fix(org-latex): rewrite tabularx filter using with-temp-buffer + replace-match

Emacs 31 changed behavior of replace-regexp-in-string with lambda replacements
containing backslashes: 'Invalid use of backslash in replacement text' error.

Fix: use with-temp-buffer + re-search-forward + replace-match (literal t t).
replace-match with LITERAL=t never processes backslash sequences.

Also fixes \end{tabular} → \end{tabularx} regex to not match existing tabularx.

config.el

@@ -326,41 +326,32 @@ Bound to cmd+v in org-mode and markdown-mode."
 ;;; ============================================================
 
 ;; LaTeX table export: tabular → tabularx{\linewidth} with lYYY column spec.
-;; Filter on rendered LaTeX string — no buffer modification, no recursion.
+;; Uses with-temp-buffer + replace-match (literal t t) — avoids backslash issues
+;; in replace-regexp-in-string lambda replacements on Emacs 31.
 ;; Uses Y column type (defined in document.org template: RaggedRight + auto-width X).
-;; Handles:
-;;   \begin{tabular}{lll}   → \begin{tabularx}{\linewidth}{lYY}
-;;   \begin{tabularx}{lll}  → \begin{tabularx}{\linewidth}{lYY}  (missing width fix)
 (defun my/org-latex-col-to-lyyy (spec)
-  "Convert tabular column SPEC to tabularx lYYY format.
-Counts l/r/c columns; first → l, rest → Y (auto-width, defined in template)."
-  (let* ((ncols (length (replace-regexp-in-string "[^lrcLRCpP]" "" spec)))
-         (ncols (max 1 ncols)))
+  "Convert tabular column SPEC to lYYY: first col l, rest Y (auto-width)."
+  (let ((ncols (max 1 (length (replace-regexp-in-string "[^lrcLRCpP]" "" spec)))))
     (if (= ncols 1) "Y"
       (concat "l" (make-string (1- ncols) ?Y)))))
 
 (defun my/org-latex-fix-tabularx (table _backend _info)
-  "Convert tabular → tabularx{\\linewidth}{lYYY} in LaTeX table output."
+  "Convert tabular/tabularx → tabularx{\\linewidth}{lYYY} in LaTeX output."
   (when (stringp table)
-    ;; Fix tabularx{colspec} missing width (some org versions omit \linewidth)
-    (setq table
-          (replace-regexp-in-string
-           "\\\\begin{tabularx}{\\([^\\\\][^}]*\\)}"
-           (lambda (m)
-             (format "\\begin{tabularx}{\\linewidth}{%s}"
-                     (my/org-latex-col-to-lyyy (match-string 1 m))))
-           table))
-    ;; Convert tabular{colspec} → tabularx{\linewidth}{lYYY}
-    (setq table
-          (replace-regexp-in-string
-           "\\\\begin{tabular}{\\([^}]*\\)}"
-           (lambda (m)
-             (format "\\begin{tabularx}{\\linewidth}{%s}"
-                     (my/org-latex-col-to-lyyy (match-string 1 m))))
-           table))
-    (setq table
-          (replace-regexp-in-string "\\\\end{tabular}" "\\\\end{tabularx}" table)))
-  table)
+    (with-temp-buffer
+      (insert table)
+      (goto-char (point-min))
+      ;; \begin{tabular}{spec} or \begin{tabularx}{spec} → \begin{tabularx}{\linewidth}{lYYY}
+      (while (re-search-forward "\\\\begin{tabular[x]?}{\\([^}]*\\)}" nil t)
+        (let* ((spec (match-string 1))
+               (new-spec (my/org-latex-col-to-lyyy spec))
+               (repl (concat "\\begin{tabularx}{\\linewidth}{" new-spec "}")))
+          (replace-match repl t t)))
+      ;; \end{tabular} → \end{tabularx}  (skip if already tabularx)
+      (goto-char (point-min))
+      (while (re-search-forward "\\\\end{tabular}" nil t)
+        (replace-match "\\end{tabularx}" t t))
+      (buffer-string))))
 
 ;; Register filter on ox-latex load AND ensure it via a pre-processing hook
 ;; (belt+suspenders: whichever fires first wins, both are idempotent).