gistfile1.el

(defun my-html-escape (start end)
  (interactive "r")
  (save-excursion
    (save-restriction
      (narrow-to-region start end)
      (goto-char (point-min))
      (while (re-search-forward "<" nil t)
        (replace-match "&lt;"))
      (goto-char (point-min))
      (while (re-search-forward ">" nil t)
        (replace-match "&gt;"))
      (while (re-search-forward "\"" nil t)
        (replace-match "&quote;"))
      (while (re-search-forward "&" nil t)
        (replace-match "&amp;")))))