using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var  regex = new Regex(@"(\d{2})");

        string aString = "040204220442040004200404020602260246";

        string replaced = regex.Replace(aString, "x$1 ");

        Console.WriteLine(replaced);

    }

}