bug#29343: 27.0.50; Match data doesn't contain elements for non-matched subgroups

[Philipp Stephani]

Am Sa., 17. März 2018 um 01:37 Uhr schrieb Noam Postavsky <npostavs@gmail.com>:
>
> Philipp Stephani <p.stephani2@gmail.com> writes:
>
> > $ emacs -Q -batch -eval '(progn (string-match "^\\(a\\)?\\(b\\)\\(c\\)?$" 
> > "b") (print (match-data)))'
> > (0 1 nil nil 0 1)
> >
> > Note that neither the `a` nor the `c` group matched, but there are
> > entries for `a` in `match-data`, but not for `c`.  This makes working
> > with the match data unnecessarily hard because its length depends on
> > whether certain optional groups have matched or not.  I haven't seen any
> > discussion about this behavior in either the manual or the docstring.  I
> > think the match data in this case should be (0 1 nil nil 0 1 nil nil).
>
> You can get that result by passing a list of the expected length as the
> REUSE argument to match-data:

True, but that also requires knowing the expected length. In the most
general case this should work for unknown regular expressions.