Replace all URL's

/**
 * This is the exact string that I need to replace all URL's on.
 *
 * The code will be used on multiple sites, whcih is why it I need to pick up every instance of
 * `http://` and then find the next '/'. It is between these that I need to replace all text
 * with 'www.mainsite.com'.
*/

<li id="page-item-65" class="page_item page-item-65 current_page_item"><a href="http://microsite.dynedrewett.com/">Home</a></li>
<li id="page-item-4314" class="page_item page-item-4314"><a href="http://microsite.dynedrewett.com/why-us/">Why us?</a></li>
<li id="page-item-6134" class="page_item page-item-6134"><a>Services</a>
<ul class='children'>
    <li id="page-item-6136" class="page_item page-item-6136"><a href="http://microsite.dynedrewett.com/sector-services/agriculture/">Agriculture</a></li>
    <li id="page-item-6139" class="page_item page-item-6139"><a href="http://microsite.dynedrewett.com/sector-services/commercial/">Commercial</a></li>
    <li id="page-item-6141" class="page_item page-item-6141"><a href="http://microsite.dynedrewett.com/sector-services/dispute-resolution/">Dispute Resolution</a></li>
    <li id="page-item-6143" class="page_item page-item-6143"><a href="http://microsite.dynedrewett.com/sector-services/family/">Family</a></li>
    <li id="page-item-6145" class="page_item page-item-6145"><a href="http://microsite.dynedrewett.com/sector-services/private-client/">Private Client</a></li>
    <li id="page-item-6147" class="page_item page-item-6147"><a href="http://microsite.dynedrewett.com/sector-services/residential-property/">Residential Property</a></li>
</ul>
</li>
<li id="page-item-67" class="page_item page-item-67"><a href="http://microsite.dynedrewett.com/news-and-events/">News</a></li>
<li id="page-item-88" class="page_item page-item-88"><a href="http://microsite.dynedrewett.com/our-people/">Our people</a></li>
<li id="page-item-162" class="page_item page-item-162"><a href="http://microsite.dynedrewett.com/careers/">Careers</a></li>
<li id="page-item-7" class="page_item page-item-7"><a href="http://microsite.dynedrewett.com/contact-us/">Contact us</a></li>