using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        String guid = "onload=309DE1FA-6C71-4C23-8BFE-06120091195E";

        Console.WriteLine(Regex.Replace(guid, "[^0-9a-fA-F\\-]", String.Empty));        

​

    }

}