$re = '/.*<a href=(.*?www\.someothersite\.com(*SKIP)(*F))?.*/m';
$str = 'blah blah blah <a href="http://www.site.com/"> blah blah
blah blah blah <a href="http://www.someothersite.com/"> blah blah
First line matches, second line doesn\'t.';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php