How to make a textbox accept only "c" and "d" letters?

How to make a textbox accept only "c" and "d" letters?

I want the textbox to accept only either a "d" or a "c" nothing else. If a user enters an "a" or "b" it must be wiped. Also, is it possible to limit the textbox to English language only? I don't want the user to enter an alphabet in another language by mistake. I want it to stick to "c" or "d" and nothing else.