481 lines
12 KiB
Groff
481 lines
12 KiB
Groff
.\" 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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.\" 4. 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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.\" $FreeBSD: src/lib/libc/regex/re_format.7,v 1.12 2008/09/05 17:41:20 keramida Exp $
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.\"
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.Dd March 20, 1994
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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 1003.2 regular expressions
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.Sh DESCRIPTION
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Regular expressions
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.Pq Dq RE Ns s ,
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as defined in
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.St -p1003.2 ,
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come in two forms:
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modern REs (roughly those of
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.Xr egrep 1 ;
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1003.2 calls these
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.Dq extended
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REs)
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and obsolete REs (roughly those of
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.Xr ed 1 ;
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1003.2
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.Dq basic
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REs).
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Obsolete REs mostly exist for backward compatibility in some old programs;
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they will be discussed at the end.
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.St -p1003.2
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leaves some aspects of RE syntax and semantics open;
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`\(dd' marks decisions on these aspects that
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may not be fully portable to other
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.St -p1003.2
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implementations.
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.Pp
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A (modern) RE is one\(dd or more non-empty\(dd
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.Em branches ,
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separated by
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.Ql \&| .
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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\(dd 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
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by a single\(dd
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.Ql \&* ,
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.Ql \&+ ,
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.Ql \&? ,
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or
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.Em bound .
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An atom followed by
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.Ql \&*
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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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.Ql \&+
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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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.Ql ?\&
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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
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.Em bound
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is
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.Ql \&{
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followed by an unsigned decimal integer,
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possibly followed by
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.Ql \&,
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possibly followed by another unsigned decimal integer,
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always followed by
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.Ql \&} .
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The integers must lie between 0 and
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.Dv RE_DUP_MAX
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(255\(dd) 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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.Em i
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and no comma matches
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a sequence of exactly
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.Em 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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.Em i
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and a comma matches
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a sequence of
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.Em 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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.Em i
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and
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.Em j
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matches
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a sequence of
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.Em i
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through
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.Em 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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.Ql ()
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(matching a match for the
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regular expression),
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an empty set of
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.Ql ()
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(matching the null string)\(dd,
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a
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.Em bracket expression
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(see below),
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.Ql .\&
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(matching any single character),
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.Ql \&^
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(matching the null string at the beginning of a line),
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.Ql \&$
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(matching the null string at the end of a line), a
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.Ql \e
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followed by one of the characters
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.Ql ^.[$()|*+?{\e
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(matching that character taken as an ordinary character),
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a
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.Ql \e
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followed by any other character\(dd
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(matching that character taken as an ordinary character,
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as if the
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.Ql \e
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had not been present\(dd),
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or a single character with no other significance (matching that character).
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A
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.Ql \&{
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followed by a character other than a digit is an ordinary
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character, not the beginning of a bound\(dd.
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It is illegal to end an RE with
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.Ql \e .
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.Pp
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A
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.Em bracket expression
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is a list of characters enclosed in
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.Ql [] .
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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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.Ql \&^ ,
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it matches any single character
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(but see below)
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.Em not
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from the rest of the list.
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If two characters in the list are separated by
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.Ql \&- ,
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this is shorthand
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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,
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.No e.g. Ql [0-9]
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in ASCII matches any decimal digit.
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It is illegal\(dd for two ranges to share an
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endpoint,
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.No e.g. Ql 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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.Ql \&]
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in the list, make it the first character
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(following a possible
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.Ql \&^ ) .
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To include a literal
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.Ql \&- ,
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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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.Ql \&-
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as the first endpoint of a range,
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enclose it in
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.Ql [.\&
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and
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.Ql .]\&
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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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.Ql \&[
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(see next paragraphs), all other special characters, including
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.Ql \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 (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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.Ql [.\&
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and
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.Ql .]\&
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stands for the
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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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.Ql ch
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collating element,
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then the RE
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.Ql [[.ch.]]*c
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matches the first five characters
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of
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.Ql chchcc .
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.Pp
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Within a bracket expression, a collating element enclosed in
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.Ql [=
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and
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.Ql =]
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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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.Ql [.\&
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and
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.Ql .] . )
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For example, if
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.Ql x
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and
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.Ql y
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are the members of an equivalence class,
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then
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.Ql [[=x=]] ,
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.Ql [[=y=]] ,
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and
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.Ql [xy]
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are all synonymous.
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An equivalence class may not\(dd be an endpoint
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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 in
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.Ql [:
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and
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.Ql :]
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stands for the list of all characters belonging to that
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class.
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Standard character class names are:
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.Pp
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.Bl -column "alnum" "digit" "xdigit" -offset indent
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.It Em "alnum digit punct"
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.It Em "alpha graph space"
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.It Em "blank lower upper"
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.It Em "cntrl print xdigit"
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.El
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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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A bracketed expression like
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.Ql [[:class:]]
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can be used to match a single character that belongs to a character
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class.
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The reverse, matching any character that does not belong to a specific
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class, the negation operator of bracket expressions may be used:
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.Ql [^[:class:]] .
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.Pp
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There are two special cases\(dd of bracket expressions:
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the bracket expressions
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.Ql [[:<:]]
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and
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.Ql [[:>:]]
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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 word characters
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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
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.St -p1003.2 ,
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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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.Ql bb*
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matches the three middle characters of
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.Ql abbbc ,
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.Ql (wee|week)(knights|nights)
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matches all ten characters of
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.Ql weeknights ,
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when
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.Ql (.*).*\&
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is matched against
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.Ql abc
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the parenthesized subexpression
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matches all three characters, and
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when
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.Ql (a*)*
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is matched against
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.Ql bc
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both the whole RE and the parenthesized
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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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.No e.g. Ql x
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becomes
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.Ql [xX] .
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When it appears inside a bracket expression, all case counterparts
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of it are added to the bracket expression, so that (e.g.)
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.Ql [x]
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becomes
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.Ql [xX]
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and
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.Ql [^x]
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becomes
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.Ql [^xX] .
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.Pp
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No particular limit is imposed on the length of REs\(dd.
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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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Obsolete
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.Pq Dq basic
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regular expressions differ in several respects.
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.Ql \&|
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is an ordinary character and there is no equivalent
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for its functionality.
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.Ql \&+
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and
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.Ql ?\&
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are ordinary characters, and their functionality
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can be expressed using bounds
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.No ( Ql {1,}
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or
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.Ql {0,1}
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respectively).
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Also note that
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.Ql x+
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in modern REs is equivalent to
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.Ql xx* .
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The delimiters for bounds are
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.Ql \e{
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and
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.Ql \e} ,
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with
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.Ql \&{
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and
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.Ql \&}
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by themselves ordinary characters.
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The parentheses for nested subexpressions are
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.Ql \e(
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and
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.Ql \e) ,
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with
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.Ql \&(
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and
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.Ql \&)
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by themselves ordinary characters.
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.Ql \&^
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is an ordinary character except at the beginning of the
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RE or\(dd the beginning of a parenthesized subexpression,
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.Ql \&$
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is an ordinary character except at the end of the
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RE or\(dd the end of a parenthesized subexpression,
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and
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.Ql \&*
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is an ordinary character if it appears at the beginning of the
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RE or the beginning of a parenthesized subexpression
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(after a possible leading
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.Ql \&^ ) .
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Finally, there is one new type of atom, a
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.Em back reference :
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.Ql \e
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followed by a non-zero decimal digit
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.Em d
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matches the same sequence of characters
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matched by the
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.Em d Ns th
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parenthesized subexpression
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(numbering subexpressions by the positions of their opening parentheses,
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left to right),
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so that (e.g.)
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.Ql \e([bc]\e)\e1
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matches
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.Ql bb
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or
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.Ql cc
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but not
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.Ql bc .
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.Sh SEE ALSO
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.Xr regex 3
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.Rs
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.%T Regular Expression Notation
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.%R IEEE Std
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.%N 1003.2
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.%P section 2.8
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.Re
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.Sh BUGS
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Having two kinds of REs is a botch.
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.Pp
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The current
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.St -p1003.2
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spec says that
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.Ql \&)
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is an ordinary character in
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the absence of an unmatched
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.Ql \&( ;
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this was an unintentional result of a wording error,
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and change is likely.
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Avoid relying on it.
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.Pp
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Back references are a dreadful botch,
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posing major problems for efficient implementations.
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They are also somewhat vaguely defined
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(does
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.Ql a\e(\e(b\e)*\e2\e)*d
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match
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.Ql abbbd ? ) .
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Avoid using them.
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.Pp
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.St -p1003.2
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specification of case-independent matching is vague.
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The
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.Dq one case implies all cases
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definition given above
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is current consensus among implementors as to the right interpretation.
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.Pp
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The syntax for word boundaries is incredibly ugly.
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