怎样把 MediaWiki 的 wikitext 转换为 GitHub 风格的 markdown

;; wikitext -> gfm converter
;;
;; 设计原则：
;; 1 所有规则按顺序执行
;; 2 每条规则对应一段代码
;; 3 不做抽象，优先可读性
;;
;; 规则：
;; 1 <code> 和 </code> -> ```
;; 2 <syntaxhighlight> -> ```
;; 3 # -> *
;; 4 ====== -> ######
;; 5 ===== -> #####
;; 6 ==== -> ####
;; 7 === -> ###
;; 8 == -> ##
;; 9 <del> -> ~~
;; 10 ---- -> ---

(defun wikitext-convert-all (&optional start end)
  "Convert simple wikitext to GFM markdown."

  (interactive
   (if (use-region-p)
       (list (region-beginning) (region-end))
     (list (point-min) (point-max))))

  (save-excursion
    (save-restriction
      (narrow-to-region start end)

      ;; --------------------------------------------------
      ;; 1. inline <code>
      ;; --------------------------------------------------

      (goto-char (point-min))
      (while (re-search-forward "<code>\\([^<\n]*\\)</code>" nil t)
        (replace-match (concat "`" (match-string 1) "`") t t))

      (goto-char (point-min))
      (while (re-search-forward "<code>" nil t)
        (replace-match "`"))

      (goto-char (point-min))
      (while (re-search-forward "</code>" nil t)
        (replace-match "`"))

      ;; --------------------------------------------------
      ;; 2. syntaxhighlight
      ;; --------------------------------------------------

      (goto-char (point-min))
      (while (re-search-forward "<syntaxhighlight\\s-+lang=[\"']\\([^\"']+\\)[\"'][^>]*>" nil t)
        (replace-match (concat "```" (match-string 1)) t t))

      (goto-char (point-min))
      (while (re-search-forward "</syntaxhighlight>" nil t)
        (replace-match "```"))

      ;; --------------------------------------------------
      ;; 3. 列表 # → *
      ;; --------------------------------------------------

      (let ((in-code-block nil))
        (goto-char (point-min))
        (while (not (eobp))

          (let ((line (thing-at-point 'line t)))

            (when (string-match "^```" line)
              (setq in-code-block (not in-code-block)))

            (unless in-code-block
              (beginning-of-line)
              (while (search-forward "#" (line-end-position) t)
                (replace-match "*"))))

          (forward-line 1)))

      ;; --------------------------------------------------
      ;; 4. headings
      ;; --------------------------------------------------

      (dolist (n '(6 5 4 3 2))
        (goto-char (point-min))
        (let ((pat (format "^=\\{%d\\}[ \t]*\\(.*?\\S-\\)[ \t]*=\\{%d\\}[ \t]*$" n n))
              (rep (concat (make-string n ?#) " \\1")))
          (while (re-search-forward pat nil t)
            (replace-match rep))))

      ;; --------------------------------------------------
      ;; 5. other
      ;; --------------------------------------------------

      (goto-char (point-min))
      (while (re-search-forward "<del>" nil t)
        (replace-match "~~"))

      (goto-char (point-min))
      (while (re-search-forward "</del>" nil t)
        (replace-match "~~"))

      (goto-char (point-min))
      (while (re-search-forward "^----[ \t]*$" nil t)
        (replace-match "---"))

      (widen))))