bug#17373: 24.3.50; match data is incorrect if there are too many groups

[Paul Eggert]

Yes, unfortunately Emacs currently has a limit of at most 256 groups of 
match data: one for the entire pattern, and 255 for parenthesized 
subpatterns.  If you go over the limit, the excess matches are silently 
discarded.  I don't see this limitation documented anywhere; it should 
be.  Or better yet, the limitation should be removed.

The limitation is wired into the representation of the 'start_memory' 
code in compiled regular expressions: this code has a one-byte operand. 
 As far as I know, the limitation is specific to Emacs, and is not 
present in the Gnulib or glibc versions of the regexp matcher.