RegeEx ProblemChains

using System;
using System.Text;
using System.Text.RegularExpressions;

namespace UseYourChainsBuddy
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var inputLine = Console.ReadLine();
            var takePattern = new Regex(@"<p>(.+?)<\/p>");// My lookaround was bad: @"(?<=<p>)(.+?)(?=<\/p>)"
            var replacePattern = @"[^a-z0-9]+";
            var result = new StringBuilder();
            var replaceFirstLetters = new Regex(@"[a-m]+");
            var replaceSecondLetters = new Regex(@"[n-z]+");

            MatchCollection matches = takePattern.Matches(inputLine);
            string currentMatch = string.Empty;

            foreach (Match match in matches)
            {
                currentMatch += match.Groups[1].Value.ToString();
            }

            currentMatch = Regex.Replace(currentMatch, replacePattern, word => " ");
            currentMatch = Regex.Replace(currentMatch, @"\s+|\n+", word => " ");

            foreach (var ch in currentMatch)
            {

                if (replaceFirstLetters.IsMatch(ch.ToString()))
                {
                    result.Append((char)(Convert.ToInt32(ch + 13)));
                }
                else if (replaceSecondLetters.IsMatch(ch.ToString()))
                {
                    result.Append((char)(Convert.ToInt32(ch - 13)));
                }
                else
                {
                    result.Append(ch);
                }
            }

            Console.WriteLine(result);
        }
    }
}