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

public class Test
{
	public static void Main()
	{
    	var pattern = @"\bisnull\((?>[^][()]|(?<x>\[[^][]*])|(?<o>)\(|(?<-o>)\))*(?(o)(?!))\)";
    	var text = "isnull(Num([abc]), [a], [b]) + sum([d])";
    	var result = Regex.Matches(text, pattern, RegexOptions.IgnoreCase)
    	          .Cast<Match>().Select(p => p.Groups["x"].Captures)
    	          .ToList();
        foreach (var coll in result)
           foreach (var v in coll)
        	Console.WriteLine(v);
	}
}