$re = '/^(1[0-2]|0?[1-9]):[0-5][0-9] (AM|PM)$/m';
$str = '11:45 PM
11:45 MP
11:4 PM
1:45 AM
11:45:AM
:45 AM
// You can show origina post on albertcolom.com/blog';
preg_match_all($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