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.
"I'd rather go naked than blow up my ass"

In reply to Re: Look Ahead: by BrowserUk
in thread Look Ahead: by nvivek

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