bug#71819: Incorrect substitution when undo replacing regexps

bug#71819: Incorrect substitution when undo replacing regexps

[Sab Pyrope]

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I encountered a bug when replacing regular expressions.
If you [u]ndo a previous change during the replacement process, subsequent
changes to the \2 substitution will use the old undone result.

Text:
---
Aaaaa. Bbbb.
Ccccc. Dddd.
Eeeee. Ffff.
Ggggg. Hhhh.
---
Input regexp: \([.]\) \([A-Z]\)
Output regexp: \1  \2
Bug:
1. Copy the text above to the buffer.
2. Enter the regular expressions to the interactive regexp replacement [
C-M-%].
3. Click [y] twice until the Ffff is highlighted.
4. Cancel the previous replacement via [u].
5. Continue replacing [y] until the end.
6. Notice that all sentences except the first and second now begin with an
incorrect letter D, which is not implied by the entered regexp, it should
be F and H, as if it had been replaced, but without being undoed in the
middle of the process.
Output:
---
Aaaaa.  Bbbb.
Ccccc.  Dddd.
Eeeee.  Dfff.
Ggggg.  Dhhh.
---

In GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
 cairo version 1.16.0) of 2024-05-20, modified by Debian built on sbuild
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.3/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/libexec
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/29.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/29.3/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils
 --with-native-compilation --with-cairo --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -ffile-prefix-map=/build/reproducible-path/emacs-29.3+1=.
-fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
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THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM GTK3 ZLIB

Important settings:
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  value of $LC_NUMERIC: ru_RU.UTF-8
  value of $LC_TIME: ru_RU.UTF-8
  value of $LANG: ru_RU.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
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  global-tab-line-mode: t
  tab-line-mode: t
  global-display-line-numbers-mode: t
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  tooltip-mode: t
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  menu-bar-mode: t
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  blink-cursor-mode: t
  column-number-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
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message sendmail dired dired-loaddefs rfc822 mml mml-sec epa derived
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mm-bodies mm-encode mail-utils mule-util gnutls network-stream url-cache
url-http url-auth mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums
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comint ansi-osc ring ansi-color gruvbox-dark-hard-theme gruvbox
autothemer color lisp-mnt dash saveplace whitespace tab-line
display-line-numbers elec-pair desktop frameset delsel cus-load
finder-inf csv-mode-autoloads info markdown-mode-autoloads
po-mode-autoloads tree-sitter-autoloads tsc-autoloads package browse-url
url url-proxy url-privacy url-expand url-methods url-history url-cookie
generate-lisp-file url-domsuf url-util mailcap url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs password-cache json subr-x
map byte-opt gv bytecomp byte-compile url-vars cl-loaddefs cl-lib
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Memory information:
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 (symbols 48 21740 133)
 (strings 32 123122 109316)
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bug#71819: Incorrect substitution when undo replacing regexps

[Eli Zaretskii]

> From: Sab Pyrope <sab.pyrope@gmail.com>
> Date: Fri, 28 Jun 2024 08:57:03 +0800
> 
> I encountered a bug when replacing regular expressions.
> If you [u]ndo a previous change during the replacement process, subsequent changes to the \2 substitution
> will use the old undone result.
> 
> Text:
> ---
> Aaaaa. Bbbb.
> Ccccc. Dddd.
> Eeeee. Ffff.
> Ggggg. Hhhh.
> ---
> Input regexp: \([.]\) \([A-Z]\)
> Output regexp: \1  \2
> Bug:
> 1. Copy the text above to the buffer.
> 2. Enter the regular expressions to the interactive regexp replacement [C-M-%].
> 3. Click [y] twice until the Ffff is highlighted.
> 4. Cancel the previous replacement via [u].
> 5. Continue replacing [y] until the end.
> 6. Notice that all sentences except the first and second now begin with an incorrect letter D, which is not
> implied by the entered regexp, it should be F and H, as if it had been replaced, but without being undoed in
> the middle of the process.
> Output:
> ---
> Aaaaa.  Bbbb.
> Ccccc.  Dddd.
> Eeeee.  Dfff.
> Ggggg.  Dhhh.
> ---

Thanks.

This is a regression between Emacs 26 and Emacs 27, perhaps as a side
effect of fixing bug#36328.  Juri, could you please take a look?

bug#71819: Incorrect substitution when undo replacing regexps

[Juri Linkov]

>> I encountered a bug when replacing regular expressions.
>> If you [u]ndo a previous change during the replacement process, subsequent changes to the \2 substitution
>> will use the old undone result.
>>
>> Text:
>> ---
>> Aaaaa. Bbbb.
>> Ccccc. Dddd.
>> Eeeee. Ffff.
>> Ggggg. Hhhh.
>> ---
>> Input regexp: \([.]\) \([A-Z]\)
>> Output regexp: \1  \2
>> Bug:
>> 1. Copy the text above to the buffer.
>> 2. Enter the regular expressions to the interactive regexp replacement [C-M-%].
>> 3. Click [y] twice until the Ffff is highlighted.
>> 4. Cancel the previous replacement via [u].
>> 5. Continue replacing [y] until the end.
>> 6. Notice that all sentences except the first and second now begin with an incorrect letter D, which is not
>> implied by the entered regexp, it should be F and H, as if it had been replaced, but without being undoed in
>> the middle of the process.
>> Output:
>> ---
>> Aaaaa.  Bbbb.
>> Ccccc.  Dddd.
>> Eeeee.  Dfff.
>> Ggggg.  Dhhh.
>> ---
>
> This is a regression between Emacs 26 and Emacs 27, perhaps as a side
> effect of fixing bug#36328.  Juri, could you please take a look?

This is caused by the commit 30c4f35a6fc8 that introduced these lines:

  (when regexp-flag
    (setq next-replacement (nth 4 elt)))

Maybe Tino knows how to fix this (Cc'ed).