$re = qr{
          \(
          (?:
            (?> [^()]+ )    # Non-parens without backtracking
           |
            (??{ $re })     # Group with matching parens
          )*
          \)
        }x;


##</code><code>##

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$