using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		string email = "fajarsyairillah@mail.com";
        string Email = maskEmail(email);
        string Email2 = maskEmailNoregex(email);
        Console.WriteLine("Email :{0}\nEmail2: {1}", Email, Email2);   
	}
	public static string maskEmail(string email)
    {
        if (string.IsNullOrEmpty(email)) return email;
        return Regex.Replace(email, @"(?<=^[^@]{2,})[^@](?=[^@]{2,}@)", "*");
    }
    public static string maskEmailNoregex(string email)
    {
    	if (string.IsNullOrEmpty(email)) return email;
    	var split = email.Split('@');
    	if (split.GetLength(0) != 2 || split[0].Length < 5) return email; // There is nothing to modify
        return split[0].Substring(0, 2) + new string('*', split[0].Length-4) + split[0].Substring(split[0].Length - 2) + "@" + split[1];
    }
}
 

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