$re = '/([A-Z][a-z]?)(\d*(?:(?:[\.|\,])\d+(?:\%)?)?)|(?:[\(|\[])([^()]*(?:(?:[\(|\[]).*(?:[\)|\]]))?[^()]*)(?:[\)|\]])(\d*(?:(?:[\.|\,]?)\d+(?:\%)?))/';
$str = 'Co3.3265(Fe(CN)6,23)2C52,0
(Fe(CN)6)2
Fe(CN)6
(CN)6
CN
CO2CH4
Co3.3265%(Fe[CN]6%)2.2%C52,23%
(Fe[CN)6)2
Fe(CN]6
(CN)6
CN
CO2CH4
';
preg_match_all($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