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

public class Test
{
	public static void Main()
	{
    	var line = "this is (a (string))";
    	var pattern = @"\(((?>[^()]+|\((?<o>)|\)(?<-o>))*(?(o)(?!)))\)|\S+";
    	var result = Regex.Matches(line, pattern)
    		.Cast<Match>()
    		.SelectMany(x => x.Groups.Cast<Group>()
    			.Where(m => !string.IsNullOrWhiteSpace(m.Value))
    			.Select(t => t.Value))
    		.ToList();
        foreach (var s in result)
        	Console.WriteLine(s);
	}
}