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

In reply to Re^3: Look Ahead: by rmflow
in thread Look Ahead: by nvivek

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