I want to find some way to simplify both the concepts and the sentence.

* *pure Python code* - as opposed to?
* *a simple mapping between URL patterns (as simple regular expressions)* - it's so simple, it said so twice! Are there complex mappings I need to worry about? Does *as simple regular expressions* refer to the mappings, or to the URL patterns?
* *callback functions* - I think I understand what those are (functions that return not a simple value or object, but another function - correct?) but I had to go and look it up

How about something like: "A URLconf is used to map URL patterns, which it describes as regular expressions, to views. Given a URL, the URL resolver runs through the URLconfs to find a matching view function."

And the following sentence could be deleted.