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.Match(text, pattern, RegexOptions.IgnoreCase)?.Groups["x"].Captures;
        Console.WriteLine(string.Join(", ", result));
	}
}