how to remove the subdomain from external video links only

how to remove the subdomain from external video links only

We have info being inputted on the subdomain (www.agent.xxxx.com) and
posting results from the editor on the main site (www.xxxx.com) everything
from the editor is displayed correct except for external video links.
Using the code here the results for video links are adding the subdomain
agent/ in front of the link causing the error and resulting in
agent/www.youtube.com/xxxxxx
We just need to correct this only on external links
<?php
$this->result->aboutme = str_replace('src="', c="agents/',
$this->result->aboutme);
echo $this->result->aboutme;
?>