$re = '/\[\[((?:(?!\[{2}).)*?)SPACE(.*?)]]/i';
$str = '..text.. [[wordoneSPACE**wordtwo**]] ..moretext..
.. [[ ..text(not to cut).. [[wordone[SPACE]**[wordtwo]**]] ..moretext..';
preg_match_all($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