From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.devel Subject: Re: regular expressions that match nothing Date: Tue, 14 May 2019 15:41:31 -0400 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="210793"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Tue May 14 21:41:52 2019 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([209.51.188.17]) by blaine.gmane.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:256) (Exim 4.89) (envelope-from ) id 1hQdJ9-000slc-It for ged-emacs-devel@m.gmane.org; Tue, 14 May 2019 21:41:51 +0200 Original-Received: from localhost ([127.0.0.1]:53305 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQdJ8-0005tR-H0 for ged-emacs-devel@m.gmane.org; Tue, 14 May 2019 15:41:50 -0400 Original-Received: from eggs.gnu.org ([209.51.188.92]:54920) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQdJ0-0005s7-CJ for emacs-devel@gnu.org; Tue, 14 May 2019 15:41:43 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hQdIz-0004iE-ET for emacs-devel@gnu.org; Tue, 14 May 2019 15:41:42 -0400 Original-Received: from [195.159.176.226] (port=38790 helo=blaine.gmane.org) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hQdIz-0004g4-7f for emacs-devel@gnu.org; Tue, 14 May 2019 15:41:41 -0400 Original-Received: from list by blaine.gmane.org with local (Exim 4.89) (envelope-from ) id 1hQdIx-000sbH-FO for emacs-devel@gnu.org; Tue, 14 May 2019 21:41:39 +0200 X-Injected-Via-Gmane: http://gmane.org/ Cancel-Lock: sha1:zIajs1Zbv3ADkg0a4KlVi+XODrg= X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 195.159.176.226 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:236514 Archived-At: > Thank you, and sorry about my bad initial attempt. I tried a few more, > like [z-a], \c* and \sq, but these were no better. The distribution is > decidedly bimodal; there seems to be no significant difference between > the 'fast' ones, Not surprised: the "fast" ones are the ones that the regexp engine recognizes as "anchored" so the search is reduced to a looking-at. > so I went with \`a\` in the attached patch. Sounds good. >> Of course this may be dependent on the internals of the specific >> regexp library at hand. I do not know enough to judge. In fact I >> believe that a solid regular expression library should provide a >> specific regular expression that matches nothing with special but >> easy treatment that guarantees best response time. > We could add a standard constant for it, like unmatchable-regexp, so Yes, please. I'd recommend a `regexp-` prefix for it. [ And I'll carefully avoid having an opinion on the rest of the name. ] > that at least people don't keep reinventing it. Not only that, but I'm pretty sure casual users will find it much easier to understand what's going on when they see `regexp-` than when they bump into "\\`a\\`". I.e. the constant's name should work as a good comment. Stefan