Bookmarked Amit Agarwal (labnol.org)

Google Forms makes it relatively easy to add such advanced date validation rules to individual fields through Regular Expressions (or regex or regexp). Think of them as search patterns and every character entered in a form field is matched against that pattern – the form can only be submitted if the patter and the user-input matches.

Amit Agarwal provides a list of Regex formulas to use to validate data within Google Forms.