4 digits

using System;
using System.Text.RegularExpressions;

    class FourDigitNumber
    {
        static void Main()
        {
           while(true)
           {
            Console.Write("Enter a 4 digit number (can't start with 0) : ");
            string str = Console.ReadLine();
            Console.WriteLine();
            int n;
            int numb = int.Parse(str);
            Regex regex = new Regex("^[1-9][0-9][0-9][0-9]*$");
            if (int.TryParse(str, out n) && (regex.IsMatch(str)))
            {
               //Calculates the sum of the digits
                int sum = 0;
                while (n != 0)
                {
                    int remainder;
                    n = Math.DivRem(n, 10, out remainder);
                    sum += remainder;
                }
                Console.WriteLine("Sum of digits : {0}", sum); // Prints the sum of the digits
                //Reverse the numbers

                int reverse = 0;
                while (numb > 0)
                {
                    int rem = numb % 10;
                    reverse = (reverse * 10) + rem;
                    numb = numb / 10;

                }
                Console.WriteLine("Reversed : {0}", reverse);
                //Last digit at first position
                char[] str2 = str.ToCharArray();
                Console.WriteLine("Last digit in front : {3}{0}{1}{2}", str2[0], str2[1], str2[2], str2[3]);
                //Second and third digits exchanged
                Console.WriteLine("second and third digits exchanged : {0}{2}{1}{3}", str2[0], str2[1], str2[2], str2[3]);
                Console.WriteLine();
            }
            else
            {
                Console.WriteLine("Not a 4 digit number : {0}", str);
                Console.WriteLine();
            }
        }
    }

}