A RadioButtonList has three items. When choosing either of the first two options, I want to verify a separate textbox has something in it (namely an integer value); the third option in the RadioButtonList will not fire the validation.

Will any of the built-in validators do that?

I created a CustomValidator for this, and also included javascript for client side validation. It all works with one thing:
I have the CustomValidator's Text property set to *. When checking one of the two radio buttons that fire the validation, it puts the cursor in the textbox and shows the * as expected. However, when I enter a valid value into the textbox and move on to another control on the form, the * stays (submitting the form does work even though * is still there; may be confusing for users).

Compare this behavior to say the RequriedFieldValidator on a textbox. When the validated textbox is empty and it loses focus the * immediately appears and the focus stays on the textbox. But when a valid value is entered and focus is moved to another form control, the * disappears. How can I make my CustomValidator remove the * after a valid value is entered?