CODE

 do
                                    {
                                        Console.WriteLine("What is the Airline Code?");

                                        AC = Console.ReadLine();
                                        if (AC.Length != 2)
                                        {
                                            Console.WriteLine("Error. Airline Code must be exactly 2 characters only.");
                                        }
                                        else
                                        {
                                            if (!r.IsMatch(AC))
                                            {
                                                Console.WriteLine("Error. Airline Code must be 2 alphanumeric characters only.");
                                                ctr = 0;
                                            }
                                            else
                                            {
                                                if (Char.IsNumber(AC[0]) && Char.IsNumber(AC[1]))
                                                {
                                                    Console.WriteLine("Error. Cannot be both numbers.");
                                                    ctr = 0;
                                                }
                                                else
                                                {
                                                    ctr = 1;
                                                }


                                            }

                                        }
                                    } while (AC.Length != 2 || ctr == 0);