Question regarding onkeypress event handler and KEYS statement

there is probably an easy way around this but I haven't found it yet.

I have a form that has a validation triggered by an onkeypress event 

" onkeypress="RunRes()" type="

I can get the event to fire only if I send the key stroke of a letter or number and not by sending not visible characters the issue is that the letter is appended to the end of the data entered into the field that I am data-driving and then messes up my validation within the application.   Is there an easy way to wipe out the char without repeating the entire text setting or other way to trigger the keypress without actually sending a character?