[am]king ~/a$ perl -we 'do { $u = 0; print "$_: ", /^((??{$u++;""})(?:a|aa))+$/ ? "hit" : "miss", ", parens called $u times\n" } for qw"aaaaaaaaa aaaaaaaaab";' aaaaaaaaa: hit, parens called 10 times aaaaaaaaab: miss, parens called 143 times