Don't use regular expressions for proccessing xml/html. This can be done very easily using the builtin dom parser:
$doc = new DOMDocument();
$doc->loadHTML($htmlAsString);
$xpath = new DOMXPath($doc);
$nodeList = $xpath->query('//a/@href');
for ($i = 0; $i < $nodeList->length; $i++) {
# Xpath query for attributes gives a NodeList containing DOMAttr objects.
# http://php.net/manual/en/class.domattr.php
echo $nodeList->item($i)->value . "
\n";
}