Dont use regex for this. You can use xpath and built in php functions to get what you want:
$xml = simplexml_load_string($myHtml);
$list = $xml->xpath("//@href");
$preparedUrls = array();
foreach($list as $item) {
$item = parse_url($item);
$preparedUrls[] = $item['scheme'] . '://' . $item['host'] . "https://stackoverflow.com/";
}
print_r($preparedUrls);