Validating a single char to be one of two chars - RESOLVED

I need a response to an input form to be P or K. How can I validate that input? I looked at the list of methods for Validator but none fit. It has to be a single char and it has to be P or K, nothing else. I can do the single char and maxlength: 1 but I do not know how to do a specific char.

Thanks for the help.

Mike