using System;
using System.Text.RegularExpressions;
 
namespace RegExApplication
{
   public class Program
   {
      public static void Main(string[] args)
      {
        
         var pattern = "^([A-Za-z0-9_]{4})_([A-Za-z0-9_]{4})$";
  	var template = "ZZZZ_ss16";
  	var replaceWith = "${1}0%$2";

  	var regex = new Regex(pattern); 

  		Console.WriteLine(regex.Replace(template, replaceWith));
      }
   }
}