bug#34100: 26.1; Incomplete ? and ?? handling in rx

[Mattias Engdegård]

In rx, the ? and ?? operators can be written verbatim as ? and ?? (space and ? 
character), or by using symbols whose leading character needs to be escaped, \? 
and \?? respectively. The names come from Olin Shivers's SRE, but ? is not a 
special character in Scheme syntax, hence the character syntax hack.

However, the symbols only partially work:

(rx (\? "x") (\?? "y"))       --> "x?y?"  ; expected "x?y??"
(rx (minimal-match (\? "x"))) --> "x??"   ; expected "x?"

While it could be argued that only the character-based syntax should be used, 
the fact is that the symbols are accepted and seem to work, just in a subtly 
broken way.

The documentation is also not clear on this point, and a programmer knowing the 
elisp syntax might very well assume that the symbols are the ones to use.

Suggested fix:

diff --git a/lisp/emacs-lisp/rx.el b/lisp/emacs-lisp/rx.el
index a39fe55c32..8b4551d0d3 100644
--- a/lisp/emacs-lisp/rx.el
+++ b/lisp/emacs-lisp/rx.el
@@ -733,8 +733,8 @@
 is non-nil."
   (rx-check form)
   (setq form (rx-trans-forms form))
-  (let ((suffix (cond ((memq (car form) '(* + ?\s)) "")
-                     ((memq (car form) '(*? +? ??)) "?")
+  (let ((suffix (cond ((memq (car form) '(* + \? ?\s)) "")
+                     ((memq (car form) '(*? +? \?? ??)) "?")
                      (rx-greedy-flag "")
                      (t "?")))
        (op (cond ((memq (car form) '(* *? 0+ zero-or-more)) "*")