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freebsd/lib/libc/regex/regex.3

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.\" Copyright (c) 1992, 1993, 1994 Henry Spencer.
.\" Copyright (c) 1992, 1993, 1994
.\" The Regents of the University of California. All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Henry Spencer.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)regex.3 8.4 (Berkeley) 3/20/94
1999-08-28 00:22:10 +00:00
.\" $FreeBSD$
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.\"
.Dd August 17, 2005
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.Dt REGEX 3
.Os
.Sh NAME
.Nm regcomp ,
.Nm regexec ,
.Nm regerror ,
.Nm regfree
.Nd regular-expression library
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In regex.h
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.Ft int
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.Fo regcomp
.Fa "regex_t * restrict preg" "const char * restrict pattern" "int cflags"
.Fc
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.Ft int
.Fo regexec
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.Fa "const regex_t * restrict preg" "const char * restrict string"
.Fa "size_t nmatch" "regmatch_t pmatch[restrict]" "int eflags"
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.Fc
.Ft size_t
.Fo regerror
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.Fa "int errcode" "const regex_t * restrict preg"
.Fa "char * restrict errbuf" "size_t errbuf_size"
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.Fc
.Ft void
.Fn regfree "regex_t *preg"
.Sh DESCRIPTION
These routines implement
.St -p1003.2
regular expressions
.Pq Do RE Dc Ns s ;
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see
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.Xr re_format 7 .
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The
.Fn regcomp
function
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compiles an RE written as a string into an internal form,
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.Fn regexec
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matches that internal form against a string and reports results,
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.Fn regerror
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transforms error codes from either into human-readable messages,
and
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.Fn regfree
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frees any dynamically-allocated storage used by the internal form
of an RE.
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.Pp
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The header
.In regex.h
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declares two structure types,
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.Ft regex_t
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and
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.Ft regmatch_t ,
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the former for compiled internal forms and the latter for match reporting.
It also declares the four functions,
a type
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.Ft regoff_t ,
and a number of constants with names starting with
.Dq Dv REG_ .
.Pp
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The
.Fn regcomp
function
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compiles the regular expression contained in the
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.Fa pattern
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string,
subject to the flags in
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.Fa cflags ,
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and places the results in the
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.Ft regex_t
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structure pointed to by
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.Fa preg .
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The
.Fa cflags
argument
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is the bitwise OR of zero or more of the following flags:
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.Bl -tag -width REG_EXTENDED
.It Dv REG_EXTENDED
Compile modern
.Pq Dq extended
REs,
rather than the obsolete
.Pq Dq basic
REs that
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are the default.
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.It Dv REG_BASIC
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This is a synonym for 0,
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provided as a counterpart to
.Dv REG_EXTENDED
to improve readability.
.It Dv REG_NOSPEC
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Compile with recognition of all special characters turned off.
All characters are thus considered ordinary,
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so the
.Dq RE
is a literal string.
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This is an extension,
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compatible with but not specified by
.St -p1003.2 ,
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and should be used with
caution in software intended to be portable to other systems.
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.Dv REG_EXTENDED
and
.Dv REG_NOSPEC
may not be used
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in the same call to
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.Fn regcomp .
.It Dv REG_ICASE
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Compile for matching that ignores upper/lower case distinctions.
See
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.Xr re_format 7 .
.It Dv REG_NOSUB
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Compile for matching that need only report success or failure,
not what was matched.
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.It Dv REG_NEWLINE
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Compile for newline-sensitive matching.
By default, newline is a completely ordinary character with no special
meaning in either REs or strings.
With this flag,
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.Ql [^
bracket expressions and
.Ql .\&
never match newline,
a
.Ql ^\&
anchor matches the null string after any newline in the string
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in addition to its normal function,
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and the
.Ql $\&
anchor matches the null string before any newline in the
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string in addition to its normal function.
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.It Dv REG_PEND
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The regular expression ends,
not at the first NUL,
but just before the character pointed to by the
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.Va re_endp
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member of the structure pointed to by
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.Fa preg .
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The
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.Va re_endp
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member is of type
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.Ft "const char *" .
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This flag permits inclusion of NULs in the RE;
they are considered ordinary characters.
This is an extension,
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compatible with but not specified by
.St -p1003.2 ,
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and should be used with
caution in software intended to be portable to other systems.
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.El
.Pp
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When successful,
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.Fn regcomp
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returns 0 and fills in the structure pointed to by
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.Fa preg .
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One member of that structure
(other than
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.Va re_endp )
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is publicized:
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.Va re_nsub ,
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of type
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.Ft size_t ,
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contains the number of parenthesized subexpressions within the RE
(except that the value of this member is undefined if the
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.Dv REG_NOSUB
flag was used).
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If
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.Fn regcomp
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fails, it returns a non-zero error code;
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see
.Sx DIAGNOSTICS .
.Pp
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The
.Fn regexec
function
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matches the compiled RE pointed to by
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.Fa preg
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against the
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.Fa string ,
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subject to the flags in
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.Fa eflags ,
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and reports results using
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.Fa nmatch ,
.Fa pmatch ,
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and the returned value.
The RE must have been compiled by a previous invocation of
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.Fn regcomp .
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The compiled form is not altered during execution of
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.Fn regexec ,
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so a single compiled RE can be used simultaneously by multiple threads.
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.Pp
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By default,
the NUL-terminated string pointed to by
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.Fa string
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is considered to be the text of an entire line, minus any terminating
newline.
The
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.Fa eflags
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argument is the bitwise OR of zero or more of the following flags:
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.Bl -tag -width REG_STARTEND
.It Dv REG_NOTBOL
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The first character of
the string
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is not the beginning of a line, so the
.Ql ^\&
anchor should not match before it.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_NOTEOL
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The NUL terminating
the string
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does not end a line, so the
.Ql $\&
anchor should not match before it.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_STARTEND
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The string is considered to start at
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.Fa string
+
.Fa pmatch Ns [0]. Ns Va rm_so
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and to have a terminating NUL located at
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.Fa string
+
.Fa pmatch Ns [0]. Ns Va rm_eo
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(there need not actually be a NUL at that location),
regardless of the value of
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.Fa nmatch .
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See below for the definition of
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.Fa pmatch
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and
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.Fa nmatch .
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This is an extension,
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compatible with but not specified by
.St -p1003.2 ,
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and should be used with
caution in software intended to be portable to other systems.
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Note that a non-zero
.Va rm_so
does not imply
.Dv REG_NOTBOL ;
.Dv REG_STARTEND
affects only the location of the string,
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not how it is matched.
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.El
.Pp
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See
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.Xr re_format 7
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for a discussion of what is matched in situations where an RE or a
portion thereof could match any of several substrings of
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.Fa string .
.Pp
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Normally,
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.Fn regexec
returns 0 for success and the non-zero code
.Dv REG_NOMATCH
for failure.
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Other non-zero error codes may be returned in exceptional situations;
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see
.Sx DIAGNOSTICS .
.Pp
If
.Dv REG_NOSUB
was specified in the compilation of the RE,
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or if
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.Fa nmatch
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is 0,
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.Fn regexec
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ignores the
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.Fa pmatch
argument (but see below for the case where
.Dv REG_STARTEND
is specified).
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Otherwise,
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.Fa pmatch
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points to an array of
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.Fa nmatch
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structures of type
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.Ft regmatch_t .
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Such a structure has at least the members
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.Va rm_so
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and
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.Va rm_eo ,
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both of type
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.Ft regoff_t
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(a signed arithmetic type at least as large as an
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.Ft off_t
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and a
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.Ft ssize_t ) ,
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containing respectively the offset of the first character of a substring
and the offset of the first character after the end of the substring.
Offsets are measured from the beginning of the
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.Fa string
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argument given to
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.Fn regexec .
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An empty substring is denoted by equal offsets,
both indicating the character following the empty substring.
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.Pp
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The 0th member of the
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.Fa pmatch
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array is filled in to indicate what substring of
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.Fa string
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was matched by the entire RE.
Remaining members report what substring was matched by parenthesized
subexpressions within the RE;
member
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.Va i
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reports subexpression
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.Va i ,
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with subexpressions counted (starting at 1) by the order of their opening
parentheses in the RE, left to right.
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Unused entries in the array (corresponding either to subexpressions that
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did not participate in the match at all, or to subexpressions that do not
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exist in the RE (that is,
.Va i
>
.Fa preg Ns -> Ns Va re_nsub ) )
have both
.Va rm_so
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and
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.Va rm_eo
set to -1.
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If a subexpression participated in the match several times,
the reported substring is the last one it matched.
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(Note, as an example in particular, that when the RE
.Ql "(b*)+"
matches
.Ql bbb ,
the parenthesized subexpression matches each of the three
.So Li b Sc Ns s
and then
an infinite number of empty strings following the last
.Ql b ,
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so the reported substring is one of the empties.)
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.Pp
If
.Dv REG_STARTEND
is specified,
.Fa pmatch
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must point to at least one
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.Ft regmatch_t
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(even if
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.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified),
to hold the input offsets for
.Dv REG_STARTEND .
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Use for output is still entirely controlled by
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.Fa nmatch ;
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if
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.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified,
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the value of
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.Fa pmatch Ns [0]
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will not be changed by a successful
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.Fn regexec .
.Pp
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The
.Fn regerror
function
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maps a non-zero
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.Fa errcode
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from either
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.Fn regcomp
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or
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.Fn regexec
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to a human-readable, printable message.
If
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.Fa preg
is
.No non\- Ns Dv NULL ,
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the error code should have arisen from use of
the
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.Ft regex_t
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pointed to by
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.Fa preg ,
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and if the error code came from
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.Fn regcomp ,
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it should have been the result from the most recent
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.Fn regcomp
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using that
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.Ft regex_t .
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The
.Fn ( regerror
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may be able to supply a more detailed message using information
from the
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.Ft regex_t . )
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The
.Fn regerror
function
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places the NUL-terminated message into the buffer pointed to by
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.Fa errbuf ,
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limiting the length (including the NUL) to at most
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.Fa errbuf_size
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bytes.
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If the whole message will not fit,
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as much of it as will fit before the terminating NUL is supplied.
In any case,
the returned value is the size of buffer needed to hold the whole
message (including terminating NUL).
If
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.Fa errbuf_size
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is 0,
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.Fa errbuf
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is ignored but the return value is still correct.
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.Pp
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If the
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.Fa errcode
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given to
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.Fn regerror
is first ORed with
.Dv REG_ITOA ,
the
.Dq message
that results is the printable name of the error code,
e.g.\&
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.Dq Dv REG_NOMATCH ,
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rather than an explanation thereof.
If
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.Fa errcode
is
.Dv REG_ATOI ,
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then
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.Fa preg
shall be
.No non\- Ns Dv NULL
and the
.Va re_endp
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member of the structure it points to
must point to the printable name of an error code;
in this case, the result in
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.Fa errbuf
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is the decimal digits of
the numeric value of the error code
(0 if the name is not recognized).
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.Dv REG_ITOA
and
.Dv REG_ATOI
are intended primarily as debugging facilities;
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they are extensions,
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compatible with but not specified by
.St -p1003.2 ,
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and should be used with
caution in software intended to be portable to other systems.
Be warned also that they are considered experimental and changes are possible.
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.Pp
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The
.Fn regfree
function
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frees any dynamically-allocated storage associated with the compiled RE
pointed to by
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.Fa preg .
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The remaining
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.Ft regex_t
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is no longer a valid compiled RE
and the effect of supplying it to
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.Fn regexec
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or
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.Fn regerror
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is undefined.
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.Pp
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None of these functions references global variables except for tables
of constants;
all are safe for use from multiple threads if the arguments are safe.
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.Sh IMPLEMENTATION CHOICES
There are a number of decisions that
.St -p1003.2
leaves up to the implementor,
either by explicitly saying
.Dq undefined
or by virtue of them being
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forbidden by the RE grammar.
This implementation treats them as follows.
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.Pp
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See
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.Xr re_format 7
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for a discussion of the definition of case-independent matching.
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.Pp
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There is no particular limit on the length of REs,
except insofar as memory is limited.
Memory usage is approximately linear in RE size, and largely insensitive
to RE complexity, except for bounded repetitions.
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See
.Sx BUGS
for one short RE using them
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that will run almost any system out of memory.
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.Pp
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A backslashed character other than one specifically given a magic meaning
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by
.St -p1003.2
(such magic meanings occur only in obsolete
.Bq Dq basic
REs)
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is taken as an ordinary character.
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.Pp
Any unmatched
.Ql [\&
is a
.Dv REG_EBRACK
error.
.Pp
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Equivalence classes cannot begin or end bracket-expression ranges.
The endpoint of one range cannot begin another.
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.Pp
.Dv RE_DUP_MAX ,
the limit on repetition counts in bounded repetitions, is 255.
.Pp
A repetition operator
.Ql ( ?\& ,
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.Ql *\& ,
.Ql +\& ,
or bounds)
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cannot follow another
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repetition operator.
A repetition operator cannot begin an expression or subexpression
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or follow
.Ql ^\&
or
.Ql |\& .
.Pp
.Ql |\&
cannot appear first or last in a (sub)expression or after another
.Ql |\& ,
i.e., an operand of
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.Ql |\&
cannot be an empty subexpression.
An empty parenthesized subexpression,
.Ql "()" ,
is legal and matches an
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empty (sub)string.
An empty string is not a legal RE.
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.Pp
A
.Ql {\&
followed by a digit is considered the beginning of bounds for a
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bounded repetition, which must then follow the syntax for bounds.
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A
.Ql {\&
.Em not
followed by a digit is considered an ordinary character.
.Pp
.Ql ^\&
and
.Ql $\&
beginning and ending subexpressions in obsolete
.Pq Dq basic
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REs are anchors, not ordinary characters.
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.Sh DIAGNOSTICS
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Non-zero error codes from
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.Fn regcomp
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and
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.Fn regexec
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include the following:
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.Pp
.Bl -tag -width REG_ECOLLATE -compact
.It Dv REG_NOMATCH
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The
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.Fn regexec
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function
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failed to match
.It Dv REG_BADPAT
invalid regular expression
.It Dv REG_ECOLLATE
invalid collating element
.It Dv REG_ECTYPE
invalid character class
.It Dv REG_EESCAPE
.Ql \e
applied to unescapable character
.It Dv REG_ESUBREG
invalid backreference number
.It Dv REG_EBRACK
brackets
.Ql "[ ]"
not balanced
.It Dv REG_EPAREN
parentheses
.Ql "( )"
not balanced
.It Dv REG_EBRACE
braces
.Ql "{ }"
not balanced
.It Dv REG_BADBR
invalid repetition count(s) in
.Ql "{ }"
.It Dv REG_ERANGE
invalid character range in
.Ql "[ ]"
.It Dv REG_ESPACE
ran out of memory
.It Dv REG_BADRPT
.Ql ?\& ,
.Ql *\& ,
or
.Ql +\&
operand invalid
.It Dv REG_EMPTY
empty (sub)expression
.It Dv REG_ASSERT
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cannot happen - you found a bug
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.It Dv REG_INVARG
invalid argument, e.g.\& negative-length string
.It Dv REG_ILLSEQ
illegal byte sequence (bad multibyte character)
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.El
2005-01-20 09:17:07 +00:00
.Sh SEE ALSO
.Xr grep 1 ,
.Xr re_format 7
.Pp
.St -p1003.2 ,
sections 2.8 (Regular Expression Notation)
and
B.5 (C Binding for Regular Expression Matching).
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.Sh HISTORY
Originally written by
.An Henry Spencer .
Altered for inclusion in the
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.Bx 4.4
distribution.
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.Sh BUGS
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This is an alpha release with known defects.
Please report problems.
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.Pp
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The back-reference code is subtle and doubts linger about its correctness
in complex cases.
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.Pp
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The
.Fn regexec
function
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performance is poor.
This will improve with later releases.
2002-12-19 09:40:28 +00:00
The
.Fa nmatch
argument
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exceeding 0 is expensive;
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.Fa nmatch
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exceeding 1 is worse.
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The
.Fn regexec
function
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is largely insensitive to RE complexity
.Em except
that back
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references are massively expensive.
RE length does matter; in particular, there is a strong speed bonus
for keeping RE length under about 30 characters,
with most special characters counting roughly double.
2001-01-12 13:35:17 +00:00
.Pp
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The
.Fn regcomp
function
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implements bounded repetitions by macro expansion,
which is costly in time and space if counts are large
or bounded repetitions are nested.
An RE like, say,
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.Ql "((((a{1,100}){1,100}){1,100}){1,100}){1,100}"
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will (eventually) run almost any existing machine out of swap space.
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.Pp
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There are suspected problems with response to obscure error conditions.
Notably,
certain kinds of internal overflow,
produced only by truly enormous REs or by multiply nested bounded repetitions,
are probably not handled well.
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.Pp
Due to a mistake in
.St -p1003.2 ,
things like
.Ql "a)b"
are legal REs because
.Ql )\&
is
a special character only in the presence of a previous unmatched
.Ql (\& .
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This cannot be fixed until the spec is fixed.
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.Pp
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The standard's definition of back references is vague.
For example, does
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.Ql "a\e(\e(b\e)*\e2\e)*d"
match
.Ql "abbbd" ?
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Until the standard is clarified,
behavior in such cases should not be relied on.
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.Pp
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The implementation of word-boundary matching is a bit of a kludge,
and bugs may lurk in combinations of word-boundary matching and anchoring.
.Pp
Word-boundary matching does not work properly in multibyte locales.