using System;

using System.Diagnostics;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var sw = Stopwatch.StartNew();

        for(var i = 0; i < 10000; i++)

        {

            var html = "<p><strong>Question #19</strong></p>" +

                "<p><strong>NOTE: The following instructions will be used for questions 16 through 20, and the same instructions will be included for each question.</strong></p>" +

                "<p><strong><img alt=\"\" height=\"488\" src=\"/Files/Images/9213/c57aac1d-b938-41e2-961d-980bff732baa.jpg\" width=\"563\" /></strong></p>" +

                "<p><strong>Question:</strong></p><p>Carry out the instructions assuming only bulb 4 is initially on.  When you stop in step 6, what is the result?</p>" + i.ToString();

            html = Regex.Replace(html, "(?<=<img.*?src=\")(/files/resources/.*?)(?=\".*?>)", $"XYZ$&", RegexOptions.IgnoreCase);

        }

        sw.Stop();

        Console.WriteLine(sw.ElapsedMilliseconds);

    }

}