using System;

using System.Globalization;

using System.Text.RegularExpressions;

​

public static class Program

{

    const string strRegGetHexNumber = @"\\u[0-9a-f]{2,}|0x[0-9a-f]{2,}|%[0-9a-f]{2,}|\u0023([0-9a-f]){1,6}|&\u0023x([0-9a-f]){1,6};|\s[0-9a-f]{2,}|[0-9a-f]{2,}|[«‹»›„‚“‟‘‛”’""""❛❜❝❞〝〞〟＂""""'‘][0-9a-f]{2,}[’'""""«‹»›„‚“‟‘‛”’""""❛❜❝❞〝〞〟＂]";

    private static readonly Regex rgxGetHex = new Regex(strRegGetHexNumber, RegexOptions.IgnoreCase | RegexOptions.CultureInvariant);

    public static void Main()

    {

        string unicodeText = "UTF-16 (hex)  0x0023 (0023)";

        //https://en.wikipedia.org/wiki/Hexadecimal --remove possible prefixes

        string prefixfree = string.Empty;

        string[] prefixHexs = new string[]{"0x", "\\u", "#", "&#", "\\x", "\\s", "U+", "X'", "16#", "#x", "#16r", "&H", "0h"};

        foreach (var pre in prefixHexs)

        {

            if (unicodeText.IndexOf(pre) > -1)

            {

                prefixfree = unicodeText.Substring(unicodeText.IndexOf(pre) + pre.Length);

                break;

            }

        }

​

        if (string.IsNullOrEmpty(prefixfree))

            prefixfree = unicodeText;

        Match m = rgxGetHex.Match(prefixfree);

        bool success = false;

        ulong number = 0;

        string hex_value = string.Empty;

        if (m.Success)

        {

            try

            {

                hex_value = m.Value;

                //long number = Convert.ToInt64(hex_value, 16); //base 16 - hex....

                //https://stackoverflow.com/questions/2801509/uint32-tryparse-hex-number-not-working -> remove prefixes

                success = ulong.TryParse(hex_value, NumberStyles.HexNumber, CultureInfo.InvariantCulture, out number);

            }

            catch (FormatException)

            {

                unicodeText = string.Format("{0} is not in the correct format for a hexadecimal number.", m.Value);

            }

            catch (OverflowException)

            {

                unicodeText = string.Format("{0} is outside the range of an Int64.", m.Value);

            }

            catch (ArgumentException)

            {

                unicodeText = string.Format("{0} is invalid in base 16.", m.Value);

            }

            catch (Exception ex)

            {

                unicodeText = string.Format("{0} return error\r\n{2}", m.Value, ex.Message);

            }

        }

        else

            unicodeText = "Could not find a hex number in \"" + unicodeText + "\". Select the hex number only.";

        if (success)

            unicodeText = string.Format("{0} from found {1:N0} hex number in string {2}", number, hex_value, unicodeText);

        else

            unicodeText = "Could not find a hex number in \"" + unicodeText + "\". Select the hex number only.";

        Console.WriteLine(unicodeText);

    }

}