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

why look ahead ?= isn't working if we give pattern after it. For ex, 
if($str=~/([0-9]+)(?=welcome)to india/) {
print "Matched\n";
}

[download]
My input string is as follows: 1welcometo india. Why it isn't getting matched? But,it gets matched when I used .* instead of "to india" after (?=welcome) in the pattern.

Replies are listed 'Best First'.
Re: Look Ahead:
by BrowserUk (Patriarch) on Apr 02, 2010 at 09:38 UTC

    Because lookaheads are "zero length" matches. That is they do not consume any of the string that they match. So, in your example, 

    1welcometo india.
^...............is matched and consumed by ([0-9]+)
 ^^^^^^^........is matched but not consumed by (?=welcome)
 ^..............so "to india" tries to match here and fails.

    [download]

    This does match though: 

    $str = '1welcometo india';;

$str =~ m/([0-9]+)(?=welcome)welcometo india/ and print 'matched';;
matched

    [download]

    It's called 'lookahead' because it looks ahead but doesn't travel there.

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
[select]
      Yeah,Thank you very much.I learnt about it and I also learnt about look behind operator in the regular expression.
[reply]
Re: Look Ahead:
by rmflow (Beadle) on Apr 02, 2010 at 08:39 UTC
    because it must be /([0-9]+)(?=welcome)welcometo india/ to match
[reply]
[d/l]
      I knew how to match the string but I want to know why it is working like that?Explain me?
[reply]
        Because sometimes you need to look ahead without actual matching.

        For example: 
        $num = scalar reverse "1000000000000";
$num =~ s/(\d\d\d(?=\d))/\1,/g;
print scalar reverse $num;

        [download]
        will print 1,000,000,000,000
[reply]
[d/l]
Re: Look Ahead:
by Anonymous Monk on Apr 02, 2010 at 08:52 UTC
    If you 
    use re 'debug';

    [download]
    you get to see how the regex engine does matching
[reply]
[d/l]

Back to Seekers of Perl Wisdom