Your (\d+) works ... if you're willing to accept bad data -- for example, data with a one-, three- or (> four)-digit year. You'd do well to read perlretut and friends re quantifiers.

Insisting on a regex is not always the best idea. That's implicit in CountZero's "absolutely, positively certain...." <Update> and in his excellent followup, written as I piddled about with this note.</update>

And, even more to the point, insisting on a regex suggests that you failed to grok the replies to your earlier String (date) to time (integer).