Add date validation

[Niek]

So, I know the regex works in playground, also in tests it passes, however when used in a project to transpile it doesn't or at least it does not on my machine and I have no clue why. I changed it as you suggested but maybe pull this branch and test it locally to see if it works for you. For now I left the workaround so it works.

Originally posted by @bnenu in #8 (comment)