$re = '/\[\s*(\s*\d{4}\s*-\s*\d{2}\s*-\s*\d{2}\s*)\s*\]|(?<=\[|-)\s*(.*?)\s*(?=-|\])|([A-Za-z].*)/m';
$str = '[category - subcategory] [some text - more text ] [2018-12-31] text title here
[category - subcategory] [some text] [ 2018 - 12 -31 ] text title here more text
[category] [some text - more text - even more] [2018-12-31] text title here more text
[category] [some text - more text - even more - some text - more text - even more ] [2018-12-31] text title here more text';
preg_match_all($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