From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: David Kastrup Newsgroups: gmane.emacs.help Subject: Re: why is this regexp invalid: "\\(?1:\\)" Date: Fri, 03 Sep 2010 09:01:04 +0200 Organization: Organization?!? Message-ID: <87aanzcnv3.fsf@lola.goethe.zz> References: <87eidbgavq.fsf@kuiper.lan.informatimago.com> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: dough.gmane.org 1291866041 31473 80.91.229.12 (9 Dec 2010 03:40:41 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Thu, 9 Dec 2010 03:40:41 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Thu Dec 09 04:40:35 2010 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1PQXMw-00034Q-5E for geh-help-gnu-emacs@m.gmane.org; Thu, 09 Dec 2010 04:40:34 +0100 Original-Received: from localhost ([127.0.0.1]:49303 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PQXMv-00013J-BT for geh-help-gnu-emacs@m.gmane.org; Wed, 08 Dec 2010 22:40:33 -0500 Original-Path: usenet.stanford.edu!goblin1!goblin.stu.neva.ru!uio.no!newsfeed1.swip.net!newsfeed.arcor.de!newsspool4.arcor-online.net!news.arcor.de.POSTED!not-for-mail Original-Newsgroups: gnu.emacs.help X-Face: 2FEFf>]>q>2iw=B6, xrUubRI>pR&Ml9=ao@P@i)L:\urd*t9M~y1^:+Y]'C0~{mAl`oQuAl \!3KEIp?*w`|bL5qr,H)LFO6Q=qx~iH4DN; i"; /yuIsqbLLCh/!U#X[S~(5eZ41to5f%E@'ELIi$t^ Vc\LWP@J5p^rst0+('>Er0=^1{]M9!p?&:\z]|;&=NP3AhB!B_bi^]Pfkw User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux) Cancel-Lock: sha1:lOnNcd4+dQI/bheQr3gz4DTh1y4= Original-Lines: 33 Original-NNTP-Posting-Date: 03 Sep 2010 09:01:04 CEST Original-NNTP-Posting-Host: cc4e77bd.newsspool2.arcor-online.net Original-X-Trace: DXC=B7ifI3YDjo1LNKYb?b>076A9EHlD; 3Yc24Fo<]lROoR18kF5MOK`al0oR:EZAT0L0MG4=`:7^[QIaW`0 Original-X-Complaints-To: usenet-abuse@arcor.de Original-Xref: usenet.stanford.edu gnu.emacs.help:181098 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:76565 Archived-At: pjb@informatimago.com (Pascal J. Bourguignon) writes: > Joel James Adamson writes: > >> Ilya Shlyakhter 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. > 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