using System;

public class Program

{

    public static void Main() {

        Console.WriteLine(("Is \'\' valid: " + IsValidTicketNumberFormat("").ToString()));

        Console.WriteLine(("Is \'1\' valid: " + IsValidTicketNumberFormat("1").ToString()));

        Console.WriteLine(("Is \'12\' valid: " + IsValidTicketNumberFormat("12").ToString()));

        Console.WriteLine(("Is \'123\' valid: " + IsValidTicketNumberFormat("123").ToString()));

        Console.WriteLine(("Is \'1234\' valid: " + IsValidTicketNumberFormat("1234").ToString()));

        Console.WriteLine(("Is \'12345\' valid: " + IsValidTicketNumberFormat("12345").ToString()));

        Console.WriteLine(("Is \'\' valid: " + IsValidTicketNumberFormat("").ToString()));

        Console.WriteLine(("Is \'a\' valid: " + IsValidTicketNumberFormat("a").ToString()));

        Console.WriteLine(("Is \'ab\' valid: " + IsValidTicketNumberFormat("ab").ToString()));

        Console.WriteLine(("Is \'abc\' valid: " + IsValidTicketNumberFormat("abc").ToString()));

        Console.WriteLine(("Is \'abcd\' valid: " + IsValidTicketNumberFormat("abcd").ToString()));

        Console.WriteLine(("Is \'abcde\' valid: " + IsValidTicketNumberFormat("abcde").ToString()));

        Console.WriteLine(("Is \'\' valid: " + IsValidTicketNumberFormat("").ToString()));

        Console.WriteLine(("Is \'a1\' valid: " + IsValidTicketNumberFormat("a1").ToString()));

        Console.WriteLine(("Is \'ab1\' valid: " + IsValidTicketNumberFormat("ab1").ToString()));

        Console.WriteLine(("Is \'abc1\' valid: " + IsValidTicketNumberFormat("abc1").ToString()));

        Console.WriteLine(("Is \'abcd1\' valid: " + IsValidTicketNumberFormat("abcd1").ToString()));

        Console.WriteLine(("Is \'abcde1\' valid: " + IsValidTicketNumberFormat("abcde1").ToString()));

        Console.WriteLine(("Is \'abc123\' valid: " + IsValidTicketNumberFormat("abc123").ToString()));

        Console.WriteLine(("Is \'abc1234\' valid: " + IsValidTicketNumberFormat("abc1234").ToString()));

        Console.WriteLine(("Is \'IN123\' valid: " + IsValidTicketNumberFormat("IN123").ToString()));

        Console.WriteLine(("Is \'INC123\' valid: " + IsValidTicketNumberFormat("INC123").ToString()));

        Console.WriteLine(("Is \'RITM123\' valid: " + IsValidTicketNumberFormat("RITM123").ToString()));

        Console.WriteLine(("Is \'CHG123\' valid: " + IsValidTicketNumberFormat("CHG123").ToString()));

        Console.WriteLine(("Is \'PRB123\' valid: " + IsValidTicketNumberFormat("PRB123").ToString()));

        Console.WriteLine(("Is \'ANY1\' valid: " + IsValidTicketNumberFormat("PRB123").ToString()));

    }

    private static bool IsValidTicketNumberFormat(string StringToCheck)

    {

        bool retval = false;

        try

        {

            char[] charArray = StringToCheck.ToCharArray();

            char stringLastCharacter = charArray[(StringToCheck.Length - 1)];

            if ((StringToCheck.Length < 4))

            {

                // At least 4 chars

                retval = false;

            }

            else if ((char.IsNumber(stringLastCharacter) == false))

            {

                // must end in a #

                retval = false;

            }

            else

            {

                retval = false;

                for (int i = 0; (i <= 2); i++)

                {

                    if (char.IsLetter(charArray[i]))

                    {

                        retval = true;

                    }

                    else

                    {

                        retval = false;

                        break;

                    }

                }

            }        

        }

        catch (Exception e) {

            retval = false;

        }

        return retval;

    }

​

}