extracting urls from page with regex in php

extracting urls from page with regex in php

How would I make a regex to extract urls from a page where I am matching
to a center part of a url
For example my urls to match would be something like:
http://m2.imageserver.net/media/thumb/66177422433.jpg
http://m4.imageserver.net/media/thumb/21717623342.jpg
http://m1.imageserver.net/media/thumb/12327722433.jpg
And I want to match by just the imageserver.net/media section of the url
as the sub domain may change.