A simple (hopfully not stupid) question

I can't say that I have the level of competence in XML as the other posters on
this list, so please excuse me if my question is naieve...

As far as I understand it, the string "foo:bar" is a valid URI as well as
being a valid URI Reference.

So if the parser (or whatever) was to absolutize the URI, what would it do to
this URI?  Would it think it is a reference?  Would it realize that it is a
complete URI?

Thanks,
  Robert