Web-based Proxy Server

I am making a web-based proxy server, and I need to make it so that it can download a webpage, and modify all of the links inside it (so that it can proxyfy all of the links, rather than the browser downloading the files from the original website).

So far the first bit is fine, but I still need to make my script modify all of the links inside the given HTML file. I know I can do it, but it wouldn’t be the most effective way of doing it. Does anyone have any suggestions on how they would search through an HTML script, and locate links?

So far what I am doing is searching for anything that one of these at the beginning of it:
href=
src=
background=

And I am changing the text that comes afterwards (thus modifying the link). But for many reasons, this is a really bad way of doing it.