BASH validate user input

Hey, im trying to validate a user input and need some help. The input needs to be just a single letter. Im using a case to so this eg:

read answer
case $answer in
     [A-Za-z]*) echo "OK"
        ;;
     [0-9]*) echo "This is a number"
                read answer
        ;;
        *)   echo "Please enter a letter ! !"
              read answer
        ;;
esac

I need to stop multiple letters and repeat letters being entered. Any ideas?? Thanks for any help.

this appears to be much the same as the question asked in

you guys doing the same course

And to answer your question...you could do it in a variety oif ways...maybe you could look at the length of the input...or use dd to "read" in one char at a time...etc

Ha thanks, that sorted it out!