if user enters HTML in Textbox - want to display message saying not allowed??

hi,
I have a textbox in my formview edit item template.
The user should not be able to enter HTML - if they do I want to warn them in a pop up or label.
 
I have read about page validate requests but this just stops the exception and the user can enter HTML.
Could somebody demonstrate a solution  in C sharp -is it possible to use regular expression control.
Many thanks
Prontonet
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