Is there such a thing as a customizable SAX Contenthandler which works as a XPath observer of sorts?

[Albretch Mueller]

SAX Content handlers do parse data in one go with very little to no
overhead whatsoever, but for each XML file with a differently layout
or if you need something different out of the same XML file, you will
need to code a corresponding content handler. There is also the
problem that not all XML-ish data is nicely written with the data
types in a serial order.
I think there should be something like what I need even if partially
(if I am making sense ;-)). Which libraries implement such a thing? Or
how would you suggest such a library should be implemented?
lbrtchx