Feathers I think, you just can't. Regexp are probably not that powerful, and as far as I know there is no way to reuse "Date 1" in "Date 2" constraints. That would be a nice feature, though, something like
date "Date 2" >"@Date 1"
Back to reality. It can probably be done with a piece of javascript. But there is no guaranty that the browser will execute it, so, in the end it must be checked in PHP. And this in turn requires some PHP code written.