using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string msg= "(((a) or (b) or (c)) and ((d) or (e)) and ((!f) or (!g)))";
        var charSetOccurences = new Regex(@"\(((?:[^()]|(?<o>\()|(?<-o>\)))+(?(o)(?!)))\)");
        var charSetMatches = charSetOccurences.Matches(msg);
        foreach (Match mainMatch in charSetMatches)
        {
        	var sets = charSetOccurences.Matches(mainMatch.Groups[1].Value);
	        foreach (Match match in sets)
	        {
	            Console.WriteLine(match.Groups[0].Value);
	        }
        }
    }
}