[Zope] Client Side Form Field Validation - How to ?

Hi, the subject says it (almost) all. Taken for granted that one should validate input "also" on the server, everybody (even me) knows many reasons (no post, faster and more user-friendly messsages, ...) for doing something on the client. The first thing that comes to (my) mind is Java (o JavaScript ?). Or would be possible to write some Python (Jython) ? Anyhting already available ? Anything "declarative" (like usign regexp) ? TIA, Luca