C#

var number = "0x08";
var integer = int.Parse(number.Substring(number.IndexOf("x") + 1), NumberStyles.HexNumber);

byte byteCode = byte.Parse(integer.ToString());

string tmpByte = "0x80H"
int integer = int.Parse(tmpByte.Substring(tmpByte.IndexOf("x") + 1), System.Globalization.NumberStyles.HexNumber);

byte converted = 0x00;
bool successful = byte.TryParse(inputString.Substring(inputString.IndexOf("x")), out converted);

if (!successful)
{
    // Uh oh
    MessageBox.Show("Problem!");
}
else
{
    // Do what you want with converted
}

Ed.Poore wrote:

Because 08H is not a valid hex number, in any language that I know of

int integer = int.Parse(tmpByte.Substring(tmpByte.IndexOf("x") + 1,<big>2</big>), System.Globalization.NumberStyles.HexNumber);

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Globalization;

namespace HexNumbers
{
    class Program
    {
        private static void Main(string[] args)
        {
            var regex = new Regex(@"(%|&?#|\\x|U\+|=|H'|16#|8'h|#|16r|16#|#x|#16r|&H|&h|$|&|0h|0H)?(?<number>[\d|A-F|a-f]+)('?|#)?(h|H)?", RegexOptions.Compiled);
            var numbers = new string[] { "%20", "&#xABCD", "#FF", "0x5A3", "\\x1B", "U+20AC", "=D1a", "FFh", "0A3CH", "$5A3", "H'ABCD'", "16#5A3#", "8'hFF", "#01AF", "16r6EF7", "16#ABCD", "#xABC", "&H5A3", "&5A3", "0hA3", "X'5A3'" };

            foreach (var number in numbers)
            {
                var isMatch = regex.IsMatch(number);
                if (isMatch)
                {
                    Console.WriteLine("{0} is a match and it's value is {1}", number, int.Parse(regex.Match(number).Groups["number"].Value, NumberStyles.HexNumber));
                }
                else
                {
                    Console.WriteLine("{0} is not a hex number", number);
                }
            }
        }
    }
}