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'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" RCS: @(#) $Id: regexp.n,v 1.1.1.1 2002-01-16 10:25:25 markom Exp $
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'\"
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.so man.macros
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.TH regexp n "" Tcl "Tcl Built-In Commands"
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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regexp \- Match a regular expression against a string
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.SH SYNOPSIS
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\fBregexp \fR?\fIswitches\fR? \fIexp string \fR?\fImatchVar\fR? ?\fIsubMatchVar subMatchVar ...\fR?
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.BE
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.SH DESCRIPTION
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.PP
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Determines whether the regular expression \fIexp\fR matches part or
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all of \fIstring\fR and returns 1 if it does, 0 if it doesn't.
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.LP
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If additional arguments are specified after \fIstring\fR then they
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are treated as the names of variables in which to return
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information about which part(s) of \fIstring\fR matched \fIexp\fR.
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\fIMatchVar\fR will be set to the range of \fIstring\fR that
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matched all of \fIexp\fR. The first \fIsubMatchVar\fR will contain
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the characters in \fIstring\fR that matched the leftmost parenthesized
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subexpression within \fIexp\fR, the next \fIsubMatchVar\fR will
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contain the characters that matched the next parenthesized
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subexpression to the right in \fIexp\fR, and so on.
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.LP
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If the initial arguments to \fBregexp\fR start with \fB\-\fR then
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they are treated as switches. The following switches are
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currently supported:
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.TP 10
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\fB\-nocase\fR
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Causes upper-case characters in \fIstring\fR to be treated as
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lower case during the matching process.
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.TP 10
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\fB\-indices\fR
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Changes what is stored in the \fIsubMatchVar\fRs.
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Instead of storing the matching characters from \fBstring\fR,
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each variable
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will contain a list of two decimal strings giving the indices
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in \fIstring\fR of the first and last characters in the matching
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range of characters.
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.TP 10
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\fB\-\|\-\fR
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Marks the end of switches. The argument following this one will
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be treated as \fIexp\fR even if it starts with a \fB\-\fR.
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.LP
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If there are more \fIsubMatchVar\fR's than parenthesized
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subexpressions within \fIexp\fR, or if a particular subexpression
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in \fIexp\fR doesn't match the string (e.g. because it was in a
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portion of the expression that wasn't matched), then the corresponding
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\fIsubMatchVar\fR will be set to ``\fB\-1 \-1\fR'' if \fB\-indices\fR
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has been specified or to an empty string otherwise.
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.SH "REGULAR EXPRESSIONS"
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.PP
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Regular expressions are implemented using Henry Spencer's package
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(thanks, Henry!),
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and much of the description of regular expressions below is copied verbatim
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from his manual entry.
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.PP
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A regular expression is zero or more \fIbranches\fR, separated by ``|''.
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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 zero or more \fIpieces\fR, 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 \fIatom\fR possibly followed by ``*'', ``+'', or ``?''.
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An atom followed by ``*'' matches a sequence of 0 or more matches of the atom.
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An atom followed by ``+'' matches a sequence of 1 or more matches of the atom.
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An atom followed by ``?'' matches a match of the atom, or the null string.
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.PP
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An atom is a regular expression in parentheses (matching a match for the
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regular expression), a \fIrange\fR (see below), ``.''
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(matching any single character), ``^'' (matching the null string at the
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beginning of the input string), ``$'' (matching the null string at the
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end of the input string), a ``\e'' followed by a single character (matching
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that character), or a single character with no other significance
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(matching that character).
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.PP
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A \fIrange\fR is a sequence of characters enclosed in ``[]''.
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It normally matches any single character from the sequence.
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If the sequence begins with ``^'',
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it matches any single character \fInot\fR from the rest of the sequence.
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If two characters in the sequence are separated by ``\-'', this is shorthand
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for the full list of ASCII characters between them
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(e.g. ``[0-9]'' matches any decimal digit).
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To include a literal ``]'' in the sequence, make it the first character
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(following a possible ``^'').
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To include a literal ``\-'', make it the first or last character.
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.SH "CHOOSING AMONG ALTERNATIVE MATCHES"
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.PP
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In general there may be more than one way to match a regular expression
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to an input string. For example, consider the command
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.CS
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\fBregexp (a*)b* aabaaabb x y\fR
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.CE
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Considering only the rules given so far, \fBx\fR and \fBy\fR could
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end up with the values \fBaabb\fR and \fBaa\fR, \fBaaab\fR and \fBaaa\fR,
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\fBab\fR and \fBa\fR, or any of several other combinations.
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To resolve this potential ambiguity \fBregexp\fR chooses among
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alternatives using the rule ``first then longest''.
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In other words, it considers the possible matches in order working
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from left to right across the input string and the pattern, and it
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attempts to match longer pieces of the input string before shorter
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ones. More specifically, the following rules apply in decreasing
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order of priority:
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.IP [1]
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If a regular expression could match two different parts of an input string
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then it will match the one that begins earliest.
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.IP [2]
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If a regular expression contains \fB|\fR operators then the leftmost
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matching sub-expression is chosen.
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.IP [3]
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In \fB*\fR, \fB+\fR, and \fB?\fR constructs, longer matches are chosen
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in preference to shorter ones.
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.IP [4]
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In sequences of expression components the components are considered
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from left to right.
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.LP
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In the example from above, \fB(a*)b*\fR matches \fBaab\fR: the \fB(a*)\fR
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portion of the pattern is matched first and it consumes the leading
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\fBaa\fR; then the \fBb*\fR portion of the pattern consumes the
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next \fBb\fR. Or, consider the following example:
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.CS
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\fBregexp (ab|a)(b*)c abc x y z\fR
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.CE
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After this command \fBx\fR will be \fBabc\fR, \fBy\fR will be
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\fBab\fR, and \fBz\fR will be an empty string.
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Rule 4 specifies that \fB(ab|a)\fR gets first shot at the input
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string and Rule 2 specifies that the \fBab\fR sub-expression
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is checked before the \fBa\fR sub-expression.
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Thus the \fBb\fR has already been claimed before the \fB(b*)\fR
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component is checked and \fB(b*)\fR must match an empty string.
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.SH KEYWORDS
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match, regular expression, string
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