Is there an event that we can register with that lets you intercept the content that gets served when you view a page on your site?

Basically we want to check the html to make sure that it doesnt contain specific phrases (which we'll be checking via regex), and if it does we want to replace them.

We thought about subscribing to the save event on a page to check and changing it at that point. The only issue with that is the content would be changed in the db, and if you go to re-edit the page you end up with the modified code.