Re: More re odditie [Was: regular expressions that match nothing]

[phs <phs@lsv.fr>]

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Hi Mattias,

On 2019/05/16 11:29, Mattias Engdegård wrote:
> 16 maj 2019 kl. 08.57 skrev phs <phs@lsv.fr>:

> Actually this one is documented:
> 
>   For historical compatibility, special characters are treated as ordinary
>   ones if they are in contexts where their special meanings make no sense.
>   For example, `*foo' treats `*' as ordinary since there is no preceding
>   expression on which the `*' can act.

I missed that. Thanks for pointing it out.

> and in any case the 'correct' behaviour would be to signal a syntax error, not repeat the empty string.

I'd rather read `*' as meaning "repeat the empty string", as with
`\(\)*', but this is a matter of taste, and historical compatibility is
very important.

BTW, can your scans of regexps tell if this compatibility is relied on a
lot? It would be safe to replace `*' and `+' with `\*' and `\+' where
this happens.  I've just grep'ed quickly through the code and only
noticed a risky use of "+" (and "[..]") in the definition of `term-word'
in term.el

> Thanks for reporting it, and you are right, that's a (known) bug in rx.

When rx is fixed, I suggest we add the following extra tests (see patch)

--phs

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diff --git a/test/lisp/emacs-lisp/rx-tests.el b/test/lisp/emacs-lisp/rx-tests.el
index 4a5919edf0..373bc9b2e3 100644
--- a/test/lisp/emacs-lisp/rx-tests.el
+++ b/test/lisp/emacs-lisp/rx-tests.el
@@ -109,5 +109,16 @@
                         (match-string 0 s))
                    "a"))))
 
+(ert-deftest rx-kleene-empty-string ()
+  ;; Test whether the repetition of empty string --that matches everywhere-- is handled correctly
+  (should (string-match (rx (zero-or-more "")) "foo"))
+  (should (string-match (rx (one-or-more "")) "foo")))
+
+(ert-deftest rx-kleene-unmatchable ()
+  ;; Test whether the repetition of regexp-unmatchable is handled correctly
+  (should (string-match (rx (zero-or-more (or))) "foo"))
+  (should-not (string-match (rx (or)) "foo"))
+  (should-not (string-match (rx (one-or-more (or))) "+")))
+
 (provide 'rx-tests)
 ;; rx-tests.el ends here.