GNU bug report logs - #30568
27.0.50; `rx' doesn't create optimal regex for (group (or ...))

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: p.stephani2 <at> gmail.com
To: bug-gnu-emacs <at> gnu.org
Subject: 27.0.50; `rx' doesn't create optimal regex for (group (or ...))
Date: Wed, 21 Feb 2018 15:31:56 +0100

emacs -Q -batch --eval=3D'(progn (princ (rx (group (or "aaa" "bbb")))) (ter=
pri))'
=3D=3D> \(\(?:aaa\|bbb\)\)

This should generate \(aaa\|bbb\) instead.  Of course, these regexes are
equivalent, but the second one is easier to read (and maybe faster).


In GNU Emacs 27.0.50 (build 16, x86_64-pc-linux-gnu, GTK+ Version 3.22.24)
 of 2018-02-21 built on localhost
Repository revision: d599dce1353ce59d134fcff21cde02c70025253d
Windowing system distributor 'The X.Org Foundation', version 11.0.11903000
System Description: Debian GNU/Linux buster/sid

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-threads --enable-gcc-warnings=3Dwarn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking --enable-check-lisp-object-type --with-modules
 'CFLAGS=3D-O0 -ggdb3''

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS NOTIFY GNUTLS FREETYPE XFT
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 MODULES JSON

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 95399 9075)
 (symbols 48 20247 1)
 (miscs 40 40 121)
 (strings 32 28348 1815)
 (string-bytes 1 757412)
 (vectors 16 14141)
 (vector-slots 8 499378 12892)
 (floats 8 50 67)
 (intervals 56 223 0)
 (buffers 992 12))

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Message #8 received at 30568 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: 30568 <at> debbugs.gnu.org
Cc: Philipp <p.stephani2 <at> gmail.com>
Subject: Re: bug#30568: 27.0.50; `rx' doesn't create optimal regex for (group
 (or ...))
Date: Fri, 13 Dec 2019 20:18:58 +0100

> (rx (group (or "aaa" "bbb")))
> ==> \(\(?:aaa\|bbb\)\)
>
> This should generate \(aaa\|bbb\) instead.  Of course, these regexes are
> equivalent, but the second one is easier to read (and maybe faster).

This remains unchanged, I'm afraid, despite rx being completely rewritten. Not that it matters much: brackets do not generate any regexp bytecode, thus matching performance isn't affected once the regexp has been compiled. When the brackets are required, there is no waste:

(rx (+ (or "aaa" "bbb"))) 
=> "\\(?:aaa\\|bbb\\)+"

Still, it's a bit untidy, and I like that you reported it. We could add a special value for the PAREN argument to regexp-opt to prevent bracketing altogether, I suppose. It isn't immediately on my to-do list, however.

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