using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = "jhon@abc.com";

        string pattern = @"(?<=[\w]{1})[\w-\._\+%]*(?=[\w]{1}@)";

        string result = Regex.Replace(input, pattern, m => new string('*', m.Length));

        Console.WriteLine(result);

    }

}