How to generate alt attributes with php / filters?

I would like to generate to generate some kind of automatic alt attributes for the images, if they don’t have alt set up.

Found this code, but it doesn’t work for some reason:

function add_alt_tags($content)
{
    global $post;
    preg_match_all('/<img (.*?)/>/', $content, $images);
    if(!is_null($images))
    {
        foreach($images[1] as $index => $value)
        {
            if(!preg_match('/alt=/', $value))
            {
                $new_img = str_replace('<img', '<img alt="'.$post->post_title.'"', $images[0][$index]);
                $content = str_replace($images[0][$index], $new_img, $content);
            }
        }
    }
    return $content;
}
add_filter('the_content', 'add_alt_tags', 99999);