How to get gallery id inserted to a post?

In single.php i put this code, from this link (thank you Wyck) https://stackoverflow.com/questions/14277794/wordpress-3-5-own-gallery-with-included-images-doesnt-work

preg_match('/\/', $post_content, $ids);
$array_id = explode(",", $ids[1]);
print_r($array_id);

Now I can use gallery shortcode