Try this:
var matches = url.match(/^https?\:\/\/([^\/?#]+)(?:[\/?#]|$)/i);
var domain = matches && matches[1]; // domain will be null if no match is found
If you want to exclude the port from your result, use this expression instead:
/^https?\:\/\/([^\/:?#]+)(?:[\/:?#]|$)/i
Edit: To prevent specific domains from matching, use a negative lookahead. (?!youtube.com)
/^https?\:\/\/(?!(?:www\.)?(?:youtube\.com|youtu\.be))([^\/:?#]+)(?:[\/:?#]|$)/i