Don’t use Regex to parse html. A correct way would be using an html parser like HmlAgilityPack
var doc = new HtmlAgilityPack.HtmlDocument();
doc.LoadHtml(htmlstring);
var links = doc.DocumentNode.SelectNodes("//a")
.Select(a => a.Attributes["href"].Value)
.ToList();
solved Regular Expression to find out tag and get the value of HREF using C# [duplicate]