Use Your Chains Buddy

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

class UseYourChainsBuddy
{
    static void Main(string[] args)
    {
        int bufSize = 8192;
        Stream inStream = Console.OpenStandardInput(bufSize);
        Console.SetIn(new StreamReader(inStream, Console.InputEncoding, false, bufSize));

        string input = Console.ReadLine();

        string pattern = @"<p>(.*?)<\/p>"; //tried this too same result @"(?<=<p>)(.*?)<?=<\/p>)"
        string[] words = Regex.Matches(input, pattern)
            .Cast<Match>()
            .Select(m => m.Groups[1].Value) //m.Value with the old regex above
            .ToArray();

        for (int word = 0; word < words.Length; word++)
        {
            words[word] = Regex.Replace(words[word], @"[^a-z\d]+", " ");

            StringBuilder builder = new StringBuilder();

            for (int letter = 0; letter < words[word].Length; letter++)
            {
                if (words[word][letter] >= 'a' && words[word][letter] <= 'm')
                {
                    builder.Append((char)(words[word][letter] + 13));
                }
                else if (words[word][letter] >= 'n' && words[word][letter] <= 'z')
                {
                    builder.Append((char)(words[word][letter] - 13));
                }
                else
                {
                    builder.Append(words[word][letter]);
                }
            }

            words[word] = builder.ToString();
        }

        string output = string.Join(" ", words);

        output = Regex.Replace(output, @"\s+|\n+", " ");
        Console.WriteLine(output);
    }
}