using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(?:(?:^|(?<=@))([^.@])|\G(?!\A))[^.@](?:([^.@])(?=[.@]))?";
        string substitution = @"$1*$2";
        string input = @"userone@domain.com
usertwo@domain.com.co";
        RegexOptions options = RegexOptions.Multiline;
        
        Regex regex = new Regex(pattern, options);
        Console.WriteLine(regex.Replace(input, substitution));
    }
}