Where do you record validation rules for form data in a web application?

Question

Say you have a web form with some fields that you want to validate to be only some subset of alphanumeric, a minimum or maximum length etc.

You can validate in the c

Answer 1

1. Client-side validation for good, responsive user interfaces
2. Server-side validation because client-side code can be bypassed or modified and so can't be trusted
3. Database validation if you have multiple apps feeding into one db. It's important here as then a change to validation is automatically propagated to all apps and you don't lose data consistency.