$re = '/^([A-Za-z0-9\\\\\/_\-]|([A-Za-z0-9\\\\\/_\-][A-Za-z0-9\\\\\/_\- ]{0,18}[A-Za-z0-9\\\\\/_\-]))$/m';
$str = 'kkdkd df sdf lk aj33
12345678901234567890
kjlf l-Ka034-/\\_ kj0
adsfdsafsdaf dsfsdfa
dlASjd83_-/\\dfdsfsda
adsfsdas sdafsdafsa
73jkjdfKJH_/\\-_saf97
dsfsdf
2
11
111
- dsf
-df
/dsaf
\\asdf
_asdf
_ sdfjlksdaflk jasdf
adsfsdas sdafsdafs#
adsfsdafsdfsdfsadds
dddddddfsdafljkasdd
$
123456789012345678901
asdf
2
2
';
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