Best way to validate a date

Hi all, I'm looking to validate a date submitted through a form and I cannot decide which would be the best way to do it. The form of the date needs to be: YYYY-MM-DD. At the moment I'm torn between using a regex, something like: 20[\d][\d]-[0-1][\d]-[0-3][\d] or using the checkdate() function. Does anyone have any pros and/or cons to implement one method over the other or other methods you may have used? Thanks, David