using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var arg = @"(?>\w*\((?>[^()]+|(?<o>)\(|(?<-o>)\))*(?(o)(?!))\)|\w+)";
		var pattern = $@"^\s*(concat)\(\s*({arg})\s*,\s*({arg})\)\s*$";
		var match = Regex.Match("concat(regex(3,4),regex(3,4))", pattern);
		if (match.Success) 
		{
		    Console.WriteLine(match.Groups[1].Value);
		    Console.WriteLine(match.Groups[2].Value);
		    Console.WriteLine(match.Groups[3].Value);
		}
	}
}

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