16进制字符串转换为byte数组

/// <summary>
        /// 16进制字符转换为byte数组
        /// </summary>
        /// <param name="hexString">偶数位，由16进制字符[0-9a-fA-F]组成</param>
        /// <returns>null为转换失败</returns>
        private byte[] HexStringToBytes(string hexString)
        {
            if (string.IsNullOrEmpty(hexString))
                return null;

            int hexStringLength = hexString.Length;
            if (hexStringLength % 2 != 0)
                return null;
           
            Regex r = new Regex(@"[0-9a-fA-F]");
            if (!r.IsMatch(hexString))
                return null;

            int bufferLength = hexStringLength / 2;
            byte[] buffer = new byte[bufferLength];

            for (int i = 0; i < bufferLength; i++)
            {
                buffer[i] = Convert.ToByte(hexString.Substring(i * 2, 2), 16); //2个16进制字符转换为一个byte
            }

            return buffer;
        }