From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.help Subject: Re: [External] : Re: Performance of `re-search-backward' vs `re-search-forward' Date: Mon, 12 Apr 2021 17:03:19 -0400 Message-ID: References: <87czv0awpq.fsf@mbork.pl> <8735vvb8c5.fsf@mbork.pl> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="31859"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) Cc: "help-gnu-emacs@gnu.org" To: Drew Adams Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Mon Apr 12 23:04:23 2021 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lW3jL-0008AD-4z for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 12 Apr 2021 23:04:23 +0200 Original-Received: from localhost ([::1]:51854 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lW3jK-0006cs-4K for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 12 Apr 2021 17:04:22 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:50556) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lW3iS-0006bg-Cq for help-gnu-emacs@gnu.org; Mon, 12 Apr 2021 17:03:28 -0400 Original-Received: from mailscanner.iro.umontreal.ca ([132.204.25.50]:39721) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lW3iP-00078B-VC for help-gnu-emacs@gnu.org; Mon, 12 Apr 2021 17:03:27 -0400 Original-Received: from pmg1.iro.umontreal.ca (localhost.localdomain [127.0.0.1]) by pmg1.iro.umontreal.ca (Proxmox) with ESMTP id 0DF16100222; Mon, 12 Apr 2021 17:03:24 -0400 (EDT) Original-Received: from mail01.iro.umontreal.ca (unknown [172.31.2.1]) by pmg1.iro.umontreal.ca (Proxmox) with ESMTP id 73EC210021B; Mon, 12 Apr 2021 17:03:21 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=iro.umontreal.ca; s=mail; t=1618261401; bh=pJga5lkdLps5qDrsLnUaUNZqohso+V6Z75zis8srMNk=; h=From:To:Cc:Subject:References:Date:In-Reply-To:From; b=GIQ/Z4XSQL6VJugoH4BrBN+SF1c/mNRLCMF9uEmE5zTcFP+kf/PkjV5q1cIA3uIQy CcSSvxqgVvG+I2PJxNDO3hyquyx6P3mUqkiXLX2av8/Ec5QYL2q/20zYZ1a32cTcSK RchfYXBQRUah9hdUmPZ3mIKTcSXfnH1gx5GfZEC0YFXqEf7DkfNFzQxwrq00tZqOTJ qe7UbEuc2BozhOvchCdghOafo7oNtnybjM8OhmflAF3iFbRjBoQINHHyTyG5bQaeRE 8GHEbNAqb87AHNQbKtoGuUKDQwATuQ5MipCEN5sQWAaXSazu8d2nTlDi5SS3F252iU QhPV8/Sx/cDkg== Original-Received: from alfajor (104-222-126-84.cpe.teksavvy.com [104.222.126.84]) by mail01.iro.umontreal.ca (Postfix) with ESMTPSA id 3B8241208A6; Mon, 12 Apr 2021 17:03:21 -0400 (EDT) In-Reply-To: (Drew Adams's message of "Mon, 12 Apr 2021 20:40:36 +0000") Received-SPF: pass client-ip=132.204.25.50; envelope-from=monnier@iro.umontreal.ca; helo=mailscanner.iro.umontreal.ca X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Original-Sender: "help-gnu-emacs" Xref: news.gmane.io gmane.emacs.help:128978 Archived-At: >> So both basically move character by character and check if the regex >> matches there (more or less)? > They don't move character by character. > They use the Boyer-Moore search algorithm. > https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore_string-search_algorithm While this is true in the special case where your regexp only matches a constant string (i.e. you could have used the non-re `search-(for|back)ward` instead), it's a rare exception. In the vast majority of cases it does move character by character. Stefan