empty a textbox after comparevalidation

How can I either hightlight or empty a textbox after compareValidation if the input is not valid. (I use C# in ASP)
For example, if a textbox is double/Integer data only, After I type in some string, it will show the error message "Must be a number etc" and highlight the text I just typed OR empty the textbox and highlight the textbox?

Thanks.
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