<?php
$text = '<div class="about_us_item"><h2>Privacy Policy</h2><div>::cck::34::/cck::<br>::introtext::<h3>Introduction</h3>
<p>John Blogs respects the privacy of each Visitor.';
$result = preg_replace('/::cck::([0-9]*)::\/cck::<br>::introtext::/', '', $text);
var_dump($result);