269 lines
10 KiB
Groff
269 lines
10 KiB
Groff
.TH PCRE2POSIX 3 "20 October 2014" "PCRE2 10.00"
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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 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 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 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 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. 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. There is no POSIX-style wrapper for PCRE2's 16-bit
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and 32-bit libraries.
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.P
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The functions described here are just wrapper functions that ultimately call
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the PCRE2 native API. Their prototypes are defined in the \fBpcre2posix.h\fP
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header file, and on Unix systems the library itself is called
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\fBlibpcre2-posix.a\fP, so can be accessed by adding \fB-lpcre2-posix\fP to the
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command for linking an application that uses them. Because the POSIX functions
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call the native ones, it is also necessary to add \fB-lpcre2-8\fP.
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.P
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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 other options that are not defined by POSIX. These have
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been added at the request of users who want to make use of certain
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PCRE2-specific features via the POSIX calling interface.
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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 header for these functions is supplied as \fBpcre2posix.h\fP to avoid any
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potential clash with other POSIX libraries. It can, of course, be renamed or
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aliased as \fBregex.h\fP, which is the "correct" name. It provides two
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structure types, \fIregex_t\fP for compiled internal forms, and
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\fIregmatch_t\fP for returning captured substrings. It also defines some
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constants whose names start with "REG_"; these are used for setting options and
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identifying error codes.
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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 \fBregcomp()\fP is called to compile a pattern into an
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internal form. The pattern is a C string terminated by a binary zero, and
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is passed in the argument \fIpattern\fP. The \fIpreg\fP argument is a pointer
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to a \fBregex_t\fP structure that is used as a base for storing information
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about the compiled regular expression.
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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_NOSUB
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.sp
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The PCRE2_NO_AUTO_CAPTURE option is set when the regular expression is passed
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for compilation to the native function. In addition, when a pattern that is
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compiled with this flag is passed to \fBregexec()\fP for matching, the
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\fInmatch\fP and \fIpmatch\fP arguments are ignored, and no captured strings
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are returned.
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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 \fBregcomp()\fP is zero on success, and non-zero otherwise. The
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\fIpreg\fP structure is filled in on success, and one member of the structure
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is public: \fIre_nsub\fP contains the number of capturing subpatterns in
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the regular expression. Various error codes are defined in the header file.
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.P
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NOTE: If the yield of \fBregcomp()\fP is non-zero, you must not attempt to
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use the contents of the \fIpreg\fP structure. If, for example, you pass it to
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\fBregexec()\fP, the result is undefined and your program is likely to 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 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 POSIX:
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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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PCRE2's behaviour is the same as Perl's, except that there is no equivalent for
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PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there is no way to stop
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newline from matching [^a].
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.P
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The default POSIX newline handling can be obtained by setting PCRE2_DOTALL and
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PCRE2_DOLLAR_ENDONLY, but there is no way to make PCRE2 behave exactly as for
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the REG_NEWLINE action.
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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 \fBregexec()\fP is called to match a compiled pattern \fIpreg\fP
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against a given \fIstring\fP, which is by default terminated by a zero byte
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(but see REG_STARTEND below), subject to the options in \fIeflags\fP. These can
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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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The string is considered to start at \fIstring\fP + \fIpmatch[0].rm_so\fP and
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to have a terminating NUL located at \fIstring\fP + \fIpmatch[0].rm_eo\fP
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(there need not actually be a NUL at that location), regardless of the value of
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\fInmatch\fP. This is a BSD extension, compatible with but not specified by
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IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software
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intended to be portable to other systems. Note that a non-zero \fIrm_so\fP does
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not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not
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how it is matched.
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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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\fBregexec()\fP are ignored.
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.P
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If the value of \fInmatch\fP is zero, or if the value \fIpmatch\fP is NULL,
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no data about any matched 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 \fBregerror()\fP function maps a non-zero errorcode from either
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\fBregcomp()\fP or \fBregexec()\fP to a printable message. If \fIpreg\fP is not
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NULL, the error should have arisen from the use of that structure. A message
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terminated by a binary zero is placed in \fIerrbuf\fP. The length of the
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message, including the zero, is limited to \fIerrbuf_size\fP. The yield of the
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function is the size of buffer needed to hold the whole message.
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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 \fBregfree()\fP frees all such
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memory, after which \fIpreg\fP may no longer be used as a compiled 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: 20 October 2014
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Copyright (c) 1997-2014 University of Cambridge.
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.fi
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