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9da5d7e90f
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227 lines
8.5 KiB
Groff
227 lines
8.5 KiB
Groff
.TH PCREPOSIX 3
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.SH NAME
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PCRE - Perl-compatible regular expressions.
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.SH "SYNOPSIS OF POSIX API"
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.rs
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.sp
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.B #include <pcreposix.h>
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.PP
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.SM
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.br
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.B int regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
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.ti +5n
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.B int \fIcflags\fP);
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.PP
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.br
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.B int regexec(regex_t *\fIpreg\fP, const char *\fIstring\fP,
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.ti +5n
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.B size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);
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.PP
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.br
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.B size_t regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,
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.ti +5n
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.B char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);
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.PP
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.br
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.B void regfree(regex_t *\fIpreg\fP);
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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 to the PCRE regular expression
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package. See the
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.\" HREF
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\fBpcreapi\fP
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.\"
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documentation for a description of PCRE'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 just wrapper functions that ultimately call
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the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fP
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header file, and on Unix systems the library itself is called
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\fBpcreposix.a\fP, so can be accessed by adding \fB-lpcreposix\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-lpcre\fP.
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.P
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I have implemented only those option bits that can be reasonably mapped to PCRE
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native options. In addition, the option REG_EXTENDED is defined with the value
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zero. This has no effect, but since programs that are written to the POSIX
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interface often use it, this makes it easier to slot in PCRE as a replacement
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library. Other POSIX options are not even defined.
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.P
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When PCRE 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 PCRE 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-byte 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 \fBpcreposix.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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.P
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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 PCRE_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 PCRE_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 PCRE_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 PCRE_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_UTF8
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.sp
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The PCRE_UTF8 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_UTF8
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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 PCRE 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 PCRE_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 . (they aren't) or by a negative class such as [^a]
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(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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.
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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 PCRE to obey POSIX semantics, but then PCRE was never
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intended to be a POSIX engine. The following table lists the different
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possibilities for matching newline characters in PCRE:
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.sp
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Default Change with
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.sp
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. matches newline no PCRE_DOTALL
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newline matches [^a] yes not changeable
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$ matches \en at end yes PCRE_DOLLARENDONLY
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$ matches \en in middle no PCRE_MULTILINE
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^ matches \en in middle no PCRE_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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PCRE's behaviour is the same as Perl's, except that there is no equivalent for
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PCRE_DOLLAR_ENDONLY in Perl. In both PCRE 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 PCRE_DOTALL and
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PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the
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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 terminated by a zero byte, subject to
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the options in \fIeflags\fP. These can be:
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.sp
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REG_NOTBOL
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.sp
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The PCRE_NOTBOL option is set when calling the underlying PCRE matching
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function.
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.sp
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REG_NOTEOL
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.sp
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The PCRE_NOTEOL option is set when calling the underlying PCRE matching
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function.
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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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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 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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Philip Hazel
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.br
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University Computing Service,
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.br
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Cambridge CB2 3QG, England.
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.P
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.in 0
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Last updated: 16 January 2006
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.br
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Copyright (c) 1997-2006 University of Cambridge.
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