#! perl

use strict;
use warnings;

my $idx;
my $old;
my $cnt;

while (<DATA>) {
                        ++$idx; # Count loop
                     $old = $_; # Save $_
    $cnt = s/(?:)/x/g; s/\n//g; # Change $_ 

    print<<__X__ if $old =~ /(?:)/;
Line $idx - seen $old ... but is now $_ 
 ... and \(?:) matched in $cnt places
__X__
}
__DATA__


Line 3

Line 5