$re = '/((?:\b(?:[1-9]|[01]\d|2[0-3])):(?:[0-5]\d)(?:\s*[AaPp][mM])?(?!([3-9]\d+:\d+)))/';
$str = 'MUST MATCH
9:30 AM - 10:30 AM
5:50AM-6:25PM
18:20-21:30
7:30-8:30
11:30-13:12
23:00-00:00
MUST NOT MATCH
33:85 - 99:20
420:69 - 69 :420
20:45AM-52:20
';
preg_match_all($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