Fontification with a specific function

[joao]

Hi all,

I'm trying to define a function to find text for fontification. This is
one of the possible elements of `font-lock-keywords'. Looking at the
emacs lisp reference manual (Font Lock Mode/Search-based Fontification),
I see that 

     When FUNCTION is called, it receives one argument, the limit of
     the search.  It should return non-`nil' if it succeeds, and set the
     match data to describe the match that was found.

I don't see where this "match data" is defined. Can anyone point me to
an example, in the emacs sources or in some package, of this usage of
fontification ?

Thanks,

Joao