PHP, WP, Lazyload images inserted into post body

add_image_placeholders

/**
 * Lazy load images inserted into the post body
 * @link http://wptheming.com/2013/03/lazy-loading-images/
 */
function add_image_placeholders($content)
{
    // Don't lazyload for feeds
    if (is_feed()) {
        return $content;
    }
    // Don't lazy-load if the content has already been run through previously
    if (false !== strpos($content, 'data-src')) {
        return $content;
    }

    // The placeholder image as data uri
    $data_uri_img = 'data:image/gif;base64,R0lGODlhAQABAIAAAMLCwgAAACH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==';
    // The regex patterns
    $regex_pattern = '#<img([^>]+?)src=[\'"]?([^\'"\s>]+)[\'"]?([^>]*)>#';
    // Placeholder image in all its glory
    $placeholder_image = apply_filters('lazyload_images_placeholder_image', $data_uri_img);
    // Replacement
    $replacement = sprintf('<img${1}src="%s" data-src="${2}"${3}><noscript><img${1}src="${2}"${3}></noscript>', $placeholder_image);
    // Do the needed regex
    $content = preg_replace($regex_pattern, $replacement, $content);
    return $content;
}
add_filter('the_content', 'add_image_placeholders', 99);