$re = '/\$.+?(?=,|\s|\])(?:\](?=\]))?/m';
$str = '[\'var1\' => $var1, \'var2\' => $obj->var1[\'asd\']]
[ \'var1\' => $var1, \'var2\' => $obj->var1]
[\'var1\' => $var1, \'var2\' => $var2]';
preg_match_all($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