I'd worry about the documentation that states, “WARNING: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice.”

So, it might not work at all on Perl 5.8 when it comes out.

If I had to write a good efficient implementation without using ??{}, I'd probably take what I showed you and do it in chunks rather than one char at a time. That is, chop the string into chunks where each chunk is a single ballancing char, or the whole run between them. Then perform the algorithm on that list, rather than every single char.

Perhaps there should be a feature that elegantly does P.D.A., not just regexp, without getting into all the complexity of full LALR grammar. Perhaps stacking could be added as regex extenstions without generalized code blocks: a way to push something, and zero-width assertions and subexpressions that can refer to the stack top, and pop.

—John