I've just implemented an experimental regex extension, which allows you to do things like match balanced parentheses, without having to embed any code in your regex. The implementation isn't in perl (yet?), but this is still a firm Perl topic, and it raises questions about what features Perl's regexes ought to have.

I wrote a web page about it, but here's the rough idea. You can treat each parenthesised group in your regex as if it were a little subroutine, and make calls to it from within your regex. The power comes from the fact that you can make recursive calls, so you can write

/^([^()]|\((?1)*\))*$/
[download]

to match strings which have balanced parentheses, for example.

My favourite so far is a regex to detect palindromic sentences, ignoring spacing and punctuation.

/^\W*(?:((.)\W*(?1)\W*\2|)|((.)\W*(?3)\W*\4|\W*.\W*))\W*$/i
[download]

If you want to try it out, follow the instructions here. Let me know what you think.

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