Search content for shortcodes and get parameters

Question

In content I can have multiple shortcodes like [book id="1"] [book id="14" page="243"]

Is there any help method with which I can search the content for that shortcode and get its parameters? I need to get IDs so I can call WP_Query and append the Custom Post Types titles at the end.

function filter_books( $content ) {

// get all shortcodes IDs and other parameters if they exists
...

return $content;
}
add_filter( 'the_content', 'filter_books', 15 );

I tried using following code but var_dump($matches) is empty and if it would work I am not sure how would I get parameters (https://stackoverflow.com/questions/23205537/wordpress-shortcode-filtering-the-content-modifies-all-posts-in-a-list)

  $shortcode = 'book';
  preg_match('/\['.$shortcode.'\]/s', $content, $matches);

Answer 1

This is working for me

  $shortcode = 'book';
  $pattern = get_shortcode_regex();

  // if shortcode 'book' exists
  if ( preg_match_all( '/'. $pattern .'/s', $post->post_content, $matches )
    && array_key_exists( 2, $matches )
    && in_array( $shortcode, $matches[2] ) )  {
   $shortcode_atts = array_keys($matches[2], $shortcode);

 // if shortcode has attributes
 if (!empty($shortcode_atts)) {
  foreach($shortcode_atts as $att) {
    preg_match('/id="(\d+)"/', $matches[3][$att], $book_id);

    // fill the id into main array
    $book_ids[] = $book_id[1];
  }
}
...

Answer 2

there is actually a much simpler way to achieve this. When the content is parsed by WP, it executes the shortcodes it finds in the content. As of WP 4.7 a new filter, do_shortcode_tag is triggered when a shortcode is replaced by its content. This is really useful to add additional content to the shortcode output based on the attributes found,

add_filter( 'do_shortcode_tag','add_my_script',10,3);
function enqueue_my_script($output, $tag, $attr){
  if('aShortcode' != $tag){ //make sure it is the right shortcode
    return $output;
  }
  if(!isset($attr['id'])){ //you can even check for specific attributes
    return $output;
  }
  $output.='<script> ... </script>';
  return $output;
}

alternatively, if you need to put something into the footer of your page, you could hook onto the wp_footer action within the above function either an an anonymous function or to fire another function in your file.