//download html

WebClient wb = new WebClient();

string html = wb.DownloadString("http://www.stackoverflow.com");

// regex test

Stopwatch regexStopwatch = Stopwatch.StartNew();

Match m = Regex.Match(html, "href\\s*=\\s*(?:[\"'](?<1>[^\"']*)[\"']|(?<1>\\S+))", RegexOptions.IgnoreCase);

List<string> ListOfURLs = new List<string>();

while (m.Success)
{
    string urlChecked = m.Groups[1].ToString().Trim();
    if (!string.IsNullOrEmpty(urlChecked)) ListOfURLs.Add(urlChecked);
    m = m.NextMatch();
}

regexStopwatch.Stop();

Console.WriteLine(regexStopwatch.ElapsedMilliseconds);

// agility test

Stopwatch agilityStopwatch = Stopwatch.StartNew();

HtmlAgilityPack.HtmlDocument htmlDoc = new HtmlAgilityPack.HtmlDocument();

htmlDoc.OptionFixNestedTags = true;

htmlDoc.LoadHtml(html);

List<string> linkList = htmlDoc.DocumentNode.SelectNodes("//a[@href]").Select(n => n.GetAttributeValue("href", string.Empty).Trim()).Where(link => !string.IsNullOrEmpty(link)).ToList();

agilityStopwatch.Stop();

Console.WriteLine(agilityStopwatch.ElapsedMilliseconds);