using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string text = "![][][]";
        var charSetOccurences = new Regex(@"!\[(.*?)*\]");
        var charSetMatches = charSetOccurences.Matches(text);
        foreach (Match match in charSetMatches)
        {
            Console.WriteLine(match.Groups[1].Value);
        }
    }
}