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

public class Test
{
	public static void Main()
	{
		var s = "My test string is ##[FirstVal][SecondVal]##";
		var myvalues = Regex.Matches(s, @"##(?:\[([^][]*)])+##")
			.Cast<Match>()
			.SelectMany(m => m.Groups[1].Captures
              .Cast<Capture>()
              .Select(t => t.Value));
        Console.WriteLine(string.Join(", ", myvalues));
	}
}