$re = '/(?:\*\*\S.+?\S\*\*|(\*\*(?:\s.+?|.+?\s)\*\*))/m';
$str = 'This line is **correct** and **properly marked down**.
But this line **is not** because ** the second bold is wrong**.
** Also** this line is wrong **even though this is right**.';
preg_match_all($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