Text Transformer

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

public class Program
{
    public static void Main()
    {
        var input = ReadInput();
        var specialSymbols = new[] { '$', '%', '&', '\'' };
        Regex rgx = new Regex(@"([$%&'])([^$%&']+)\1");
        StringBuilder output = new StringBuilder();
        Match m = rgx.Match(input);
        while (m.Success)
        {
            var message = m.Value;
            if (message.Length > 2)
            {
                var messageBody = message.Substring(1, message.Length - 2);
                var charWeight = Array.IndexOf(specialSymbols, message[0]) + 1;
                var decrypted = DecryptMessage(messageBody, charWeight);
                output.Append($"{decrypted} ");
            }
            m = rgx.Match(input, m.Index + m.Length);
        }


        Console.WriteLine(output.ToString());
    }

    private static string ReadInput()
    {
        var sb = new StringBuilder();
        string input;
        while (!(input = Console.ReadLine()).Equals("burp"))
        {
            sb.Append(input);
        }

        string allLines = sb.ToString();
        allLines = Regex.Replace(allLines, @"\s{2,}", " ");
        return allLines;
    }
    private static string DecryptMessage(string messageBody, int charWeight)
    {
        StringBuilder result = new StringBuilder(messageBody.Length);
        foreach (var letter in messageBody)
        {
            result.Append((char)(letter + charWeight));
            charWeight = -charWeight;
        }
        return result.ToString();
    }
}