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));
        }
	}
}

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