using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string[] strings = { "File name without 3 digit end.jpg",
                "username1@example.com",
                "username@example.com",                
                "username1@example@example.com",
                "username1@",
                "test",
                };

        String pattern = @"\G([^\s@])[^\s@](?=[^\s@]*@[^\s@]+(?!\S))";
        Regex regex = new Regex(pattern);
        string substitution = @"$1*";
        foreach(string str in strings)
        {
            Console.WriteLine(regex.Replace(str, substitution));
        }
	}
}