From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org
X-Spam-Level: 
X-Spam-Status: No, score=-1.0 required=5.0 tests=MAILING_LIST_MULTI
	autolearn=ham autolearn_force=no version=3.4.4
Received: (qmail 30508 invoked from network); 4 Mar 2023 10:07:24 -0000
Received: from minnie.tuhs.org (2600:3c01:e000:146::1)
  by inbox.vuxu.org with ESMTPUTF8; 4 Mar 2023 10:07:24 -0000
Received: from minnie.tuhs.org (localhost [IPv6:::1])
	by minnie.tuhs.org (Postfix) with ESMTP id C7D554129B;
	Sat,  4 Mar 2023 20:07:22 +1000 (AEST)
Received: from relay05.pair.com (relay05.pair.com [216.92.24.67])
	by minnie.tuhs.org (Postfix) with ESMTPS id 41CC641255
	for <coff@tuhs.org>; Sat,  4 Mar 2023 20:07:19 +1000 (AEST)
Received: from orac.inputplus.co.uk (unknown [87.112.122.222])
	by relay05.pair.com (Postfix) with ESMTP id 7CE231A275F
	for <coff@tuhs.org>; Sat,  4 Mar 2023 05:07:18 -0500 (EST)
Received: from orac.inputplus.co.uk (orac.inputplus.co.uk [IPv6:::1])
	by orac.inputplus.co.uk (Postfix) with ESMTP id E8F882021A
	for <coff@tuhs.org>; Sat,  4 Mar 2023 10:07:17 +0000 (GMT)
From: Ralph Corderoy <ralph@inputplus.co.uk>
To: coff@tuhs.org
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
In-reply-to: <c4e87cee-d04f-50be-cb91-edf1b24eaf07@spamtrap.tnetconsulting.net>
References: <8d1de5c8-1f34-3d37-395d-0f1da7b062ec@spamtrap.tnetconsulting.net> <c4e87cee-d04f-50be-cb91-edf1b24eaf07@spamtrap.tnetconsulting.net>
Date: Sat, 04 Mar 2023 10:07:17 +0000
Message-Id: <20230304100717.E8F882021A@orac.inputplus.co.uk>
Message-ID-Hash: ECUD33676QZRMC7CE3GLT7BPHZR4ULSD
X-Message-ID-Hash: ECUD33676QZRMC7CE3GLT7BPHZR4ULSD
X-MailFrom: ralph@inputplus.co.uk
X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header
X-Mailman-Version: 3.3.6b1
Precedence: list
Subject: [COFF] Re: Requesting thoughts on extended regular expressions in grep.
List-Id: Computer Old Farts Forum <coff.tuhs.org>
Archived-At: <https://www.tuhs.org/mailman3/hyperkitty/list/coff@tuhs.org/message/ECUD33676QZRMC7CE3GLT7BPHZR4ULSD/>
List-Archive: <https://www.tuhs.org/mailman3/hyperkitty/list/coff@tuhs.org/>
List-Help: <mailto:coff-request@tuhs.org?subject=help>
List-Owner: <mailto:coff-owner@tuhs.org>
List-Post: <mailto:coff@tuhs.org>
List-Subscribe: <mailto:coff-join@tuhs.org>
List-Unsubscribe: <mailto:coff-leave@tuhs.org>

Hi Grant,

> Suppose I have the following two lines:
>
>     aaa aaa
>     aaa bbb
>
> Does the following RE w/ back-reference introduce a big performance
> penalty?
>
>     (aaa|bbb) \1
>
> As in:
>
>     % echo "aaa aaa" | egrep "(aaa|bbb) \1"
>     aaa aaa

You could measure the number of CPU instructions and experiment.

    $ echo xyzaaa aaaxyz >f
    $ ticks() { LC_ALL=C perf stat -e instructions egrep "$@"; }
    $
    $ ticks '(aaa|bbb) \1' <f
    xyzaaa aaaxyz

     Performance counter stats for 'egrep (aaa|bbb) \1':

	       2790889      instructions:u                                              

	   0.009146904 seconds time elapsed

	   0.009178000 seconds user
	   0.000000000 seconds sys


    $

Bear in mind that egreps differ, even within GNU egrep, say, over time.

    $ LC_ALL=C perf stat -e instructions egrep '(aaa|bbb) \1' f
    xyzaaa aaaxyz
    ...
	       2795836      instructions:u                                              
    ...
    $ LC_ALL=C perf stat -e instructions perl -ne '/(aaa|bbb) \1/ and print' f
    xyzaaa aaaxyz
    ...
	       2563488      instructions:u                                              
    ...
    $ LC_ALL=C perf stat -e instructions sed -nr '/(aaa|bbb) \1/p' f
    xyzaaa aaaxyz
    ...
		610213      instructions:u                                              
    ...
    $

-- 
Cheers, Ralph.