using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = "natthavat28@gmail.com";

        //var result = string.Format("{0}****{1}",input[0], input.Substring(input.IndexOf('@')-1));

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

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

        Console.WriteLine("input: " + input);

        Console.WriteLine("output: " + result);

    }

}