<?php
 
$re = '~\bhref=(["\'])(?!http://(?:www\.)*mysite\.com)(?<link>(?:(?!\1).)*)~i'; 
$str = "<a href=\"http://w...content-available-to-author-only...e.com\">\n<a href=\"http://m...content-available-to-author-only...e.com\">\n<a href='http://m...content-available-to-author-only...e.com'>\n<a href='http://w...content-available-to-author-only...e.com'>\n<a href='http://w...content-available-to-author-only...e.com'>\n<a href='http://w...content-available-to-author-only...n.com/id.php?site=mysite.com'>"; 
preg_match_all($re, $str, $matches);
print_r($matches["link"]);

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