negative lookbehind heartburn

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

How can I match a string which contains "weird" but does not have "odd" somewhere in the string before it? I have tried negative lookbehind as shown below with no success. The example only works if "odd" immediately precedes "weird".

my $str = "odd an very weird receiver";

if ($str =~ /(?<!odd)weird/)
{
    print "hit";
}
[download]

Many Thanks!

Comment on negative lookbehind heartburn Download Code

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•Re: negative lookbehind heartburn by merlyn (Sage) on May 17, 2002 at 21:48 UTC
That's a tough thing to specify. You have to "inchworm": `my $str = "..."; if ($str =~ /^(?:(?!odd).)*weird/s) { # we found "weird" without looking at odd }` [download] -- Randal L. Schwartz, Perl hacker	[reply] [d/l]
Re: •Re: negative lookbehind heartburn by amazotron (Novice) on May 17, 2002 at 22:17 UTC
Yup, that did it. Many thanks, Randall!	[reply]
Re: negative lookbehind heartburn by perlplexer (Hermit) on May 17, 2002 at 22:08 UTC
Do you have to use lookbehinds? Sometimes negating the problem statement simplifies things a little `unless ($str =~ /odd.*?weird/){ print "hit!\n"; }` [download] --perlplexer	[reply] [d/l]
Re: Re: negative lookbehind heartburn by thelenm (Vicar) on May 17, 2002 at 22:38 UTC
Unfortunately this will also match strings that don't contain the substring "weird". Without using ~~lookbehind~~ lookahead, this problem will probably require two tests (one to test for "weird" and one to test for "odd"), since we only want strings containing "weird" but not "odd" before "weird". That was an odd sentence, how weird. :-)	[reply]
Re: Re: Re: negative lookbehind heartburn by perlplexer (Hermit) on May 17, 2002 at 22:49 UTC
Yep, you are right. Thanks for pointing that out. --perlplexer	[reply]