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("ab", "Hello ");

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

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

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

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

        Messages.Add("extrabits", "%ab%I'm 100% good, thanks.");

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

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

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

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

    }

​

    private static string Message(string code)

    {

        if (Messages.TryGetValue(code, out string message))

        {

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

            foreach (Match match in matches)

            {

                string replacement = Message(match.ToString().Trim('%')) ?? "";

                message = replacement.Length > 0 ? message.Replace(match.ToString(), replacement) : message;

            }

        }

​

        return message;

    }

}