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From: des0mpw@colman.newcastle.ac.uk (M.P. Ward)
Newsgroups: comp.lang.perl
Subject: Re: Matching parentheses
Message-ID: <1990Nov29.150720.14766@newcastle.ac.uk>
Date: 29 Nov 90 15:07:20 GMT
References: <1990Nov28.150131.28981@ugle.unit.no>
Sender: news@newcastle.ac.uk
Organization: Computing Laboratory, University of Newcastle upon Tyne, UK, NE1 7RU
Lines: 55

The trick is to repetedly remove parantheses (and their contents) which don't
contain parentheses. If the result has any parantheses left then the match
fails. Another approach is to count +1 for (, -1 for ) and 0 for anything
else. The count must never go negative and must finish at zero.

Here's my perl code for checking {} balancing in LaTeX files:
(I have a similar program to check \begin...\end matching)

#! /usr/bin/perl
# check begin/end pairs in latex file

($myname = $0) =~ s|(.*/)*||;   # strip path component from name
$Usage = "Usage: $myname [file]\n";

if ($#ARGV == 0) {
  $file = "< $ARGV[0]";
  if (! -f $ARGV[0]) {
    die "File not found: $ARGV[0]\n";
  }
} elsif ($#ARGV == -1) {
  $file = "";
} else {
  die "$Usage";
}
open (GOODS,"qtoa $file | egrep -n '{|}' |") ||
                die "qtoa/egrep failed: $!";
undef $/;
$* = 1;
$_ = <GOODS>;   # slurp in begin/end pairs
close GOODS;

# remove \{ and \} (which don't have to match):
s/\\{//g;
s/\\}//g;
 
# repeatedly collapse {...} pairs
while (s/{[^{}]*}//g) {
  1;
}

# eliminate junk lines (with no { or }):
while (s/^[^{}]*\n//g) {
  1;
}
 
# print the result:
print;


			Martin.

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