Change What The User Types In A Text Field?

I'm trying to change what the user types in a text field if they hit a certain key combination. For example, "cx".

This code checks for the letter "x" to be pressed, then checks to see if the second to last letter is the letter "c". If it is, then it deletes the letter "x" and replaces the "c" with the letter "ĉ".

Code:
// This code checks for a certain keypress combo. For example cx, Cx, gx, Gx, etc...
// The hard to type esperanto characters we aim to insert.
var esperanto_c = "ĉ";

[Code].....