On: I hate the Pumping Lemma

[kingkilr]

A conversation I had the other week:

Him: Do you know how to check if it's possible to write a regular expression for this?

Me: Either create an automata and then it's demonstrably possible, or apply the pumping lemma to prove it's impossible?

Him: No. Dare Stackoverflow to write it.

[TheZenPsycho → kingkilr]

the answer if you're using perl or pcre, is probably yes, but for the love of djikstra don't ever do it. This was on HN a while back:

http://nikic.github.io/2012/06/15/The-true-power-of-regular-...

[mcguire → kingkilr]

This is a trivial corollary of the Basic Technique For Finding the Correct Answer to a Question on the Internet:

Strongly assert a wrong answer on the Internet.

The correct answer will appear shortly.

[im3w1l → kingkilr]

Some regexp dialects are actually Turing complete from what I've heard.

EDIT: Yes this seems to be wrong

[kingkilr → im3w1l]

I'm not sure if they're turing complete, however many many many common regexp dialects (as seen in perl, ruby, python, etc.) are more powerful than what you learned as "regular expressions" in CS class, they're at least as powerful as a PDA.

[chimeracoder → kingkilr]

> they're at least as powerful as a PDA.

PCREs (ie, the regular expressions that Perl uses) are NP-hard, since they allow backreferences.

[kazagistar → chimeracoder]

I think you are confusing complexity and computability class.

[anaphor → kingkilr]

Do you mean that some regex implementations allow you to parse context-free grammars?

[hyperpape → anaphor]

Most regex implementations allow you to parse some context-free grammars, and even some non-context-free grammars. But they don't let you parse all context-free grammars.

reference: http://cstheory.stackexchange.com/questions/1047/where-do-mo...

[anaphor → hyperpape]

Yeah, I should've said "all". I know that you can describe regular grammars using languages designed for CFGs, like BNF.