using System;
using System.Text.RegularExpressions;
using System.IO;
public class Test
{
	public static void Main()
	{
		var s = "This is a {test} %String%. %Stack% {Overflow}";
		var regex = "(?:(?<p>%)|(?<b>{))(?<v>.*?)(?(p)%|})";
		var matches = Regex.Matches(s, regex, RegexOptions.Singleline); 
		foreach (Match match in matches) 
		{
		    Console.WriteLine(match.Groups["v"].Value);
		}
	}
}