My favourite online resource
My favourite list or newsgroup would have to be the Regex list over at ASPAlliance:
http://www.aspalliance.com/lists/SignUp/list.aspx?l=68&c=16
I've found that it's the best place to get involved in interesting regex discussions - and some debates too :).
A couple of new faces have appeared of late. One notable name is Wayne King (Microsoft). It's always good to test your knowledge with people that have Wayne's level of knowledge.