Problem solution for "Balanced smileys" at Facebook Hacker Cup 2013 Qualification round. https://www.facebook.com/hackercup/problems.php?pid=403525256396727&round=185564241586420

smileys.pl

use strict;
use warnings;
use 5.10.00;


my $regexp = qr/ 
^( 
(?: 
[a-z :]|:\(|:\)|\((?1)\)
)* 
)$ 
/x;

<>;
my $i;
for(<>){
    $i++;
    say /$regexp/?"Case #$i: YES":"Case #$i: NO";
}