Uppercase Words

/* Write a program to reverse the letters of all uppercase words in a text. In case an uppercase word stays unchanged after reversing its letters, then double each of its letters. A word is a sequence of Latin letters separated by non-letter characters (e.g. punctuation characters or digits).
 * The output should hold the result text. Ensure you escape correctly the HTML special characters in the output with the SecurityElement.Escape() method. */

namespace _15.Uppercase_Words
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Security;
    using System.Text.RegularExpressions;

    class UppercaseWords
    {
        static void Main(string[] args)
        {
            string textLine = Console.ReadLine();
            while (textLine != "END")
            {
                ReverseAndReplaceUppercaseWords(textLine);
                textLine = Console.ReadLine();
            }
        }

        private static void ReverseAndReplaceUppercaseWords(string text)
        {
            const string pattern = @"(?:^|\W)\b[A-Z]+\d*[A-Z]*\b";
            string resultText = text;
            HashSet<string> matchedWords = new HashSet<string>();
            MatchCollection matches = Regex.Matches(text, pattern);
            foreach (Match match in matches)
            {
                string word = match.Groups[0].Value;
                if (matchedWords.Contains(word))
                {
                    continue;
                }

                matchedWords.Add(word);
                Dictionary<string, string> matchedGroups = new Dictionary<string, string>();
                MatchCollection uppercases = Regex.Matches(word, @"[A-Z]+");
                foreach (Match uppercase in uppercases)
                {
                    string upperWord = uppercase.Groups[0].Value;
                    if (matchedGroups.ContainsKey(upperWord))
                    {
                        continue;
                    }

                    char[] letters = upperWord.ToCharArray();
                    Array.Reverse(letters);
                    string reversed = new string(letters);
                    if (reversed == upperWord)
                    {
                        char[] doubleReverse = new char[reversed.Length * 2];
                        for (int i = 0; i < reversed.Length; i++)
                        {
                            doubleReverse[2 * i] = reversed[i];
                            doubleReverse[2 * i + 1] = reversed[i];
                        }

                        reversed = new string(doubleReverse);
                    }

                    matchedGroups.Add(upperWord, reversed);
                }

                string replacement = matchedGroups
            .Aggregate(word, (current, @group) => current.Replace(@group.Key, @group.Value));

                resultText = resultText.Replace(word, replacement);
            }

            Console.WriteLine(SecurityElement.Escape(resultText));
        }
    }
}