Dynamically change the regular expression in RegularExpressionValidator?

[BH]

I have a textbox allows user to enter either an integer number or a string
based on the values from two radio boxes (e.g. search by ID, or search by
name). Is there a way to change the regular expression in the
RegularExpressionValidator that validates user input in the textbox based on
which radio box is selected? I'm looking for a way to perform the
validation on the client side in IE.

Thanks