pcre2/doc/pcre2posix.3

.TH PCRE2POSIX 3 "05 June 2017" "PCRE2 10.30"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "SYNOPSIS"
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.B #include <pcre2posix.h>
.PP
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.B int regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
.B "     int \fIcflags\fP);"
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.B int regexec(const regex_t *\fIpreg\fP, const char *\fIstring\fP,
.B "     size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);"
.sp
.B "size_t regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,"
.B "     char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);"
.sp
.B void regfree(regex_t *\fIpreg\fP);
.fi
.
.SH DESCRIPTION
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This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. See the
.\" HREF
\fBpcre2api\fP
.\"
documentation for a description of PCRE2's native API, which contains much
additional functionality. There are no POSIX-style wrappers for PCRE2's 16-bit
and 32-bit libraries.
.P
The functions described here are just wrapper functions that ultimately call
the PCRE2 native API. Their prototypes are defined in the \fBpcre2posix.h\fP
header file, and on Unix systems the library itself is called
\fBlibpcre2-posix.a\fP, so can be accessed by adding \fB-lpcre2-posix\fP to the
command for linking an application that uses them. Because the POSIX functions
call the native ones, it is also necessary to add \fB-lpcre2-8\fP.
.P
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.
.P
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.
.P
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.
.P
The header for these functions is supplied as \fBpcre2posix.h\fP to avoid any
potential clash with other POSIX libraries. It can, of course, be renamed or
aliased as \fBregex.h\fP, which is the "correct" name. It provides two
structure types, \fIregex_t\fP for compiled internal forms, and
\fIregmatch_t\fP for returning captured substrings. It also defines some
constants whose names start with "REG_"; these are used for setting options and
identifying error codes.
.
.
.SH "COMPILING A PATTERN"
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The function \fBregcomp()\fP 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 \fIpreg\fP argument is a pointer to a
\fBregex_t\fP 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.)
.P
The argument \fIcflags\fP is either zero, or contains one or more of the bits
defined by the following macros:
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  REG_DOTALL
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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.
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  REG_ICASE
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The PCRE2_CASELESS option is set when the regular expression is passed for
compilation to the native function.
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  REG_NEWLINE
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The PCRE2_MULTILINE option is set when the regular expression is passed for
compilation to the native function. Note that this does \fInot\fP mimic the
defined POSIX behaviour for REG_NEWLINE (see the following section).
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  REG_NOSUB
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When a pattern that is compiled with this flag is passed to \fBregexec()\fP for
matching, the \fInmatch\fP and \fIpmatch\fP arguments are ignored, and no
captured 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 back references.
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  REG_PEND
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If this option is set, the \fBreg_endp\fP field in the \fIpreg\fP structure 
(which has the type const char *) must be set to point to the character beyond 
the end of the pattern before calling \fBregcomp()\fP. The pattern itself may
now contain binary zeroes, which are treated as data characters. Without
REG_PEND, a binary zero terminates the pattern and the \fBre_endp\fP 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.
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  REG_UCP
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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 \ed, \ew, etc., instead of just recognizing ASCII values. Note
that REG_UCP is not part of the POSIX standard.
.sp
  REG_UNGREEDY
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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.
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  REG_UTF
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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.
.P
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
\fIsome\fP 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).
.P
The yield of \fBregcomp()\fP is zero on success, and non-zero otherwise. The
\fIpreg\fP structure is filled in on success, and one other member of the
structure (as well as \fIre_endp\fP) is public: \fIre_nsub\fP contains the
number of capturing subpatterns in the regular expression. Various error codes
are defined in the header file.
.P
NOTE: If the yield of \fBregcomp()\fP is non-zero, you must not attempt to
use the contents of the \fIpreg\fP structure. If, for example, you pass it to
\fBregexec()\fP, the result is undefined and your program is likely to crash.
.
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.SH "MATCHING NEWLINE CHARACTERS"
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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:
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                          Default   Change with
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  . matches newline          no     PCRE2_DOTALL
  newline matches [^a]       yes    not changeable
  $ matches \en at end        yes    PCRE2_DOLLAR_ENDONLY
  $ matches \en in middle     no     PCRE2_MULTILINE
  ^ matches \en in middle     no     PCRE2_MULTILINE
.sp
This is the equivalent table for a POSIX-compatible pattern matcher:
.sp
                          Default   Change with
.sp
  . matches newline          yes    REG_NEWLINE
  newline matches [^a]       yes    REG_NEWLINE
  $ matches \en at end        no     REG_NEWLINE
  $ matches \en in middle     no     REG_NEWLINE
  ^ matches \en in middle     no     REG_NEWLINE
.sp
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].
.P
Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and
PCRE2_DOLLAR_ENDONLY when calling \fBpcre2_compile()\fP 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 \fBregcomp()\fP function
causes PCRE2_MULTILINE to be passed to \fBpcre2_compile()\fP, and REG_DOTALL
passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
.
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.SH "MATCHING A PATTERN"
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The function \fBregexec()\fP is called to match a compiled pattern \fIpreg\fP
against a given \fIstring\fP, which is by default terminated by a zero byte
(but see REG_STARTEND below), subject to the options in \fIeflags\fP. These can
be:
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  REG_NOTBOL
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The PCRE2_NOTBOL option is set when calling the underlying PCRE2 matching
function.
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  REG_NOTEMPTY
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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.
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  REG_NOTEOL
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The PCRE2_NOTEOL option is set when calling the underlying PCRE2 matching
function.
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  REG_STARTEND
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When this option is set, the subject string is starts at \fIstring\fP +
\fIpmatch[0].rm_so\fP and ends at \fIstring\fP + \fIpmatch[0].rm_eo\fP, which
should point to the first character beyond the string. There may be binary 
zeroes within the subject string, and indeed, using REG_STARTEND is the only 
way to pass a subject string that contains a binary zero.
.P
Whatever the value of \fIpmatch[0].rm_so\fP, the offsets of the matched string
and any captured substrings are still given relative to the start of
\fIstring\fP itself. (Before PCRE2 release 10.30 these were given relative to
\fIstring\fP + \fIpmatch[0].rm_so\fP, but this differs from other
implementations.)
.P
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 \fIrm_so\fP 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 \fIpmatch\fP as NULL
are mutually exclusive; the error REG_INVARG is returned.
.P
If the pattern was compiled with the REG_NOSUB flag, no data about any matched
strings is returned. The \fInmatch\fP and \fIpmatch\fP arguments of
\fBregexec()\fP are ignored (except possibly as input for REG_STARTEND).
.P
The value of \fInmatch\fP may be zero, and the value \fIpmatch\fP may be NULL
(unless REG_STARTEND is set); in both these cases no data about any matched
strings is returned.
.P
Otherwise, the portion of the string that was matched, and also any captured
substrings, are returned via the \fIpmatch\fP argument, which points to an
array of \fInmatch\fP structures of type \fIregmatch_t\fP, containing the
members \fIrm_so\fP and \fIrm_eo\fP. 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 \fIstring\fP 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.
.P
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.
.
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.SH "ERROR MESSAGES"
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The \fBregerror()\fP function maps a non-zero errorcode from either
\fBregcomp()\fP or \fBregexec()\fP to a printable message. If \fIpreg\fP is not
NULL, the error should have arisen from the use of that structure. A message
terminated by a binary zero is placed in \fIerrbuf\fP. If the buffer is too
short, only the first \fIerrbuf_size\fP - 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
\fIerrbuf_size\fP if the message was truncated.
.
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.SH MEMORY USAGE
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Compiling a regular expression causes memory to be allocated and associated
with the \fIpreg\fP structure. The function \fBregfree()\fP frees all such
memory, after which \fIpreg\fP may no longer be used as a compiled expression.
.
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.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge, England.
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.SH REVISION
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Last updated: 05 June 2017
Copyright (c) 1997-2017 University of Cambridge.
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