$re = '/(?:{{([\w-]+(?:\h+[\w-]+)*)|(?!\A)\G)(?:\h*\[([^]]+)]|\h*}})/';
$str = '{{word with spaces}}
{{word with spaces [sub1]}}
{{word with spaces [sub1] [sub 2] [Sub-3] }}';
preg_match_all($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