$re = '/([ap]\.m\.)\h(?=–\h\d+(:\d\d)?\h\1)/m';
$str = '11 a.m. – 10:30 a.m.
6 a.m. – 9 a.m.
5 p.m. – 8:15 p.m.';
preg_match_all($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