<sarcasm>
Write a regex that determines if an arbitrary set of parentheses are balanced within a string. :-)
</sarcasm>

/\/\averick

$re = qr/(?:(?>[^()]*)|\((??{$re})\))+/;
print 'I (pity (the (fool) who) dont) know (regexes)' =~ /^$re$/;
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print