How can I limit user input to a single alpha character in c#?

Question

I'm using the following code to accept user input. I want to limit user input to a single alpha (a-z) character only. I'm finding a lot of validations using IsNumber to validate integer input, and a lot of information about using a regex on an input String, but I've been unable to uncover how I would be able to restrict input possibilities with this code. Can someone point me in the right direction?

    public char promptForGuess()
    {
        Console.Write("\nGuess a letter: ");
        String pre = Console.ReadKey().Key.ToString();
        string pre2 = pre.ToUpper();
        char pre3 = Convert.ToChar(pre2);
    }

Answer 1

You cannot limit the user only put in a-z chars on the console - you have to check the input, he can write in any character (just think about when the input is redirected to your program from a file with <, e.g. yourapp.exe < input.dat ).

But its easy to check a character is lowercase a-z letter. E.g. with plain, ASCII, C tactic (I will use your defined variables):

if('A' <= pre3 && pre3 <'Z') { // pre3 was made upper in your code
    // input OK
} else {
    // input NOK
}

With regex:

Regex r = new Regex(@"^[a-zA-Z]$");
return r.IsMatch(pre);

If you cannot allow case-insensitive characters, just change the code I wrote.

Anyway, I think you need Console.Read() (ReadKey also read keys like arrows, F1-F12 etc..., so ALL keys, even tab and caps lock). Refer to MSDN: http://msdn.microsoft.com/en-us/library/system.console.read.aspx

And maybe you should use this function, if you would support unicode letters: http://msdn.microsoft.com/en-us/library/yyxz6h5w.aspx Note that unicode letters are usually not one bytes! But char can store it. These letters are for example beautiful Hungarian letters with acutes and these king of things: á, é, ő, ű, ö, ü etc (but also French have a lot, and also Dutch etc...)

Answer 2

For judging a valid string, you could judge by

str.length() == 1 && str[0] >= 'a' && str[1] <= 'z'

and for restricting input possibilities, you could write a loop that loops if the input is invalid.

pre = read();
while (!valid(pre))
  pre = read();

Answer 3

why don't you use Regex

if (Regex.IsMatch(pre[0].ToString(), @"[A-Za-z]"))
   {
       //do someting
   }
   else
   {
       //do someting
   }