using System;

​

using System.Text.RegularExpressions;

public class Program

{

    public static void Main() {

​

        string s2 = Regex.Replace("abcdefghik", @"b(.)d", cb =>

                                      " hahah " + cb.Groups[1].Value + " heheh "

                                  );

        Console.WriteLine(s2);      

    }

}