using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string input = "root/element1/element2[@attr1='abc']/element3[attr3='xyz']/element4/element5[attr4='pqr']/element6/element7";

        Regex regex = new Regex(@"(\[((?!attr4)[^\]])*\])");

        var output = regex.Replace(input, "");

        Console.WriteLine(output);

    }

}