;; (assert (string= (tags-sort-tags-string "zee bee   cee") "bee cee zee") t "Tags not sorted.")
(defun tags-sort-tags-string (string)
  "Return an alphabetically-sorted list of words in this string."
  (save-match-data
    (mapconcat 'identity
               (sort (mapcar 'downcase
                             (split-string string))
                     'string<)
               " ")))

(defun tags-sort-tags ()
  "Find the first line beginning with Tags: tag tag tag and sort the tags alphabetically."
  (interactive)
  (goto-char (point-min))
  (when (re-search-forward "^Tags: \\(.+\\)" nil t)
    (replace-match (tags-sort-tags-string (match-string 1)) t t nil 1)))

;; (assert (string-match (tags-make-emacs-search-regexp '("bee")) "bee cee"))
;; (assert (string-match (tags-make-emacs-search-regexp '("cee")) "bee cee"))
;; (assert (string-match (tags-make-emacs-search-regexp '("cee" "bee")) "bee cee"))
;; (assert (null (string-match (tags-make-emacs-search-regexp '("zee" "cee")) "bee cee")))
;; (assert (null (string-match (tags-make-emacs-search-regexp '("cee" "b")) "bee cee")))

(defun tags-make-emacs-search-regexp (tags)
  "Convert TAGS to a regular expression."
  (concat
   "\\b"
   (mapconcat 'downcase
             (sort tags 'string<)
             "\\b+\\(.+\\b\\)*")
   "\\b"))

(defun tags-make-grep-search-regexp (tags)
  "Convert TAGS to a regular expression."
  (concat
   "\b"
   (mapconcat 'downcase
             (sort tags 'string<)
             "\b+\(.+\b\)*")
   "\b"))