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) {