We all know by now that regexes have problems with arbitrarily delimited data. This little snippet demonstrates a small subroutine that determines whether or not your delimiters are balanced. It also allows you specify an escape character, if needed.

Just pass the sub your string, left delimiter, right delimeter, and an optional escape character.

As a side note, any regexophiles want to tell me how I can get that frickin' dot star out of there? I worked on it for hours, but that was the easiest solution :(

I remember seeing an earlier version of this in the form of while ( $str =~ s/\(([^)]+)\)/$1/g ){};It didn't allow for escaped characters and I can't remember where I found it. If any monks know the origin, I'd like to be able to credit the person who had the initial idea.

#!/usr/bin/perl -w
use strict;
my @strings = ( '(a(a(b)a)a)',         # balanced
                '(a(a(b)a)a)a)',       # too many right parens
                '(a(a(b)a\)a)a)',      # balanced -- one right paren i
+s escaped
                '(a(a\(a(a(b)a)a)',    # unbalanced -- still have too 
+many left parens
                '(a(a\(a(b)a)a)' );    # balanced -- one left paren is
+ escaped
foreach my $string ( @strings ) {
    my $balanced = balanced_delimiters( $string, "(", ")", "\\" );
    print "Delimiters in $string are ";
    if ( $balanced ){
        print "balanced.\n";
    } else {
        print "not balanced.\n";
    } 
}
sub balanced_delimiters {
    # $escape is optional.  If not supplied, no escape character will
    # be recognized.
    my $str    = shift;
    my $left   = quotemeta shift;
    my $right  = quotemeta shift;
    my $escape = quotemeta shift;
    my $unescapedLeft  = "(?<!$escape)$left";
    my $unescapedRight = "(?<!$escape)$right";
    my $middle = ".";  # AAARRRRGGGGHHHH!!!!
    my $regex  = "$unescapedLeft($middle*?)$unescapedRight";
    while ($str =~ s/$regex/$1/gs){};
    return $str =~ /$unescapedLeft|$unescapedRight/ ? 0 : 1;
}

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In reply to Determining if you have balanced delimiters by Ovid

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