Dude, you asked the question I've been working on for a couple of weeks. I invite those commenting to give a date that breaks this. Now note that this works for the years 1000-9999, is Proleptic Gregorian and assumes that we won't change how leap-years work until the year 9999 ;)
^(?:(?:(?:0[1-9]|1\d|2[0-8])(?:0[1-9]|1[0-2])|(?:29|30)(?:0[13-9]|1[0-2])|31(?:0[13578]|1[02]))[1-9]\d{3}|2902(?:[1-9]\d(?:0[48]|[2468][048]|[13579][26])|(?:[2468][048]|[13579][26])00))$
Debuggex Demo
