$re = '/(?:^|[])] +)(?P<title>\w+)(?: +[[【(]|$)/m';
$str = '(event) (tag) [group (artist)] title (form) [addition1] [addition2]
(event) [group (artist)] title (form) [addition1]
[event] [group (artist)] title (form) (addition1)
(tag) [group (artist)] title
[group (artist)] title
title
【tag】 [group (artist)] title 【form】
[group (artist)] title
[group] title
[artist] title
(artist) title
';
preg_match_all($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