using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var pattern1 = @"^(.*?)\.mytest.nl/(.*?)$";

        var pattern2 = @"$2.masc.nl/$1";

        var input = "www.mytest.nl/this-is-a-test";

        var output = Regex.Replace(input, pattern1, pattern2);

        Console.WriteLine(input);

        Console.WriteLine(output);

    }

}