From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via Users list for the GNU Emacs text editor Newsgroups: gmane.emacs.help Subject: Re: How are regexen implemented in Emacs? Date: Tue, 13 Dec 2022 11:16:12 -0500 Message-ID: References: <87a63see6m.fsf@mbork.pl> <877cywe8w9.fsf@mbork.pl> <875yege53t.fsf@mbork.pl> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="7368"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:XDqzGlFZnS4/FpdohlhWROCtuaA= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Tue Dec 13 17:17:02 2022 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 1p57xm-0001iD-8Q for geh-help-gnu-emacs@m.gmane-mx.org; Tue, 13 Dec 2022 17:17:02 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1p57xH-0005AR-DD; Tue, 13 Dec 2022 11:16:31 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1p57xB-00056Q-1W for help-gnu-emacs@gnu.org; Tue, 13 Dec 2022 11:16:29 -0500 Original-Received: from ciao.gmane.io ([116.202.254.214]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1p57x7-0002yC-8U for help-gnu-emacs@gnu.org; Tue, 13 Dec 2022 11:16:23 -0500 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1p57x3-0000mz-7K for help-gnu-emacs@gnu.org; Tue, 13 Dec 2022 17:16:17 +0100 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=geh-help-gnu-emacs@m.gmane-mx.org; helo=ciao.gmane.io X-Spam_score_int: -15 X-Spam_score: -1.6 X-Spam_bar: - X-Spam_report: (-1.6 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.25, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.29 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-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.help:141721 Archived-At: > Perl does the latter. That has saved my bacon from time to time. > So that seems possible with a backtracker, too. Don't ask me how, > though :-) Indeed, it's not more difficulty to allow to run ELisp/Perl code in the middle of the backtracking match than a non-backtracking one, but the difference is in the number of times the ELisp/Perl code may need to be run, which changes the performance profile (and hence the potential use-cases) significantly. Stefan PS: Regarding non-backtracking matchers are backreferences, most of the uses of backreferences I'm familiar with *can* be implemented without backtracking because the subgroup to which the backref points has only one possible way to match. It won't handle cases like `diff-merge-strings`, but it's still an important subset of cases.