Regular expression (regEx) is a string of text that specifies the format of a desired response. Regular expression rules can be written to limit responses to a 4-digit zip code (39387), the format of a phone number (123-456-7890), or state abbreviations (FL).
Response validation in Google Forms supports regular expression rules which means you can write a rule for virtually any type of response.
To get started using regular expressions, check out RegExLib
for a list of common rules.