I would like to catch the URL handler after it fails to find any matching page, but before it handles the response as a 404.
Then, based on whether I match a regex in the URL, I might like to generate dynamic content to be returned as the result. If I don't match the URL, then I would allow the 404 page.
For example, assume my users each have a special unique ID, such as "x1234" or "x5533". I want example/x5533 to result in my custom function being called so I can generate content related to my user (such as a limited public profile). I don't want the URL to change from the viewer's perspective - I just want to generate content for that one request.
If no such user exists, or the URL doesn't match anything else on the site (first), then I would like Wordpress to send the usual 404 page.
I have seen the URL rewrite function, but I don't believe I want to rewrite the URL. I just want to handle it and dynamically generate output.
