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From: phs <phs@lsv.fr>
To: "Mattias Engdegård" <mattiase@acm.org>
Cc: Michael Heerdegen <michael_heerdegen@web.de>,
	Stefan Monnier <monnier@iro.umontreal.ca>,
	emacs-devel@gnu.org
Subject: Re: More re odditie [Was: regular expressions that match nothing]
Date: Thu, 16 May 2019 12:59:19 +0200	[thread overview]
Message-ID: <4f88e61c-77d0-96fd-9deb-c35686757c99@lsv.fr> (raw)
In-Reply-To: <ABAFE6B8-9CAE-4CEA-A516-6A636CAA1132@acm.org>

[-- Attachment #1: Type: text/plain, Size: 1205 bytes --]

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

[-- Attachment #2: more-rx-tests.patch --]
[-- Type: text/plain, Size: 875 bytes --]

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.

  reply	other threads:[~2019-05-16 10:59 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-14  7:25 regular expressions that match nothing philippe schnoebelen
2019-05-14 10:14 ` Mattias Engdegård
2019-05-14 19:41   ` Stefan Monnier
2019-05-15 16:21     ` Mattias Engdegård
2019-05-15 19:41       ` Alan Mackenzie
2019-05-16 10:54         ` Mattias Engdegård
2019-05-16 23:18           ` Phil Sainty
2019-05-17  9:43             ` Alan Mackenzie
2019-05-17 10:17               ` Mattias Engdegård
2019-05-17 12:53               ` Stefan Monnier
2019-05-15 20:17       ` Michael Heerdegen
2019-05-15 21:06         ` Stefan Monnier
2019-05-15 21:07         ` Mattias Engdegård
2019-05-15 21:38           ` Michael Heerdegen
2019-05-16  6:57           ` More re odditie [Was: regular expressions that match nothing] phs
2019-05-16  9:29             ` Mattias Engdegård
2019-05-16 10:59               ` phs [this message]
2019-05-16 12:31                 ` Stefan Monnier
2019-05-16 18:35             ` Michael Heerdegen
2019-05-16 20:31               ` Mattias Engdegård
2019-05-16 21:01                 ` Global and local definitions of non-functions/variable (was: More re odditie [Was: regular expressions that match nothing]) Stefan Monnier
2019-05-20 16:26           ` Bootstrap/autoload policy (was Re: regular expressions that match nothing) Mattias Engdegård
2019-05-22 14:02             ` Stefan Monnier
2019-05-22 14:07               ` Mattias Engdegård
2019-05-22 14:24                 ` Stefan Monnier
2019-05-22 15:06                   ` Mattias Engdegård
2019-05-22 15:53                     ` Stefan Monnier
2019-05-22 16:40                       ` Mattias Engdegård
2019-05-22 19:08                         ` Stefan Monnier
2019-05-26 12:05                         ` Basil L. Contovounesios
2019-05-16 18:12       ` regular expressions that match nothing Eric Abrahamsen
2019-05-19  4:30         ` 回复: " net june
2019-05-19  5:00           ` HaiJun Zhang
2019-05-19  7:32             ` Mattias Engdegård
2019-05-20  7:56               ` philippe schnoebelen
2019-05-20 23:19                 ` Richard Stallman
2019-05-19 14:12           ` 回复: " Drew Adams

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