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<?php
function give_linked_images_class($html, $id, $caption, $title, $align, $url, $size, $alt = '' ) {
      // Separate classes with spaces, e.g. 'img image-link'
      $classes = 'wplightbox';

      // check if there are already classes assigned to the anchor
      if ( preg_match('/<a.? class=".?">/', $html) ) { $html = preg_replace('/(<a.? class=".?)(".\?>)/', '$1 ' . $classes . '$2', $html); } else { $html = preg_replace('/(<a.*?)>/', '$1 class="' . $classes . '" >', $html); }

      return $html;
  }

  add_filter('the_content', 'give_linked_images_class', 10, 8);
?>