using System;
using System.IO;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Linq;
public class Test
{
	public static void Main()
	{
		var variablesDictionary = new Dictionary<string, string>();
		variablesDictionary.Add("$$@Key", "Value");
		var pattern = @"\$\$@[a-zA-Z0-9_]+\b";
		var stringVariableMatches = Regex.Replace("$$@Unknown and $$@Key", pattern, 
		        m => variablesDictionary.ContainsKey(m.Value) ? variablesDictionary[m.Value] : m.Value);
		Console.WriteLine(stringVariableMatches);
	}
	
	
}