WordPress is a large application. According to Westi’s XREF of trunk, WordPress defines 188 classes, 3840 functions, and 302 constants (since the xref counts variables of all scopes, I’m not including them here). For comparison, depending on your version of PHP, there are anywhere from 2-4 thousand native functions in the language. With so many functions, WordPress is bound to have some really cool ones floating around; and unless you either read core or are active on trac and/or #wordpress-dev, you might not have seen these before.
You’re probably looking for some code about custom permalinks. Don’t. Just don’t. I’ve taken it down. Look on some other website.