using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string result = MaskAnyWords("zoey_fffffffchen@17life.com");

        Console.WriteLine(result);

    }

    public static string MaskAnyWords(string words)

    {

      Regex re = new Regex("", RegexOptions.None);

      int wordslen = words.Length;

      switch (wordslen)

      {

          case 5:

              re = new Regex("(\\w{3})(\\w{2})", RegexOptions.None);

              words = re.Replace(words, "$1****");

              break;

          case 6:

              re = new Regex("(\\w{3})(\\d{3})", RegexOptions.None);

              words = re.Replace(words, "$1****");

              break;

          case 7:

              re = new Regex("(\\w{3})(\\w{4})", RegexOptions.None);

              words = re.Replace(words, "$1****");

              break;

          case 8:

              re = new Regex("(\\w{3})(\\w{4})(\\w{1})", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

          case 9:

              re = new Regex("(\\w{3})(\\w{4})(\\w{2})", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

          case 10:

              re = new Regex("(\\w{3})(\\w{4})(\\w{3})", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

          case 11:

              re = new Regex("(\\w{3})(\\w{4})(\\w{4})", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

          case 12:

              re = new Regex("(\\w{3})(\\w{4})(\\w{5})", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

          default:

              re = new Regex("(\\w{3})(\\w{4})(\\w)", RegexOptions.None);

              words = re.Replace(words, "$1****$3");

              break;

      }           

      return words;

    }

}