using System;

using System.Text;

using System.Text.RegularExpressions;                   

public class Program

{

    public static void Main()

    {

        string input = "DK5AW-109045";

        string SubString;

        Console.WriteLine(Regex.Match(input,@"(.{4})\s*$"));

        // or

        Console.WriteLine(input.Substring(input.Length - 5));

​

         SubString = input.Substring(input.Length-3);

​

​

        Console.WriteLine(input.Substring(input.Length-3));

    }

}