;; based on select-safe-coding-system in mule-cmds.el of Emacs 20.7.

(require 'unicode)

(defun charref-regeon (from to &optional default-coding-system)
  "Convert characters of unsafe coding-system into XML character references."
  (interactive "r")
  (or default-coding-system
      (setq default-coding-system buffer-file-coding-system))
  (let* ((charsets (if (stringp from) (find-charset-string from)
		     (find-charset-region from to)))
	 (safe-coding-systems (find-coding-systems-for-charsets charsets)))
    (if (or (not enable-multibyte-characters)
	    (eq (car safe-coding-systems) 'undecided)
	    (eq default-coding-system 'no-conversion)
	    (and default-coding-system
		 (memq (coding-system-base default-coding-system)
		       safe-coding-systems)))
	default-coding-system

      ;; At first, change each coding system to the corresponding
      ;; mime-charset name if it is also a coding system.
      (let ((l safe-coding-systems)
	    mime-charset)
	(while l
	  (setq mime-charset (coding-system-get (car l) 'mime-charset))
	  (if (and mime-charset (coding-system-p mime-charset))
	      (setcar l mime-charset))
	  (setq l (cdr l))))

      (let ((non-safe-chars (find-multibyte-characters
			     from to 3
			     (and default-coding-system
				  (coding-system-get default-coding-system
						     'safe-charsets))))
	    show-position)
	(save-excursion
	  (when (integerp from)
	    (goto-char from)
	    (let ((found nil))
	      (while (and (not found)
			  (re-search-forward "[^\000-\177]" to t))
		(setq found (assq (char-charset (preceding-char))
				  non-safe-chars))))
	    (forward-line -1)
	    (setq show-position (point))
	    (save-excursion
	      (while (re-search-forward "[^\000-\177]" to t)
		(let* ((char (preceding-char))
		       (charset (char-charset char)))
		  (when (assq charset non-safe-chars)
		    (let* ((ucs (char-to-ucs char))
			   (ref (if (integerp ucs)
				    (format "&#%d;" ucs)
				  char)))
		      (delete-char -1)
		      (insert ref)
		      (setq to (+ to (length ref))))))))))))))