$re = '/^(?=.{8}$)(?:(?:[a-z]+|[0-9]+)(?:-|$)){3}/m';
$str = 'aa-99-99
99-23-bd
02-xx-04
ab-99-cz
sc-xd-49
99-xav-4
48-xyz-09
9-abc-01
xs-899-x
sss-99-x
10-b1-c2
aa-bb-01
10-11-ac';
preg_match_all($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