client side validation

here is a nice article about client side validation.

form data validation is very important. a mix of client and server side validation gets you the best of both worlds. for users with javascript enabled (the vast majority), it gives immediate response and saves the server from trying to process obviously bad data. however, server side validation will catch errors for those who have turned off javascript. additionally, more complex validation can be done (say, a credit card number) on the server side.