$re = '/(?!.*internal).*auditor[\'’s]*.*(?=report)|(?!.*internal)(?<=report).*auditor[\'’s]*.*/mi';
$str = 'report of auditor
report of external auditor
auditor external report
in auditor report
auditor report
internal report of auditor
report of internal auditor
auditor internal report';
preg_match_all($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