Subject: bug#16046: closed (Re: bug#16046: Bug with Regexp Containing only a Character Class with a Caret (PATCH))

From: [email-masked] (GNU bug Tracking System)
Date: Thu, 05 Dec 2013 20:27:02 +0000

Your bug report

#16046: Bug with Regexp Containing only a Character Class with a Caret

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
16046: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16046
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#16046: Bug with Regexp Containing only a Character Class with a Caret (PATCH)
From: Stefan Monnier <[email-masked]>
To: Cameron Desautels <[email-masked]>
Cc: [email-masked]
Date: Thu, 05 Dec 2013 15:26:38 -0500
List-Id: <debbugs-submit.debbugs.gnu.org>

> After further experimentation, I suspect that "[^]" is simply not
> a valid regular expression.

Indeed, according to the documentation, for ^ to be treated as itself,
it needs to be "not the first char", but since we have nothing else to
put there, we're kind of screwed.

> This suggests that the broken behavior is within
> `regexp-opt-charset`.  I've attached a patch for that function.

Thank you for tracking down the problem and providing a fix.  I just
installed it in trunk, closing,

        Stefan

Subject: Bug with Regexp Containing only a Character Class with a Caret
From: Cameron Desautels <[email-masked]>
To: [email-masked]
Date: Tue, 3 Dec 2013 22:57:56 -0600
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi all,

I've run across a dilemma, in the most literal sense: either there's a
problem in Emacs's regexp engine or there's an issue with
`regexp-opt-charset`---I'm not sure which.

The issue has to do with regular expressions containing character
classes with only a caret character.  I know this seems like a rather
silly case (why not just use "\\^"?) but it came up in the context of
trying to track down a bug in ruby-mode, so it does occur in real (and
particularly *programmatic*) settings.

The simplest case to reproduce is the following:

    (re-search-forward "[^]")
    ; => Debugger entered--Lisp error: (invalid-regexp "Unmatched [ or [^")
    ;   re-search-forward("[^]")
    ;   eval((re-search-forward "[^]") nil)
    ;   eval-last-sexp-1(t)
    ;   eval-last-sexp(t)
    ;   eval-print-last-sexp()
    ;   call-interactively(eval-print-last-sexp record nil)
    ;   command-execute(eval-print-last-sexp record)
    ;   execute-extended-command(nil "eval-print-last-sexp")
    ;   call-interactively(execute-extended-command nil nil)

Now, you can make a compelling case that that's not a valid regexp
(and the Emacs Lisp Reference Manual doesn't seem to *directly*
contradict this argument), but that presents a problem when paired
with `regexp-opt-charset`:

    (regexp-opt-charset '(?^))
    => "[^]"

Note that that produces the problem regexp; which is to say that the
following code is bound to fail when it should succeed:

    (re-search-forward (regexp-opt-charset '(?^)))

What's the correct behavior? I'd be happy to offer a patch for either
side of the equation but I'm not sure which one to target.

All the best.

-- Cameron

In GNU Emacs 24.3.1 (x86_64-apple-darwin11.4.2, Carbon Version 1.6.0
AppKit 1138.51)
 of 2013-05-13 on atago
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