Look Ahead:

[nvivek]

why look ahead ?= isn't working if we give pattern after it. For ex,

if($str=~/([0-9]+)(?=welcome)to india/) {
print "Matched\n";
}

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.

[BrowserUk]

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.

This does match though:

$str = '1welcometo india';;

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

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

[nvivek]

Yeah,Thank you very much.I learnt about it and I also learnt about look behind operator in the regular expression.

[rmflow]

because it must be /([0-9]+)(?=welcome)welcometo india/ to match

[nvivek]

I knew how to match the string but I want to know why it is working like that?Explain me?

[rmflow]

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;

will print 1,000,000,000,000

[Anonymous Monk]

If you

use re 'debug';

you get to see how the regex engine does matching