HTML is not regular so using regular expressions to parse html might not be a good idea... @csabinho's answer ^(?!<!--).*"(.*www.mysite\/embed\/+[\w\-]*) won't work if the URL you want to match is in middle of a page, it simply checks if line doesn't begin with a comment.

Best practice would be to create DOM and use XPath to query XML-like contents.

Edit:

By the way you can use following code first to remove comments.

System.Text.RegularExpressions;
...
string pattern = @"(<!--(.+?)-->)";
var res = Regex.Replace(input, pattern, "", RegexOptions.Singleline);

and then use a simple pattern to extract the URL from result