$re = '/(?<=href=")([^"]*)/im';
$str = '<a href="test.com">test</a> lorum <a href="ola.com">ipsum</a>';
$subst = "http://ourdomain.com?url=$1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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