Albi,
Welcome to Bytes!
I see you are talking about input masks. I am not sure there is a way to do that with input masks unless you know exactly how many letters the input string is. That is, if you know that the input string is eight characters and only eight characters, you could do this. However, if the string is a random length, you would have to examine the string after the fact, and change the last character. You would have to determine the ASCII value of the last character and then adjust accordingly. You would adjust that character by subtracting 32 from its ASCII value.