in reply to back translating regular expressions
". . . and then I fed it 'a.*b' and my computer just locked up." See also "Halting Problem, The". :)
Having said that, if you can wait a few more weeks until Higher Order Perl (ISBN 1558607013) comes out I believe one of the stream generator examples is generating possible matches for a regex. That lets you generate matches one at a time in a manner where you can stop at any time.