using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

​

public class Program

{

    private static Dictionary<string, string> Messages

    {

        get;

        set;

    }

​

    public static void Main()

    {

        Messages = new Dictionary<string, string>();

        Messages.Add("abc", "Hello %bca%");

        Messages.Add("bca", "world, %cde%%dca%");

        Messages.Add("cde", "how are you ");

        Messages.Add("dca", "today%edc%");

        Messages.Add("edc", "? %message%");

        Messages.Add("message", "I'm good, thanks.");

        Console.WriteLine(Message("abc"));

        //Console.WriteLine(Message("abc"));

        //Console.WriteLine(Message("abc").Replace("%message%", Message("extrabits")));

        //Console.WriteLine(Message("alsonotfound"));

    }

​

    private static string Message(string code)

    {

        var message = "%" + code + "%";

        bool replaced;

        do

        {

            replaced = false;

            var matches = new Regex(@"%.[^%]*%").Matches(message);

            foreach (Match match in matches)

            {

                var matchStr = match.ToString();

                code = matchStr.Trim('%');

                string replaceStr;

                if (Messages.TryGetValue(code, out replaceStr) && replaceStr != "")

                {

                    message = message.Replace(matchStr, replaceStr);

                    replaced = true;

                }

            }

        }

        while (replaced);

        return code == message.Trim('%') ? null : message;

    }

}