RegularExpressionValidator in popup windows

i am using RegularExpressionValidator inside Ajax popup.
inside this poupup i have a cancel button which will exist the popup in whatever state it is.
problem: when ValidationExpression is false i am not able to exits the popup by hitting cancel button.
i always have to remove the vale from the textbox and exist from poupup.
question:
how can i achieve my goal, not always emptying the textbox...?
thanx 
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