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

From: Noam Postavsky <[email-masked]>
Date: Fri, 16 Mar 2018 20:37:41 -0400

Philipp Stephani <[email-masked]> 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:

(progn
  (string-match "^\\(a\\)?\\(b\\)\\(c\\)?$" "b")
  (match-data t (make-list 8 nil)))
  ;=> (0 1 nil nil 0 1 nil nil)