string msg = "我的邮箱是zxh@itcast.cn的邮箱是yzk365@chezhihui.com减肥看到了撒steve_zhao@163.com发动机是啦";
            //通过委托实现的字符串替换
            msg = Regex.Replace(msg, @"([-a-zA-Z0-9_.]+)(@[-a-zA-Z0-9]+(\.[a-zA-Z0-9]+){1,})", GetReplaceString);
            Console.WriteLine(msg);
            Console.Read();

   public static string GetReplaceString(Match match)
        {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < match.Groups[1].Length; i++)
            {
                sb.Append("*");
            }
            sb.Append(match.Groups[2].Value);
            return sb.ToString();
        }