Thanks, Arash. Agreed, on all counts. Revised patch attached.

Best, David.

On Thu, 16 May 2024 at 08:54, Arash Esbati <arash@gnu.org> wrote:
>
> David Fussner <dfussner@googlemail.com> writes:
>
> > +(defun tex-expl-buffer-parse ()
> > +  "Identify buffers where expl3 syntax is always active."
> > +  (save-excursion
> > +    (goto-char (point-min))
> > +    (when (tex-search-noncomment
> > +        (re-search-forward
> > +         "\\(?:\\\\\\(?:ExplFile\\|ProvidesExpl\\|__xparse_file\\)\\)"
>
> Is the outer grouping necessary?  Why not just:
>
> "\\\\\\(?:ExplFile\\|ProvidesExpl\\|__xparse_file\\)"
>
> > +         nil t))
> > +      (setq tex-expl-buffer-p t))))
> > +
> > +(defun tex-expl-region-set (_beg _end)
> > +  "Create a list of regions where expl3 syntax is active.
> > +This function updates the list whenever `syntax-propertize' runs, and
> > +stores it in the buffer-local variable `tex-expl-region-list'.  The
> > +list will always be nil when the buffer visits an expl3 file, e.g., an
> > +expl3 class or package, where expl3 syntax is always active."
> > +  (unless syntax-ppss--updated-cache;; Stop forward search running twice.
> > +    (setq tex-expl-region-list nil)
> > +    ;; Leaving this test here allows users to set `tex-expl-buffer-p'
> > +    ;; independently of the mode's automatic detection of an expl3 file.
> > +    (unless tex-expl-buffer-p
> > +      (goto-char (point-min))
> > +      (while (tex-search-noncomment
> > +              (re-search-forward "\\ExplSyntaxOn" nil t))
>
> This looks wrong, I think you want `search-forward'.
>
> > +        (let ((new-beg (point))
> > +              (new-end (or (tex-search-noncomment
> > +                            (re-search-forward "\\ExplSyntaxOff" nil t))
>
> Same here.
>
> > +                           (point-max))))
> > +          (push (cons new-beg new-end) tex-expl-region-list))))))
>
> Best, Arash