using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var input = "X1 OR ( X2 AND( X3 AND X4 ) AND X5 ) OR X6";
		var filtered = Regex.Replace(input, @"\((?>[^()]|(?<o>)\(|(?<-o>)\))*\)(?(o)(?!))", string.Empty);
		var result = filtered.Split(new[] { ' ' }, 
        	StringSplitOptions.RemoveEmptyEntries)
            .Where(element => element == "OR" || element == "AND");    
		var temp = string.Join(" ", result);
		Console.WriteLine(temp);
	}
}