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NAME | SYNOPSIS | DESCRIPTION | USING THE POSIX FUNCTIONS | COMPILING A PATTERN | MATCHING NEWLINE CHARACTERS | MATCHING A PATTERN | ERROR MESSAGES | MEMORY USAGE | AUTHOR | REVISION | COLOPHON |
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PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3)
PCRE2 - Perl-compatible regular expressions (revised API)
#include <pcre2posix.h>
int pcre2_regcomp(regex_t *preg, const char *pattern,
int cflags);
int pcre2_regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t pcre2_regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void pcre2_regfree(regex_t *preg);
This set of functions provides a POSIX-style API for the PCRE2
regular expression 8-bit library. There are no POSIX-style
wrappers for PCRE2's 16-bit and 32-bit libraries. See the pcre2api
documentation for a description of PCRE2's native API, which
contains much additional functionality.
IMPORTANT NOTE: The functions described here are NOT thread-safe,
and should not be used in multi-threaded applications. They are
also limited to processing subjects that are not bigger than 2GB.
Use the native API instead.
These functions are wrapper functions that ultimately call the
PCRE2 native API. Their prototypes are defined in the pcre2posix.h
header file, and they all have unique names starting with pcre2_.
However, the pcre2posix.h header also contains macro definitions
that convert the standard POSIX names such regcomp() into
pcre2_regcomp() etc. This means that a program can use the usual
POSIX names without running the risk of accidentally linking with
POSIX functions from a different library.
On Unix-like systems the PCRE2 POSIX library is called
libpcre2-posix, so can be accessed by adding -lpcre2-posix to the
command for linking an application. Because the POSIX functions
call the native ones, it is also necessary to add -lpcre2-8.
On Windows systems, if you are linking to a DLL version of the
library, it is recommended that PCRE2POSIX_SHARED is defined
before including the pcre2posix.h header, as it will allow for a
more efficient way to invoke the functions by adding the
__declspec(dllimport) decorator.
Although they were not defined as prototypes in pcre2posix.h,
releases 10.33 to 10.36 of the library contained functions with
the POSIX names regcomp() etc. These simply passed their arguments
to the PCRE2 functions. These functions were provided for
backwards compatibility with earlier versions of PCRE2, which had
only POSIX names. However, this has proved troublesome in
situations where a program links with several libraries, some of
which use PCRE2's POSIX interface while others use the real POSIX
functions. For this reason, the POSIX names have been removed
since release 10.37.
Calling the header file pcre2posix.h avoids any conflict with
other POSIX libraries. It can, of course, be renamed or aliased as
regex.h, which is the "correct" name, if there is no clash. 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.
Note that these functions are just POSIX-style wrappers for
PCRE2's native API. They do not give POSIX regular expression
behaviour, and they are not thread-safe or even POSIX compatible.
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 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 or
to add BSD or GNU functionality.
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 descriptions below use the actual names of the functions, but,
as described above, the standard POSIX names (without the pcre2_
prefix) may also be used.
The function pcre2_regcomp() is called to compile a pattern into
an internal form. By default, the pattern is a C string terminated
by a binary zero (but see REG_PEND below). 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. It is also used
for input when REG_PEND is set. The regex_t structure used by
pcre2_regcomp() is defined in pcre2posix.h and is not the same as
the structure used by other libraries that provide POSIX-style
matching.
The argument cflags is either zero, or contains one or more of the
bits defined by the following macros:
REG_DOTALL
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 POSIX standard.
REG_ICASE
The PCRE2_CASELESS option is set when the regular expression is
passed for compilation to the native function.
REG_NEWLINE
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).
REG_NOSPEC
The PCRE2_LITERAL option is set when the regular expression is
passed for compilation to the native function. This disables all
meta characters in the pattern, causing it to be treated as a
literal string. The only other options that are allowed with
REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and REG_UTF. Note
that REG_NOSPEC is not part of the POSIX standard.
REG_NOSUB
When a pattern that is compiled with this flag is passed to
pcre2_regexec() for matching, the nmatch and pmatch arguments are
ignored, and no captured strings are returned. Versions of the
PCRE2 library prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE
compile option, but this no longer happens because it disables the
use of backreferences.
REG_PEND
If this option is set, the reg_endp field in the preg structure
(which has the type const char *) must be set to point to the
character beyond the end of the pattern before calling
pcre2_regcomp(). The pattern itself may now contain binary zeros,
which are treated as data characters. Without REG_PEND, a binary
zero terminates the pattern and the re_endp field is ignored. This
is a GNU extension to the POSIX standard and should be used with
caution in software intended to be portable to other systems.
REG_UCP
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 matching \d, \w, etc., instead of just
recognizing ASCII values. Note that REG_UCP is not part of the
POSIX standard.
REG_UNGREEDY
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 POSIX standard.
REG_UTF
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 that 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 pcre2_regcomp() is zero on success, and non-zero
otherwise. The preg structure is filled in on success, and one
other member of the structure (as well as re_endp) 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 pcre2_regcomp() is non-zero, you must not
attempt to use the contents of the preg structure. If, for
example, you pass it to pcre2_regexec(), the result is undefined
and your program is likely to crash.
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, PCRE2's behaviour is the same as Perl's,
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 PCRE2's pcre2_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.
The function pcre2_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 be:
REG_NOTBOL
The PCRE2_NOTBOL option is set when calling the underlying PCRE2
matching function.
REG_NOTEMPTY
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.
REG_NOTEOL
The PCRE2_NOTEOL option is set when calling the underlying PCRE2
matching function.
REG_STARTEND
When this option is set, the subject string starts at string +
pmatch[0].rm_so and ends at string + pmatch[0].rm_eo, which should
point to the first character beyond the string. There may be
binary zeros within the subject string, and indeed, using
REG_STARTEND is the only way to pass a subject string that
contains a binary zero.
Whatever the value of pmatch[0].rm_so, the offsets of the matched
string and any captured substrings are still given relative to the
start of string itself. (Before PCRE2 release 10.30 these were
given relative to string + pmatch[0].rm_so, but this differs from
other implementations.)
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 and length 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 pcre2_regexec() are ignored (except possibly as input for
REG_STARTEND).
The value of nmatch may be zero, and the value pmatch may be NULL
(unless REG_STARTEND is set); in both these cases 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.
regmatch_t as well as the regoff_t typedef it uses are defined in
pcre2posix.h and are not warranted to have the same size or layout
as other similarly named types from other libraries that provide
POSIX-style matching.
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 pcre2_regerror() function maps a non-zero errorcode from
either pcre2_regcomp() or pcre2_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 pcre2_regfree()
frees all such memory, after which preg may no longer be used as a
compiled expression.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 27 November 2024
Copyright (c) 1997-2024 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2025-08-11. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for the
page, or you have corrections or improvements to the information
in this COLOPHON (which is not part of the original manual page),
send a mail to man-pages@man7.org
PCRE2 10.46-DEV 27 November 2024 PCRE2POSIX(3)
Pages that refer to this page: pcre2api(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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