bug#81327: 14.1.0; texmathp returns nil inside $...$ math that follows "\\" (e.g. "$a$\\$b$")

Gabriele Nicolardi via bug-auctex via Bug reporting list for AUCTeX Mon, 29 Jun 2026 00:37:05 -0700

Hi,


texmathp misreports math mode when a math-opening "$" is immediately
preceded by "\\" (a TeX line break). This is valid LaTeX and common in
tabular/array cells and multi-line TikZ node labels.

Tested against current git master (texmathp.el, copyright 1998-2024).

How to reproduce (emacs -Q, AUCTeX loaded):

  Put point on the "b" of the second math group and run M-x texmathp.

    \begin{tabular}{c}$a$\\$b$\end{tabular}
                          ^ point here

  Expected: math-mode is on (point is inside $...$).
  Actual:   texmathp returns nil ("math-mode is off").

  Minimal trigger: in a buffer containing  $a$\\$b$  , point on "b"
  returns nil. The first group $a$ is detected correctly; everything
  after the "\\$" has inverted parity.

Root cause:

  texmathp counts unescaped "$" toggles via `texmathp-toggle-regexp',
  built in `texmathp-compile' as

      (concat "\\([^\\$]\\|\\`\\)" (regexp-opt togglers t))

  The "[^\\$]" prefix is meant to skip an escaped dollar "\$", by
  requiring the char before a counted "$" to be neither "\" nor "$".
  But it also rejects a *genuine* math "$" that follows "\\": in
  "$a$\\$b$" the opening "$" of the second group is preceded by the
  second backslash of "\\", so it is never counted. The dollar parity
  is then wrong for the rest of the paragraph, and texmathp returns the
  inverted result on alternating math groups.

  The correct rule is backslash *parity*: a "$" is a real toggle iff it
  is preceded by an even number of backslashes ("\$" = literal, skip;
  "\\$" = line break + math open, count).

Proposed fix (texmathp-compile):

  -          (concat "\\([^\\$]\\|\\`\\)"

+ (concat"\$\\(?:\\(?:[^\\$]\\|\\`\$\$?:\\\\\\\\\$*\\)\\|\$?:\\\\\\\\\$+\\)"

                     (regexp-opt togglers t))

  i.e. group 1 becomes "(anchor char or BOL) followed by zero or more
  backslash pairs" OR "one or more backslash pairs" (the second branch
  handles the case where the toggling "$" directly follows a previously
  consumed "$", as in "$...$\\$..."). Group numbering is unchanged, so
  the (match-string 2)/(match-beginning 2) references in `texmathp'
  still refer to the dollar(s).

Verified with the patch:

    $a$\\$b$            point on b  -> t    (was nil)   FIXED
    tabular cell above  point on b  -> t    (was nil)   FIXED
    $x+y$               -> t                (unchanged)
    $$a+b$$             -> t                (unchanged)
    \$5 e testo         -> nil              (literal $, unchanged)
    \$5 \$6 testo       -> nil              (between two \$, unchanged)

For full transparency: the root-cause analysis and the suggested fix
were developed with the help of an AI assistant (Claude Code). I have
independently reproduced the issue with "emacs -Q" and checked the
patch against the regression cases listed above. Please review the
patch on its own merits; feel free to adopt a different formulation if
you prefer.

Thanks,

Gabriele Nicolardi

Emacs : GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version3.24.41, cairo version 1.18.0)

 of 2026-02-03, modified by Debian
Package: 14.1.0

current state:
==============
(setq
 window-system 'x
 LaTeX-version "2e"

TeX-style-path '("~/.emacs.d/auctex/""/home/gabriele/.emacs.d/elpa/auctex-14.1.0/style" "/home/gabriele/.emacs.d/auctex/auto""/home/gabriele/.emacs.d/auctex/style" "auto" "style")

 TeX-auto-save nil
 TeX-parse-self nil
 TeX-master t

TeX-command-list '(("TeX" "%(PDF)%(tex) %(file-line-error)%`%(extraopts) %S%(PDFout)%(mode)%' %(output-dir) %t" TeX-run-TeX nil (plain-TeX-mode AmSTeX-mode Texinfo-mode):help "Run plain TeX") ("LaTeX" "%`%l%(mode)%' %T" TeX-run-TeX nil (LaTeX-modedocTeX-mode) :help "Run LaTeX") ("Makeinfo" "makeinfo %(extraopts) %(o-dir) %t"TeX-run-compile nil (Texinfo-mode) :help

             "Run Makeinfo with Info output")

("Makeinfo HTML" "makeinfo %(extraopts) %(o-dir) --html %t"TeX-run-compile nil

             (Texinfo-mode) :help "Run Makeinfo with HTML output")

("AmSTeX" "amstex %(PDFout) %`%(extraopts) %S%(mode)%'%(output-dir) %t" TeX-run-TeX nil

             (AmSTeX-mode) :help "Run AMSTeX")

("ConTeXt" "%(cntxcom) --once %(extraopts) %(execopts)%t"TeX-run-TeX nil (ConTeXt-mode)

             :help "Run ConTeXt once")

("ConTeXt Full" "%(cntxcom) %(extraopts) %(execopts)%t"TeX-run-TeX nil (ConTeXt-mode) :help

             "Run ConTeXt until completion")
            ("BibTeX" "bibtex %(O?aux)" TeX-run-BibTeX nil

(plain-TeX-mode LaTeX-mode docTeX-mode AmSTeX-modeTexinfo-mode ConTeXt-mode) :help

             "Run BibTeX")
            ("Biber" "biber %(output-dir) %s" TeX-run-Biber nil

(plain-TeX-mode LaTeX-mode docTeX-mode AmSTeX-modeTexinfo-mode) :help "Run Biber") ("Texindex" "texindex %s.??" TeX-run-command nil(Texinfo-mode) :help "Run Texindex") ("Texi2dvi" "%(PDF)texi2dvi %t" TeX-run-command nil(Texinfo-mode) :help

             "Run Texi2dvi or Texi2pdf")

("View" "%V" TeX-run-discard-or-function t t :help "RunViewer")

            ("Print" "%p" TeX-run-command t t :help "Print the file")

("Queue" "%q" TeX-run-background nil t :help "View theprinter queue" :visible

             TeX-queue-command)
            ("File" "%(o?)dvips %d -o %f " TeX-run-dvips t