$re = '/[ap]\.m\.(*SKIP)(*FAIL)|\./i';
$str = 'Applicants can check the final result of Admissions through the online enquiry system. The online enquiry system will be available from 10:00 a.m. on November 16 (Wednesday).';
preg_match_all($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