matching
The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 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 PCRE2_NOTEOL option is set when calling the underlying PCRE2 matching
The string is considered to start at string + pmatch.rm_so and
to have a terminating NUL located at string + pmatch.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. Setting REG_STARTEND and passing pmatch as NULL are
mutually exclusive; the error REG_INVARG is returned.
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 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 byte 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. If the buffer is too
short, only the first errbuf_size - 1 characters of the error message are
used. The yield of the function is the size of buffer needed to hold the whole
message, including the terminating zero. This value is greater than
errbuf_size if the message was truncated.
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.
University Computing Service
Last updated: 29 November 2015
Copyright (c) 1997-2015 University of Cambridge.
|PCRE2 10.21 ||PCRE2POSIX (3) ||29 November 2015 |
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