How does Google Reader extract news items from a web page?

Question

I was wondering how Google Reader extracts news items from a web page.

Does any of you know how it works? Or how someone can build a similar system to extract the same information from the HTML of a web page.

Obviously it is not using a standard (nor is it only reading RSS/ATOM), because Google Reader proves that it can read the content of the page regardless of how the markup looks like.

Answer 1

Google Reader does not currently do any kind of extraction of content from raw web pages. It used to have a "track changes to arbitrary pages" feature, but that was removed more than a year ago.

When given an URL that is not that of a feed, Google Reader fetches its contents. If the contents are HTML, it looks for an autodiscovery element of the form <link rel="alternate" type="application/atom+xml" href="feed.xml">. If found, it subscribes to the feed.

Answer 2

You already answered your question by tagging your question with "RSS".

Anyway, Google Reader like all other RSS/Atom-Readers read an RSS or an Atom feed. You may want to have a look at the corresponding wikipedia article: http://en.wikipedia.org/wiki/RSS