电话号码 马赛克*号 string类扩展

        /// <summary>
        /// 字符串马赛克
        /// </summary>
        /// <param name="source"></param>
        /// <param name="mask">马赛克</param>
        /// <param name="left">左边长度</param>
        /// <param name="length">打码长度</param>
        /// <returns></returns>
        public static string ToMask(this string source, char mask, int left, int length)
        {
            if (source.IsNullOrEmpty())
            {
                return source;
            }
            var pattern = string.Format(@"(?<=^.{{{0}}}).{{{1}}}", left, length);
            return Regex.Replace(source, pattern, mask.Pad(length));
        }

string类扩张 ，可运用于电话号码打* 号