public class Inputs
{
   /**
      Gets the choice that the user provided, or null if the
      user didn't provide a valid choice.
      @param input the user input
      @param maxChoice the maximum valid choice, e.g. "D" if there
      are four choices.
      @return the user input if it was a valid choice (i.e. length 1
      and between "A" and maxChoice), null otherwise
   */
   public String getChoice(String input, String maxChoice)
   {
      String regex = "[A-" + maxChoice + "]";
      return (input.matches(regex)) ? input : null;
   }
}