Regular Expression Question

VingInMedina has asked for the wisdom of the Perl Monks concerning the following question:

Can anyone please explain to me what the first regular expression test will match, but the second one won't?

#!/usr/bin/perl

use strict;
use warnings;

my $a = 'abc == 123';
if ($a =~ /^(.*?)==(.*)$/) {
   print "This will match\n";
   print "First: $1\nLast: $2\n";
}
if ($a =~ /^(.*?)\b==\b(.*)$/) {
   print "This will not match\n";
   print "First: $1\nLast: $2\n";
}
[download]

I need to use the non-greedy match in the first part of the expression becuase I need to find the first occurrence of the '=='.

Comment on Regular Expression Question Download Code

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Re: Regular Expression Question by davido (Cardinal) on May 02, 2012 at 18:36 UTC
The second one requires a word boundary to exist immediately to the left and to the right of the `==`. Your string, "`abc == 123`" has white-space immediately to the left and right of the `==`. Consequently, the word boundaries are between 'c' and 'space', and again between 'space' and '1'. There is no boundary between 'space' and '=', nor between '=' and 'space'. Thus, the second RE cannot match against your target string. To put it a little more clearly (I hope): `abc[boundary](space)==(space)[boundary]123 <---- What your string loo +ks like. abc(space)[boundary]==[boundary](space)123 <---- What you're trying t +o match.` [download] That second case is impossible; there can never be a word boundary between a space and an equals sign. Rather than using non-greedy quantifiers, why don't you instead specify what would be considered legal on the left-hand side of the `==`. It's likely that your left-hand side needs to be an identifier of some sort. If that's the case, specify what characters are legal: `m/^(\w+)\s==\s(\w+)$/` [download] If either side can contain quoted literals that may embed '==', you're going to have to use a real parser instead. Dave	[reply] [d/l] [select]
Re^2: Regular Expression Question by VingInMedina (Acolyte) on May 02, 2012 at 19:52 UTC
Thanks Dave I think I got it	[reply]