PHP Regex - Remove second-level domain

Question

I am using this code which works fine in stripping the http:// part.

<?php $str = get_field('fl_website');$str = preg_replace('#^https?://www.#', '', rtrim($str,'/'));echo $str; ?>

However I need to also strip any second-level links and only keep the main domain name.

For example:

https://www.example.com/sites/annabel/2016/03

needs to be converted to just:

example.com