using System;

using System.Globalization;

using System.Text.RegularExpressions;

​

public static class Program

{

​

    const string strRegHexPrefixCandidates = @"0x[0-9a-f]{2,}|%x[0-9a-f]{2,}|\\u[0-9a-f]{2,}|&#x([0-9a-f]){1,6};|&#([0-9a-f]){1,6};|\\x[0-9a-f]{2,}|\\s[0-9a-f]{2,}|U\+[0-9a-f]{2,}|X'[0-9a-f]{2,}|16#([0-9a-f]){2,}|#x([0-9a-f]){2,}|#16r([0-9a-f]){2,6}|&H([0-9a-f]){2,}|0h([0-9a-f]){2,}|#([0-9a-f]){1,6}|%[0-9a-f]{2,}";

    const string strRegGetHexNumber = @"[0-9a-f]{2,}|[«‹»›„‚“‟‘‛”’""""❛❜❝❞〝〞〟＂""""'‘][0-9a-f]{2,}[’'""""«‹»›„‚“‟‘‛”’""""❛❜❝❞〝〞〟＂]";

    private static readonly Regex rgxHexPre = new Regex(strRegHexPrefixCandidates, RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.CultureInvariant | RegexOptions.Compiled);

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

    public static void Main()

    {

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

        string unicodeText = @"In XML and XHTML, characters can be expressed as hexadecimal numeric character references using the notation &#xcode;, for instance ’ represents the character U+2019 (the right single quotation mark). If there is no x the number is decimal (thus ’ is the same character).[3]";

        //string unicodeText = "8E2";

        //string unicodeText = "this is the end";

​

        string firstCandidateHexVal = string.Empty;

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

        foreach (Match p in rgxHexPre.Matches(unicodeText))

        {

            if (p.Success) { 

                firstCandidateHexVal = p.Value;

                break;

            }

        }

​

        string prefixfree = string.Empty;

        if (!string.IsNullOrEmpty(firstCandidateHexVal)) 

        { 

            //same prefixes as in Regex

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

            foreach (var pre in prefixHexs)

            {

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

                {

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

                    break;

                }

            }

        }

​

        string finalHexCandy = string.Empty;

​

        if (string.IsNullOrEmpty(prefixfree))

            finalHexCandy = unicodeText;

        else  

            finalHexCandy = prefixfree;

​

        Match m = rgxGetHexAgressive.Match(finalHexCandy);

        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 (!string.IsNullOrEmpty(firstCandidateHexVal))

            hex_value = firstCandidateHexVal; 

​

        if (success)

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

        else

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

        Console.WriteLine(unicodeText);

    }

}