If you just need a quick regex for matching some uncertainly-delimited numeric dates out of a file or something, (and I take it you are not super familiar with regular expressions) check out: http://www.regular-expressions.info/dates.html

(Btw, that example matches valid dates. I can't really think of why you wouldn't want them to be valid, since someone else is already writing the expression for you :) )

Note that that site also has some other useful info about regular expressions.

If you are doing anything beyond some very basic matching, definitely go after the Date:: modules as mentioned above!