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757 lines
18 KiB
Groff
757 lines
18 KiB
Groff
.\" $OpenBSD: re_format.7,v 1.14 2007/05/31 19:19:30 jmc Exp $
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.\"
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.\" Copyright (c) 1997, Phillip F Knaack. All rights reserved.
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.\"
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.\" Copyright (c) 1992, 1993, 1994 Henry Spencer.
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.\" Copyright (c) 1992, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Henry Spencer.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)re_format.7 8.3 (Berkeley) 3/20/94
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.\"
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.Dd $Mdocdate: May 31 2007 $
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.Dt RE_FORMAT 7
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.Os
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.Sh NAME
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.Nm re_format
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.Nd POSIX regular expressions
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.Sh DESCRIPTION
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Regular expressions (REs),
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as defined in
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.St -p1003.1-2004 ,
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come in two forms:
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basic regular expressions
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(BREs)
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and extended regular expressions
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(EREs).
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Both forms of regular expressions are supported
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by the interfaces described in
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.Xr regex 3 .
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Applications dealing with regular expressions
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may use one or the other form
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(or indeed both).
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For example,
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.Xr ed 1
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uses BREs,
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whilst
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.Xr egrep 1
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talks EREs.
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Consult the manual page for the specific application to find out which
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it uses.
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.Pp
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POSIX leaves some aspects of RE syntax and semantics open;
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.Sq **
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marks decisions on these aspects that
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may not be fully portable to other POSIX implementations.
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.Pp
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This manual page first describes regular expressions in general,
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specifically extended regular expressions,
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and then discusses differences between them and basic regular expressions.
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.Sh EXTENDED REGULAR EXPRESSIONS
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An ERE is one** or more non-empty**
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.Em branches ,
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separated by
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.Sq \*(Ba .
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It matches anything that matches one of the branches.
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.Pp
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A branch is one** or more
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.Em pieces ,
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concatenated.
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It matches a match for the first, followed by a match for the second, etc.
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.Pp
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A piece is an
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.Em atom
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possibly followed by a single**
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.Sq * ,
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.Sq + ,
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.Sq ?\& ,
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or
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.Em bound .
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An atom followed by
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.Sq *
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matches a sequence of 0 or more matches of the atom.
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An atom followed by
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.Sq +
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matches a sequence of 1 or more matches of the atom.
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An atom followed by
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.Sq ?\&
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matches a sequence of 0 or 1 matches of the atom.
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.Pp
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A bound is
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.Sq {
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followed by an unsigned decimal integer,
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possibly followed by
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.Sq ,\&
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possibly followed by another unsigned decimal integer,
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always followed by
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.Sq } .
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The integers must lie between 0 and
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.Dv RE_DUP_MAX
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(255**) inclusive,
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and if there are two of them, the first may not exceed the second.
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An atom followed by a bound containing one integer
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.Ar i
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and no comma matches
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a sequence of exactly
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.Ar i
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matches of the atom.
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An atom followed by a bound
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containing one integer
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.Ar i
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and a comma matches
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a sequence of
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.Ar i
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or more matches of the atom.
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An atom followed by a bound
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containing two integers
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.Ar i
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and
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.Ar j
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matches a sequence of
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.Ar i
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through
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.Ar j
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(inclusive) matches of the atom.
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.Pp
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An atom is a regular expression enclosed in
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.Sq ()
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(matching a part of the regular expression),
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an empty set of
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.Sq ()
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(matching the null string)**,
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a
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.Em bracket expression
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(see below),
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.Sq .\&
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(matching any single character),
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.Sq ^
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(matching the null string at the beginning of a line),
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.Sq $
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(matching the null string at the end of a line),
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a
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.Sq \e
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followed by one of the characters
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.Sq ^.[$()|*+?{\e
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(matching that character taken as an ordinary character),
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a
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.Sq \e
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followed by any other character**
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(matching that character taken as an ordinary character,
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as if the
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.Sq \e
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had not been present**),
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or a single character with no other significance (matching that character).
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A
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.Sq {
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followed by a character other than a digit is an ordinary character,
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not the beginning of a bound**.
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It is illegal to end an RE with
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.Sq \e .
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.Pp
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A bracket expression is a list of characters enclosed in
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.Sq [] .
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It normally matches any single character from the list (but see below).
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If the list begins with
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.Sq ^ ,
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it matches any single character
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.Em not
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from the rest of the list
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(but see below).
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If two characters in the list are separated by
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.Sq - ,
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this is shorthand for the full
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.Em range
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of characters between those two (inclusive) in the
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collating sequence, e.g.\&
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.Sq [0-9]
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in ASCII matches any decimal digit.
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It is illegal** for two ranges to share an endpoint, e.g.\&
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.Sq a-c-e .
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Ranges are very collating-sequence-dependent,
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and portable programs should avoid relying on them.
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.Pp
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To include a literal
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.Sq ]\&
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in the list, make it the first character
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(following a possible
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.Sq ^ ) .
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To include a literal
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.Sq - ,
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make it the first or last character,
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or the second endpoint of a range.
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To use a literal
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.Sq -
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as the first endpoint of a range,
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enclose it in
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.Sq [.
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and
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.Sq .]
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to make it a collating element (see below).
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With the exception of these and some combinations using
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.Sq [
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(see next paragraphs),
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all other special characters, including
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.Sq \e ,
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lose their special significance within a bracket expression.
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.Pp
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Within a bracket expression, a collating element
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(a character,
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a multi-character sequence that collates as if it were a single character,
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or a collating-sequence name for either)
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enclosed in
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.Sq [.
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and
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.Sq .]
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stands for the sequence of characters of that collating element.
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The sequence is a single element of the bracket expression's list.
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A bracket expression containing a multi-character collating element
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can thus match more than one character,
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e.g. if the collating sequence includes a
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.Sq ch
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collating element,
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then the RE
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.Sq [[.ch.]]*c
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matches the first five characters of
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.Sq chchcc .
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.Pp
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|
Within a bracket expression, a collating element enclosed in
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.Sq [=
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and
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.Sq =]
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is an equivalence class, standing for the sequences of characters
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of all collating elements equivalent to that one, including itself.
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(If there are no other equivalent collating elements,
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the treatment is as if the enclosing delimiters were
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.Sq [.
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and
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.Sq .] . )
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For example, if
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.Sq x
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and
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.Sq y
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are the members of an equivalence class,
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then
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.Sq [[=x=]] ,
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.Sq [[=y=]] ,
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and
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.Sq [xy]
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are all synonymous.
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An equivalence class may not** be an endpoint of a range.
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.Pp
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Within a bracket expression, the name of a
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.Em character class
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enclosed
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in
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.Sq [:
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and
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.Sq :]
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|
stands for the list of all characters belonging to that class.
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|
Standard character class names are:
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.Bd -literal -offset indent
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alnum digit punct
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alpha graph space
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blank lower upper
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cntrl print xdigit
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|
.Ed
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.Pp
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|
These stand for the character classes defined in
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.Xr ctype 3 .
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A locale may provide others.
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A character class may not be used as an endpoint of a range.
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.Pp
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There are two special cases** of bracket expressions:
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the bracket expressions
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.Sq [[:<:]]
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and
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.Sq [[:>:]]
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match the null string at the beginning and end of a word, respectively.
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|
A word is defined as a sequence of
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|
characters starting and ending with a word character
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which is neither preceded nor followed by
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word characters.
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|
A word character is an
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|
.Em alnum
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character (as defined by
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.Xr ctype 3 )
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or an underscore.
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|
This is an extension,
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compatible with but not specified by POSIX,
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and should be used with
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caution in software intended to be portable to other systems.
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.Pp
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|
In the event that an RE could match more than one substring of a given
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string,
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the RE matches the one starting earliest in the string.
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If the RE could match more than one substring starting at that point,
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it matches the longest.
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Subexpressions also match the longest possible substrings, subject to
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the constraint that the whole match be as long as possible,
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with subexpressions starting earlier in the RE taking priority over
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ones starting later.
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|
Note that higher-level subexpressions thus take priority over
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their lower-level component subexpressions.
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.Pp
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Match lengths are measured in characters, not collating elements.
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A null string is considered longer than no match at all.
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|
For example,
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.Sq bb*
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matches the three middle characters of
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.Sq abbbc ;
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|
.Sq (wee|week)(knights|nights)
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|
matches all ten characters of
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.Sq weeknights ;
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|
when
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|
.Sq (.*).*
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|
is matched against
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.Sq abc ,
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|
the parenthesized subexpression matches all three characters;
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and when
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.Sq (a*)*
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is matched against
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.Sq bc ,
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both the whole RE and the parenthesized subexpression match the null string.
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.Pp
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If case-independent matching is specified,
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the effect is much as if all case distinctions had vanished from the
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alphabet.
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When an alphabetic that exists in multiple cases appears as an
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ordinary character outside a bracket expression, it is effectively
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transformed into a bracket expression containing both cases,
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|
e.g.\&
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.Sq x
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becomes
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.Sq [xX] .
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When it appears inside a bracket expression,
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all case counterparts of it are added to the bracket expression,
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so that, for example,
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.Sq [x]
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becomes
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|
.Sq [xX]
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and
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|
.Sq [^x]
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becomes
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.Sq [^xX] .
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.Pp
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No particular limit is imposed on the length of REs**.
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Programs intended to be portable should not employ REs longer
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than 256 bytes,
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as an implementation can refuse to accept such REs and remain
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POSIX-compliant.
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.Pp
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The following is a list of extended regular expressions:
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.Bl -tag -width Ds
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.It Ar c
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Any character
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.Ar c
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not listed below matches itself.
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.It \e Ns Ar c
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Any backslash-escaped character
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.Ar c
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matches itself.
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.It \&.
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Matches any single character that is not a newline
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.Pq Sq \en .
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.It Bq Ar char-class
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Matches any single character in
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.Ar char-class .
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To include a
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.Ql \&]
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in
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.Ar char-class ,
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it must be the first character.
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A range of characters may be specified by separating the end characters
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of the range with a
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.Ql - ;
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e.g.\&
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.Ar a-z
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specifies the lower case characters.
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The following literal expressions can also be used in
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.Ar char-class
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to specify sets of characters:
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.Bd -unfilled -offset indent
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[:alnum:] [:cntrl:] [:lower:] [:space:]
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|
[:alpha:] [:digit:] [:print:] [:upper:]
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[:blank:] [:graph:] [:punct:] [:xdigit:]
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|
.Ed
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.Pp
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If
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.Ql -
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appears as the first or last character of
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.Ar char-class ,
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|
then it matches itself.
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All other characters in
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.Ar char-class
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match themselves.
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.Pp
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Patterns in
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.Ar char-class
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|
of the form
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.Eo [.
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.Ar col-elm
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|
.Ec .]\&
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|
or
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.Eo [=
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.Ar col-elm
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.Ec =]\& ,
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where
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.Ar col-elm
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is a collating element, are interpreted according to
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.Xr setlocale 3
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|
.Pq not currently supported .
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.It Bq ^ Ns Ar char-class
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Matches any single character, other than newline, not in
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.Ar char-class .
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.Ar char-class
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is defined as above.
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.It ^
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If
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.Sq ^
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is the first character of a regular expression, then it
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|
anchors the regular expression to the beginning of a line.
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|
Otherwise, it matches itself.
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|
.It $
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|
If
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.Sq $
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|
is the last character of a regular expression,
|
|
it anchors the regular expression to the end of a line.
|
|
Otherwise, it matches itself.
|
|
.It [[:<:]]
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|
Anchors the single character regular expression or subexpression
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|
immediately following it to the beginning of a word.
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|
.It [[:>:]]
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|
Anchors the single character regular expression or subexpression
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|
immediately following it to the end of a word.
|
|
.It Pq Ar re
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|
Defines a subexpression
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|
.Ar re .
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|
Any set of characters enclosed in parentheses
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|
matches whatever the set of characters without parentheses matches
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|
(that is a long-winded way of saying the constructs
|
|
.Sq (re)
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|
and
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|
.Sq re
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|
match identically).
|
|
.It *
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|
Matches the single character regular expression or subexpression
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|
immediately preceding it zero or more times.
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|
If
|
|
.Sq *
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|
is the first character of a regular expression or subexpression,
|
|
then it matches itself.
|
|
The
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|
.Sq *
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|
operator sometimes yields unexpected results.
|
|
For example, the regular expression
|
|
.Ar b*
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|
matches the beginning of the string
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|
.Qq abbb
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|
(as opposed to the substring
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|
.Qq bbb ) ,
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|
since a null match is the only leftmost match.
|
|
.It +
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|
Matches the singular character regular expression
|
|
or subexpression immediately preceding it
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|
one or more times.
|
|
.It ?
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|
Matches the singular character regular expression
|
|
or subexpression immediately preceding it
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|
0 or 1 times.
|
|
.Sm off
|
|
.It Xo
|
|
.Pf { Ar n , m No }\ \&
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|
.Pf { Ar n , No }\ \&
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|
.Pf { Ar n No }
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|
.Xc
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|
.Sm on
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|
Matches the single character regular expression or subexpression
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|
immediately preceding it at least
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|
.Ar n
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|
and at most
|
|
.Ar m
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|
times.
|
|
If
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|
.Ar m
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|
is omitted, then it matches at least
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|
.Ar n
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|
times.
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|
If the comma is also omitted, then it matches exactly
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|
.Ar n
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|
times.
|
|
.It \*(Ba
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|
Used to separate patterns.
|
|
For example,
|
|
the pattern
|
|
.Sq cat\*(Badog
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|
matches either
|
|
.Sq cat
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|
or
|
|
.Sq dog .
|
|
.El
|
|
.Sh BASIC REGULAR EXPRESSIONS
|
|
Basic regular expressions differ in several respects:
|
|
.Bl -bullet -offset 3n
|
|
.It
|
|
.Sq \*(Ba ,
|
|
.Sq + ,
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|
and
|
|
.Sq ?\&
|
|
are ordinary characters and there is no equivalent
|
|
for their functionality.
|
|
.It
|
|
The delimiters for bounds are
|
|
.Sq \e{
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|
and
|
|
.Sq \e} ,
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|
with
|
|
.Sq {
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|
and
|
|
.Sq }
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|
by themselves ordinary characters.
|
|
.It
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|
The parentheses for nested subexpressions are
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|
.Sq \e(
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|
and
|
|
.Sq \e) ,
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|
with
|
|
.Sq (
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|
and
|
|
.Sq )\&
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|
by themselves ordinary characters.
|
|
.It
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|
.Sq ^
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|
is an ordinary character except at the beginning of the
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|
RE or** the beginning of a parenthesized subexpression.
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|
.It
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|
.Sq $
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|
is an ordinary character except at the end of the
|
|
RE or** the end of a parenthesized subexpression.
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|
.It
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|
.Sq *
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|
is an ordinary character if it appears at the beginning of the
|
|
RE or the beginning of a parenthesized subexpression
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|
(after a possible leading
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|
.Sq ^ ) .
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|
.It
|
|
Finally, there is one new type of atom, a
|
|
.Em back-reference :
|
|
.Sq \e
|
|
followed by a non-zero decimal digit
|
|
.Ar d
|
|
matches the same sequence of characters matched by the
|
|
.Ar d Ns th
|
|
parenthesized subexpression
|
|
(numbering subexpressions by the positions of their opening parentheses,
|
|
left to right),
|
|
so that, for example,
|
|
.Sq \e([bc]\e)\e1
|
|
matches
|
|
.Sq bb\&
|
|
or
|
|
.Sq cc
|
|
but not
|
|
.Sq bc .
|
|
.El
|
|
.Pp
|
|
The following is a list of basic regular expressions:
|
|
.Bl -tag -width Ds
|
|
.It Ar c
|
|
Any character
|
|
.Ar c
|
|
not listed below matches itself.
|
|
.It \e Ns Ar c
|
|
Any backslash-escaped character
|
|
.Ar c ,
|
|
except for
|
|
.Sq { ,
|
|
.Sq } ,
|
|
.Sq \&( ,
|
|
and
|
|
.Sq \&) ,
|
|
matches itself.
|
|
.It \&.
|
|
Matches any single character that is not a newline
|
|
.Pq Sq \en .
|
|
.It Bq Ar char-class
|
|
Matches any single character in
|
|
.Ar char-class .
|
|
To include a
|
|
.Ql \&]
|
|
in
|
|
.Ar char-class ,
|
|
it must be the first character.
|
|
A range of characters may be specified by separating the end characters
|
|
of the range with a
|
|
.Ql - ;
|
|
e.g.\&
|
|
.Ar a-z
|
|
specifies the lower case characters.
|
|
The following literal expressions can also be used in
|
|
.Ar char-class
|
|
to specify sets of characters:
|
|
.Bd -unfilled -offset indent
|
|
[:alnum:] [:cntrl:] [:lower:] [:space:]
|
|
[:alpha:] [:digit:] [:print:] [:upper:]
|
|
[:blank:] [:graph:] [:punct:] [:xdigit:]
|
|
.Ed
|
|
.Pp
|
|
If
|
|
.Ql -
|
|
appears as the first or last character of
|
|
.Ar char-class ,
|
|
then it matches itself.
|
|
All other characters in
|
|
.Ar char-class
|
|
match themselves.
|
|
.Pp
|
|
Patterns in
|
|
.Ar char-class
|
|
of the form
|
|
.Eo [.
|
|
.Ar col-elm
|
|
.Ec .]\&
|
|
or
|
|
.Eo [=
|
|
.Ar col-elm
|
|
.Ec =]\& ,
|
|
where
|
|
.Ar col-elm
|
|
is a collating element, are interpreted according to
|
|
.Xr setlocale 3
|
|
.Pq not currently supported .
|
|
.It Bq ^ Ns Ar char-class
|
|
Matches any single character, other than newline, not in
|
|
.Ar char-class .
|
|
.Ar char-class
|
|
is defined as above.
|
|
.It ^
|
|
If
|
|
.Sq ^
|
|
is the first character of a regular expression, then it
|
|
anchors the regular expression to the beginning of a line.
|
|
Otherwise, it matches itself.
|
|
.It $
|
|
If
|
|
.Sq $
|
|
is the last character of a regular expression,
|
|
it anchors the regular expression to the end of a line.
|
|
Otherwise, it matches itself.
|
|
.It [[:<:]]
|
|
Anchors the single character regular expression or subexpression
|
|
immediately following it to the beginning of a word.
|
|
.It [[:>:]]
|
|
Anchors the single character regular expression or subexpression
|
|
immediately following it to the end of a word.
|
|
.It \e( Ns Ar re Ns \e)
|
|
Defines a subexpression
|
|
.Ar re .
|
|
Subexpressions may be nested.
|
|
A subsequent backreference of the form
|
|
.Pf \e Ns Ar n ,
|
|
where
|
|
.Ar n
|
|
is a number in the range [1,9], expands to the text matched by the
|
|
.Ar n Ns th
|
|
subexpression.
|
|
For example, the regular expression
|
|
.Ar \e(.*\e)\e1
|
|
matches any string consisting of identical adjacent substrings.
|
|
Subexpressions are ordered relative to their left delimiter.
|
|
.It *
|
|
Matches the single character regular expression or subexpression
|
|
immediately preceding it zero or more times.
|
|
If
|
|
.Sq *
|
|
is the first character of a regular expression or subexpression,
|
|
then it matches itself.
|
|
The
|
|
.Sq *
|
|
operator sometimes yields unexpected results.
|
|
For example, the regular expression
|
|
.Ar b*
|
|
matches the beginning of the string
|
|
.Qq abbb
|
|
(as opposed to the substring
|
|
.Qq bbb ) ,
|
|
since a null match is the only leftmost match.
|
|
.Sm off
|
|
.It Xo
|
|
.Pf \e{ Ar n , m No \e}\ \&
|
|
.Pf \e{ Ar n , No \e}\ \&
|
|
.Pf \e{ Ar n No \e}
|
|
.Xc
|
|
.Sm on
|
|
Matches the single character regular expression or subexpression
|
|
immediately preceding it at least
|
|
.Ar n
|
|
and at most
|
|
.Ar m
|
|
times.
|
|
If
|
|
.Ar m
|
|
is omitted, then it matches at least
|
|
.Ar n
|
|
times.
|
|
If the comma is also omitted, then it matches exactly
|
|
.Ar n
|
|
times.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr ctype 3 ,
|
|
.Xr regex 3
|
|
.Sh STANDARDS
|
|
.St -p1003.1-2004 :
|
|
Base Definitions, Chapter 9 (Regular Expressions).
|
|
.Sh BUGS
|
|
Having two kinds of REs is a botch.
|
|
.Pp
|
|
The current POSIX spec says that
|
|
.Sq )\&
|
|
is an ordinary character in the absence of an unmatched
|
|
.Sq ( ;
|
|
this was an unintentional result of a wording error,
|
|
and change is likely.
|
|
Avoid relying on it.
|
|
.Pp
|
|
Back-references are a dreadful botch,
|
|
posing major problems for efficient implementations.
|
|
They are also somewhat vaguely defined
|
|
(does
|
|
.Sq a\e(\e(b\e)*\e2\e)*d
|
|
match
|
|
.Sq abbbd ? ) .
|
|
Avoid using them.
|
|
.Pp
|
|
POSIX's specification of case-independent matching is vague.
|
|
The
|
|
.Dq one case implies all cases
|
|
definition given above
|
|
is the current consensus among implementors as to the right interpretation.
|
|
.Pp
|
|
The syntax for word boundaries is incredibly ugly.
|