Get a first image of a post with preg_match_all

Hi there,

I would need a help with preg_match_all function. I try to get first image of a post for related posts plugin. Currently I use following function:

function get_first_image() {
 global $post, $posts;
 $first_img = '';
 $output = preg_match_all('/< *img[^>]*src *= *["\']?([^"\']*)/', $post->post_content, $matches);
 $first_img = $matches [1][0];
 return $first_img;
}

This function is working without a problem. The problem is that some posts are not in the wp_posts table so it doesn’t show any picture for some items. Some posts contents are in wp_post_settings table created by a plugin where new posts contents are saved. How should the function looks now to work for both (for classic post content in the wp_posts as well as for wp_post_settings.

Thanks.