Writing regular expressions

[Rakesh]

Im using a cutom control to try to validate a text box to ensure that on the client side a validation expression is present to prevent the user from posting back html in their text box. I was only partially successful with some of the regular expressions i came up with. There are a few contraints. There can be no roundtrip to the server. The server process halts reports possible dangerous Html. Ive thought about using javascript to combat the problem
Any suggestions???