c# 正则验证域名

/// <summary>
        /// 验证字符串是否是域名
        /// </summary>
        /// <param name="str">指定字符串</param>
        /// <returns></returns>
        public bool IsDomain(string str)
        {
            string pattern = @"^[a-zA-Z0-9][-a-zA-Z0-9]{0,62}(\.[a-zA-Z0-9][-a-zA-Z0-9]{0,62})+$";
            return IsMatch(pattern, str);
        }
        /// <summary>
        /// 判断一个字符串，是否匹配指定的表达式(区分大小写的情况下)
        /// </summary>
        /// <param name="expression">正则表达式</param>
        /// <param name="str">要匹配的字符串</param>
        /// <returns></returns>
        public bool IsMatch(string expression, string str)
        {
            Regex reg = new Regex(expression);
            if (string.IsNullOrEmpty(str))
                return false;
            return reg.IsMatch(str);
        }