for example, a zip code field only contains digits and up to 5 digits

I hope that's not a compulsory field - don't forget about those of us outside the USA who have different systems for our zip codes, for example in the UK a valid post code is 1* or 2 letters, followed by 1 or two digits, a space, a digit and 2 more letters. If I were trying to register for your site and you made me enter a zip code your way, I'd just have to make something up ;)

* Normally 2 letters, but London just had to be different