$re = '~<a[^>]*>[^<]*</a>(*SKIP)(*FAIL)|https?:\S+~m';
$str = ' https://vimeo.com/id1 //this line should get replaced with embed code by module
<a href="https://vimeo.com/id1"> Check out this video </a> //here, anything that is in href="" should not be replaced
on <a href="https://vimeo.com"> Vimeo </a>';
preg_match_all($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