[Zope-dev] PATH_INFO reliable?

I want to strip off the SERVER_URL of an absolute url (which I have as a string) without doing string manipulations. Is there some built-in function to do this? So what I'm looking for is actual the reverse of absolute_url. The request variable PATH_INFO looks very good, but I don't know whether this is _always_ created. Thx in advance, Danny P.S.: Maybe I just got something completely wrong here. Help!