327 lines
14 KiB
Groff
327 lines
14 KiB
Groff
.TH PCRE2POSIX 3 "30 January 2019" "PCRE2 10.33"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH "SYNOPSIS"
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.rs
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.sp
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.B #include <pcre2posix.h>
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.PP
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.nf
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.B int pcre2_regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
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.B " int \fIcflags\fP);"
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.sp
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.B int pcre2_regexec(const regex_t *\fIpreg\fP, const char *\fIstring\fP,
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.B " size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);"
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.sp
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.B "size_t pcre2_regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,"
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.B " char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);"
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.sp
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.B void pcre2_regfree(regex_t *\fIpreg\fP);
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.fi
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.
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.SH DESCRIPTION
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.rs
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.sp
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This set of functions provides a POSIX-style API for the PCRE2 regular
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expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 16-bit
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and 32-bit libraries. See the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation for a description of PCRE2's native API, which contains much
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additional functionality.
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.P
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The functions described here are wrapper functions that ultimately call the
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PCRE2 native API. Their prototypes are defined in the \fBpcre2posix.h\fP header
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file, and they all have unique names starting with \fBpcre2_\fP. However, the
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\fBpcre2posix.h\fP header also contains macro definitions that convert the
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standard POSIX names such \fBregcomp()\fP into \fBpcre2_regcomp()\fP etc. This
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means that a program can use the usual POSIX names without running the risk of
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accidentally linking with POSIX functions from a different library.
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.P
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On Unix-like systems the PCRE2 POSIX library is called \fBlibpcre2-posix\fP, so
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can be accessed by adding \fB-lpcre2-posix\fP to the command for linking an
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application. Because the POSIX functions call the native ones, it is also
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necessary to add \fB-lpcre2-8\fP.
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.P
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Although they are not defined as protypes in \fBpcre2posix.h\fP, the library
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does contain functions with the POSIX names \fBregcomp()\fP etc. These simply
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pass their arguments to the PCRE2 functions. These functions are provided for
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backwards compatibility with earlier versions of PCRE2, so that existing
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programs do not have to be recompiled.
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.P
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Calling the header file \fBpcre2posix.h\fP avoids any conflict with other POSIX
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libraries. It can, of course, be renamed or aliased as \fBregex.h\fP, which is
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the "correct" name, if there is no clash. It provides two structure types,
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\fIregex_t\fP for compiled internal forms, and \fIregmatch_t\fP for returning
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captured substrings. It also defines some constants whose names start with
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"REG_"; these are used for setting options and identifying error codes.
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.
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.
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.SH "USING THE POSIX FUNCTIONS"
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.rs
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.sp
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Those POSIX option bits that can reasonably be mapped to PCRE2 native options
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have been implemented. In addition, the option REG_EXTENDED is defined with the
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value zero. This has no effect, but since programs that are written to the
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POSIX interface often use it, this makes it easier to slot in PCRE2 as a
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replacement library. Other POSIX options are not even defined.
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.P
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There are also some options that are not defined by POSIX. These have been
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added at the request of users who want to make use of certain PCRE2-specific
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features via the POSIX calling interface or to add BSD or GNU functionality.
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.P
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When PCRE2 is called via these functions, it is only the API that is POSIX-like
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in style. The syntax and semantics of the regular expressions themselves are
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still those of Perl, subject to the setting of various PCRE2 options, as
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described below. "POSIX-like in style" means that the API approximates to the
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POSIX definition; it is not fully POSIX-compatible, and in multi-unit encoding
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domains it is probably even less compatible.
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.P
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The descriptions below use the actual names of the functions, but, as described
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above, the standard POSIX names (without the \fBpcre2_\fP prefix) may also be
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used.
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.
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.
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.SH "COMPILING A PATTERN"
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.rs
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.sp
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The function \fBpcre2_regcomp()\fP is called to compile a pattern into an
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internal form. By default, the pattern is a C string terminated by a binary
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zero (but see REG_PEND below). The \fIpreg\fP argument is a pointer to a
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\fBregex_t\fP structure that is used as a base for storing information about
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the compiled regular expression. (It is also used for input when REG_PEND is
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set.)
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.P
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The argument \fIcflags\fP is either zero, or contains one or more of the bits
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defined by the following macros:
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.sp
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REG_DOTALL
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.sp
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The PCRE2_DOTALL option is set when the regular expression is passed for
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compilation to the native function. Note that REG_DOTALL is not part of the
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POSIX standard.
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.sp
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REG_ICASE
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.sp
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The PCRE2_CASELESS option is set when the regular expression is passed for
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compilation to the native function.
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.sp
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REG_NEWLINE
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.sp
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The PCRE2_MULTILINE option is set when the regular expression is passed for
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compilation to the native function. Note that this does \fInot\fP mimic the
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defined POSIX behaviour for REG_NEWLINE (see the following section).
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.sp
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REG_NOSPEC
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.sp
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The PCRE2_LITERAL option is set when the regular expression is passed for
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compilation to the native function. This disables all meta characters in the
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pattern, causing it to be treated as a literal string. The only other options
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that are allowed with REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and
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REG_UTF. Note that REG_NOSPEC is not part of the POSIX standard.
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.sp
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REG_NOSUB
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.sp
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When a pattern that is compiled with this flag is passed to
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\fBpcre2_regexec()\fP for matching, the \fInmatch\fP and \fIpmatch\fP arguments
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are ignored, and no captured strings are returned. Versions of the PCRE library
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prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile option, but this
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no longer happens because it disables the use of backreferences.
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.sp
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REG_PEND
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.sp
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If this option is set, the \fBreg_endp\fP field in the \fIpreg\fP structure
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(which has the type const char *) must be set to point to the character beyond
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the end of the pattern before calling \fBpcre2_regcomp()\fP. The pattern itself
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may now contain binary zeros, which are treated as data characters. Without
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REG_PEND, a binary zero terminates the pattern and the \fBre_endp\fP field is
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ignored. This is a GNU extension to the POSIX standard and should be used with
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caution in software intended to be portable to other systems.
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.sp
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REG_UCP
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.sp
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The PCRE2_UCP option is set when the regular expression is passed for
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compilation to the native function. This causes PCRE2 to use Unicode properties
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when matchine \ed, \ew, etc., instead of just recognizing ASCII values. Note
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that REG_UCP is not part of the POSIX standard.
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.sp
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REG_UNGREEDY
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.sp
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The PCRE2_UNGREEDY option is set when the regular expression is passed for
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compilation to the native function. Note that REG_UNGREEDY is not part of the
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POSIX standard.
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.sp
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REG_UTF
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.sp
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The PCRE2_UTF option is set when the regular expression is passed for
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compilation to the native function. This causes the pattern itself and all data
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strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF
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is not part of the POSIX standard.
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.P
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In the absence of these flags, no options are passed to the native function.
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This means the the regex is compiled with PCRE2 default semantics. In
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particular, the way it handles newline characters in the subject string is the
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Perl way, not the POSIX way. Note that setting PCRE2_MULTILINE has only
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\fIsome\fP of the effects specified for REG_NEWLINE. It does not affect the way
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newlines are matched by the dot metacharacter (they are not) or by a negative
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class such as [^a] (they are).
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.P
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The yield of \fBpcre2_regcomp()\fP is zero on success, and non-zero otherwise.
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The \fIpreg\fP structure is filled in on success, and one other member of the
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structure (as well as \fIre_endp\fP) is public: \fIre_nsub\fP contains the
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number of capturing subpatterns in the regular expression. Various error codes
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are defined in the header file.
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.P
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NOTE: If the yield of \fBpcre2_regcomp()\fP is non-zero, you must not attempt
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to use the contents of the \fIpreg\fP structure. If, for example, you pass it
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to \fBpcre2_regexec()\fP, the result is undefined and your program is likely to
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crash.
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.
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.
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.SH "MATCHING NEWLINE CHARACTERS"
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.rs
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.sp
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This area is not simple, because POSIX and Perl take different views of things.
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It is not possible to get PCRE2 to obey POSIX semantics, but then PCRE2 was
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never intended to be a POSIX engine. The following table lists the different
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possibilities for matching newline characters in Perl and PCRE2:
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.sp
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Default Change with
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.sp
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. matches newline no PCRE2_DOTALL
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newline matches [^a] yes not changeable
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$ matches \en at end yes PCRE2_DOLLAR_ENDONLY
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$ matches \en in middle no PCRE2_MULTILINE
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^ matches \en in middle no PCRE2_MULTILINE
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.sp
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This is the equivalent table for a POSIX-compatible pattern matcher:
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.sp
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Default Change with
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.sp
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. matches newline yes REG_NEWLINE
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newline matches [^a] yes REG_NEWLINE
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$ matches \en at end no REG_NEWLINE
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$ matches \en in middle no REG_NEWLINE
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^ matches \en in middle no REG_NEWLINE
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.sp
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This behaviour is not what happens when PCRE2 is called via its POSIX
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API. By default, PCRE2's behaviour is the same as Perl's, except that there is
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no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there
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is no way to stop newline from matching [^a].
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.P
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Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and
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PCRE2_DOLLAR_ENDONLY when calling \fBpcre2_compile()\fP directly, but there is
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no way to make PCRE2 behave exactly as for the REG_NEWLINE action. When using
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the POSIX API, passing REG_NEWLINE to PCRE2's \fBpcre2_regcomp()\fP function
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causes PCRE2_MULTILINE to be passed to \fBpcre2_compile()\fP, and REG_DOTALL
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passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
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.
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.
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.SH "MATCHING A PATTERN"
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.rs
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.sp
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The function \fBpcre2_regexec()\fP is called to match a compiled pattern
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\fIpreg\fP against a given \fIstring\fP, which is by default terminated by a
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zero byte (but see REG_STARTEND below), subject to the options in \fIeflags\fP.
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These can be:
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.sp
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REG_NOTBOL
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.sp
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The PCRE2_NOTBOL option is set when calling the underlying PCRE2 matching
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function.
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.sp
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REG_NOTEMPTY
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.sp
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The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 matching
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function. Note that REG_NOTEMPTY is not part of the POSIX standard. However,
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setting this option can give more POSIX-like behaviour in some situations.
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.sp
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REG_NOTEOL
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.sp
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The PCRE2_NOTEOL option is set when calling the underlying PCRE2 matching
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function.
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.sp
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REG_STARTEND
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.sp
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When this option is set, the subject string starts at \fIstring\fP +
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\fIpmatch[0].rm_so\fP and ends at \fIstring\fP + \fIpmatch[0].rm_eo\fP, which
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should point to the first character beyond the string. There may be binary
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zeros within the subject string, and indeed, using REG_STARTEND is the only
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way to pass a subject string that contains a binary zero.
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.P
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Whatever the value of \fIpmatch[0].rm_so\fP, the offsets of the matched string
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and any captured substrings are still given relative to the start of
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\fIstring\fP itself. (Before PCRE2 release 10.30 these were given relative to
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\fIstring\fP + \fIpmatch[0].rm_so\fP, but this differs from other
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implementations.)
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.P
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This is a BSD extension, compatible with but not specified by IEEE Standard
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1003.2 (POSIX.2), and should be used with caution in software intended to be
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portable to other systems. Note that a non-zero \fIrm_so\fP does not imply
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REG_NOTBOL; REG_STARTEND affects only the location and length of the string,
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not how it is matched. Setting REG_STARTEND and passing \fIpmatch\fP as NULL
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are mutually exclusive; the error REG_INVARG is returned.
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.P
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If the pattern was compiled with the REG_NOSUB flag, no data about any matched
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strings is returned. The \fInmatch\fP and \fIpmatch\fP arguments of
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\fBpcre2_regexec()\fP are ignored (except possibly as input for REG_STARTEND).
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.P
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The value of \fInmatch\fP may be zero, and the value \fIpmatch\fP may be NULL
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(unless REG_STARTEND is set); in both these cases no data about any matched
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strings is returned.
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.P
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Otherwise, the portion of the string that was matched, and also any captured
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substrings, are returned via the \fIpmatch\fP argument, which points to an
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array of \fInmatch\fP structures of type \fIregmatch_t\fP, containing the
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members \fIrm_so\fP and \fIrm_eo\fP. These contain the byte offset to the first
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character of each substring and the offset to the first character after the end
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of each substring, respectively. The 0th element of the vector relates to the
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entire portion of \fIstring\fP that was matched; subsequent elements relate to
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the capturing subpatterns of the regular expression. Unused entries in the
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array have both structure members set to -1.
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.P
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A successful match yields a zero return; various error codes are defined in the
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header file, of which REG_NOMATCH is the "expected" failure code.
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.
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.
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.SH "ERROR MESSAGES"
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.rs
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.sp
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The \fBpcre2_regerror()\fP function maps a non-zero errorcode from either
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\fBpcre2_regcomp()\fP or \fBpcre2_regexec()\fP to a printable message. If
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\fIpreg\fP is not NULL, the error should have arisen from the use of that
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structure. A message terminated by a binary zero is placed in \fIerrbuf\fP. If
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the buffer is too short, only the first \fIerrbuf_size\fP - 1 characters of the
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error message are used. The yield of the function is the size of buffer needed
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to hold the whole message, including the terminating zero. This value is
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greater than \fIerrbuf_size\fP if the message was truncated.
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.
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.
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.SH MEMORY USAGE
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.rs
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.sp
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Compiling a regular expression causes memory to be allocated and associated
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with the \fIpreg\fP structure. The function \fBpcre2_regfree()\fP frees all
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such memory, after which \fIpreg\fP may no longer be used as a compiled
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expression.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 30 January 2019
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Copyright (c) 1997-2019 University of Cambridge.
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.fi
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