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pcre2posix(3)
PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2posix.h>
int regcomp(regex_t *preg, const char *pattern,
int cflags);
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);
DESCRIPTION
This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. See the pcre2api documentation for a descrip‐
tion of PCRE2's native API, which contains much additional functional‐
ity. There are no POSIX-style wrappers for PCRE2's 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 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 expres‐
sions 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 reg‐
match_t for returning captured substrings. It also defines some con‐
stants 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. 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 com‐
piled regular expression. (It is also used for input when REG_PEND is
set.)
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 sec‐
tion).
REG_NOSPEC
The PCRE2_LITERAL option is set when the regular expression is passed
for compilation to the native function. This disables all meta charac‐
ters 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 regexec()
for matching, the nmatch and pmatch arguments are ignored, and no cap‐
tured strings are returned. Versions of the PCRE 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 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 matchine \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 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 metacharac‐
ter (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 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 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, 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 reg‐
comp() function causes PCRE2_MULTILINE to be passed to pcre2_compile(),
and REG_DOTALL passes PCRE2_DOTALL. There is no way to pass PCRE2_DOL‐
LAR_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 be:
REG_NOTBOL
The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match‐
ing 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 behav‐
iour in some situations.
REG_NOTEOL
The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match‐
ing 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
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.
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. 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.
MEMORY USAGE
Compiling a regular expression causes memory to be allocated and asso‐
ciated with the preg structure. The function regfree() frees all such
memory, after which preg may no longer be used as a compiled expres‐
sion.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 15 June 2017
Copyright (c) 1997-2017 University of Cambridge.
PCRE2 10.30 15 June 2017 PCRE2POSIX(3)