GNU bug report logs - #34823
27.0.50; re-search-forward infloop

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

From: Vitalie Spinu <spinuvit <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 27.0.50; re-search-forward infloop
Date: Tue, 12 Mar 2019 11:49:40 +0100

Hi,

In the context of markdown-mode I am seeing a font-lock infloop with a
relatively simple regexp:

 1) emacs -Q
 2) open a buffer with the following line

   "$_\t$tmp\t$cat\t$donor\t$stage\t$pool\t$rep\t$time\t$comment"'

 3) at bol eval (re-search-forward "\\(\\w[^\n]*\\)+}")
    which infloops

I see it with emacs 25, 26 and 27.0.5.

  Vitalie

----

In GNU Emacs 27.0.50 (build 7, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-08-31 built on Galago
Repository revision: 244b6827257fb0ec9c14f19b9dd01a0e1bee1d75
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 18.04.1 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
(New file)
Mark set
Quit [2 times]
Mark activated [4 times]
ido-read-internal: Command attempted to use minibuffer while in minibuffer
Quit [3 times]
ido-read-internal: Command attempted to use minibuffer while in minibuffer
Quit [2 times]
ad-Advice-delete-char: Text is read-only [3 times]

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LCMS2

Important settings:
  value of $LC_MONETARY: nl_NL.UTF-8
  value of $LC_NUMERIC: nl_NL.UTF-8
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
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  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  size-indication-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/vspinu/Dropbox/ELPA/math-symbol-lists-20170221.1353/math-symbol-lists hides ~/dev/math-symbol-lists/math-symbol-lists
/home/vspinu/Dropbox/ELPA/math-symbol-lists-20170221.1353/msl-build hides ~/dev/math-symbol-lists/msl-build
/home/vspinu/Dropbox/ELPA/key-combo-20150324.1439/key-combo hides ~/dev/key-combo/key-combo
/home/vspinu/Dropbox/ELPA/company-math-20171016.1514/company-math hides ~/VC/company-math/company-math
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens hides ~/VC/smartparens/smartparens
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-html hides ~/VC/smartparens/smartparens-html
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-lua hides ~/VC/smartparens/smartparens-lua
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/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-clojure hides ~/VC/smartparens/smartparens-clojure
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-config hides ~/VC/smartparens/smartparens-config
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-ruby hides ~/VC/smartparens/smartparens-ruby
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-rust hides ~/VC/smartparens/smartparens-rust
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-python hides ~/VC/smartparens/smartparens-python
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-racket hides ~/VC/smartparens/smartparens-racket
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-haskell hides ~/VC/smartparens/smartparens-haskell
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-latex hides ~/VC/smartparens/smartparens-latex
/home/vspinu/Dropbox/ELPA/smartparens-20181007.1501/smartparens-pkg hides ~/VC/smartparens/smartparens-pkg
/home/vspinu/Dropbox/ELPA/spinner-1.7.3/spinner hides ~/VC/spinner.el/spinner
/home/vspinu/Dropbox/ELPA/epl-20180205.2049/epl hides ~/VC/epl/epl
/home/vspinu/Dropbox/ELPA/dash-20180903.1042/dash hides ~/VC/dash.el/dash
/home/vspinu/Dropbox/ELPA/pkg-info-20150517.1143/pkg-info hides ~/VC/pkg-info.el/pkg-info
/home/vspinu/Dropbox/ELPA/projectile-20180824.2239/projectile hides ~/VC/projectile/projectile
/home/vspinu/Dropbox/ELPA/auto-complete-20170125.245/auto-complete-pkg hides ~/VC/auto-complete/auto-complete-pkg
/home/vspinu/Dropbox/ELPA/auto-complete-20170125.245/auto-complete-config hides ~/VC/auto-complete/auto-complete-config
/home/vspinu/Dropbox/ELPA/auto-complete-20170125.245/auto-complete hides ~/VC/auto-complete/auto-complete
/home/vspinu/Dropbox/ELPA/color-theme-solarized-20171024.1525/solarized-theme hides ~/VC/solarized-emacs/solarized-theme
/home/vspinu/Dropbox/ELPA/queue-0.2/queue hides ~/Dropbox/.emacs.d/queue
~/VC/markdown-mode/markdown-mode hides ~/Dropbox/.emacs.d/markdown-mode
/home/vspinu/Dropbox/ELPA/rebox2-20121113.2100/rebox2 hides ~/Dropbox/.emacs.d/rebox2
~/Dropbox/.emacs.d/allout-widgets hides /home/vspinu/bin/emacs/lisp/allout-widgets
~/Dropbox/.emacs.d/allout hides /home/vspinu/bin/emacs/lisp/allout
/home/vspinu/Dropbox/ELPA/less-css-mode-20161001.453/less-css-mode hides /home/vspinu/bin/emacs/lisp/textmodes/less-css-mode
~/Dropbox/.emacs.d/org-protocol hides /home/vspinu/bin/emacs/lisp/org/org-protocol
~/Dropbox/.emacs.d/tabulated-list hides /home/vspinu/bin/emacs/lisp/emacs-lisp/tabulated-list

Features:
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Memory information:
((conses 16 972819 26432)
 (symbols 48 68247 1)
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 (strings 32 246201 16927)
 (string-bytes 1 6790923)
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Message #8 received at 34823 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: 34823 <at> debbugs.gnu.org
Subject: 27.0.50; re-search-forward infloop
Date: Fri, 29 Mar 2019 14:28:18 +0100

It is not actually an infinite loop; it just takes a very long time.

Your case is, more or less, essentially

(search-string "\\(?:a*\\)*b" "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")

Try trimming the string, and you will see that the match does terminate (and fail); it is just super-linear in the length of the text.
This is a shortcoming of the general type of regexp engine used by Emacs.

To work around your problem, you should make the regexp less prone to backtracking. Try asking on the list help-gnu-emacs <at> gnu.org for advice. Describe what text you wish to match as precisely as possible.

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