Provided by: libpcre2-dev_10.21-1_amd64 
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 description of PCRE2's native API, which contains much additional
functionality. There is no POSIX-style wrapper 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 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:
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_NOSUB
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 are returned.
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
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, 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 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 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
The string is considered to start at string + pmatch[0].rm_so and to have a terminating NUL located at
string + pmatch[0].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.
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 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, England.
REVISION
Last updated: 29 November 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE2 10.21 29 November 2015 PCRE2POSIX(3)