$re = '/(\[[0-9a-fA-F]{6}\])?(?:[^\S\r\n]*(.+?))(?=\[[0-9a-fA-F]{6}\]|$)/m';
$str = '[00B800][!] Green - [004DB8] Blue does not match!!
[FFFFff][!] White
[e60000][!] Red
123456
Test string, another test abcdef';
preg_match_all($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