08. Use Your Chains, Buddy

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace UseYourChainsBuddy
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.SetIn(new StreamReader(Console.OpenStandardInput(8192)));

            // Decrypted manual
            List<string> manual = new List<string>();
            string input = Console.ReadLine();

            // Find text inside <p> tags
            string tagPattern = @"<p>(.+?)<\/p>";
            MatchCollection inputMatches = Regex.Matches(input, tagPattern);
            foreach (Match match in inputMatches)
            {
                string rawText = match.Groups[1].Value;
                // Leave only small letters and digits - the rest replace with a space
                string alphanumPattern = @"[^a-z0-9]+";
                rawText = Regex.Replace(rawText, alphanumPattern, " ");

                // Do a ROT13
                StringBuilder rot13Text = new StringBuilder(rawText);
                for (int i = 0; i < rot13Text.Length; i++)
                {
                    char letter = rot13Text[i];
                    if (letter >= 'a' && letter <= 'm')
                    {
                        rot13Text[i] = (char)(letter + 13);
                    }
                    else if ( letter > 'm' && letter <= 'z' )
                    {
                        rot13Text[i] = (char)(letter - 13);
                    }
                }

                // Add decrypted text to manual
                manual.Add(rot13Text.ToString().Trim());
            }

            Console.WriteLine(string.Join(" ", manual));
        }
    }
}