From: pjb@informatimago.com (Pascal J. Bourguignon)
To: help-gnu-emacs@gnu.org
Subject: Re: why is this regexp invalid: "\\(?1:\\)"
Date: Fri, 03 Sep 2010 13:51:14 +0200 [thread overview]
Message-ID: <87k4n3hwp9.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: 87aanzcnv3.fsf@lola.goethe.zz
David Kastrup <dak@gnu.org> writes:
> pjb@informatimago.com (Pascal J. Bourguignon) writes:
>
>> Joel James Adamson <adamsonj@email.unc.edu> writes:
>>
>>> Ilya Shlyakhter <ilya_shl@alum.mit.edu> writes:
>>>
>>>> (string-match "\\(?1:\\)" "")
>>>> (string-match "\\(?:\\)" "")
>>>
>>> I get 0 from both.
>>
>> Which is expected. These regexps are not invalid, they're perfectly
>> valid, and match perfectly what they have to match.
>>
>> Read (info "(elisp)Regexp Backslash").
>>
>> Notice that the two regexps have a different meaning.
>>
>> "\\(?1:\\)" matches exactly 0 characters, and assign them the group number 1.
>>
>> "\\(?:\\)" matches exactly 0 characters, if it can (it's shy), and
>> assign them the next group number (which is 1 since it's the first
>> group).
>
> I recommend you check again what a shy group is supposed to be.
Right. I confounded it with non-greedy.
"\\(?:\\)" matches exactly 0 characters, and do not assign them to any
group number.
>> In both case, when matching the empty string, the result should be 0,
>> since there are 0 character at the position 0 in the empty string.
>
> That much is correct.
>
> --
> David Kastrup
--
__Pascal Bourguignon__ http://www.informatimago.com/
next prev parent reply other threads:[~2010-09-03 11:51 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-01 19:40 why is this regexp invalid: "\\(?1:\\)" Ilya Shlyakhter
2010-09-02 18:27 ` Joel James Adamson
[not found] ` <mailman.0.1283452083.4022.help-gnu-emacs@gnu.org>
2010-09-02 20:15 ` Pascal J. Bourguignon
2010-09-03 7:01 ` David Kastrup
2010-09-03 11:51 ` Pascal J. Bourguignon [this message]
[not found] <mailman.2.1283370006.27482.help-gnu-emacs@gnu.org>
2010-09-02 8:17 ` Stefan Monnier
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