Thank you very much for your comments, Stefan. As you point out, the
previous code is found to fail when regular expressions and sexp
comments exist in the same line.

Now I put the logic to scan the beginning of sexp comments and regular
expressions in syntax-propertize-rules macro. I use different functions
to be invoked by them, because the beginning of regular expressions
need to be canceled when they are already in normal strings or
comments. (This kind of logic might be required also for sexp comment,
but the current implementation does not do that and does not seem to
cause any harms.)

Therefore, if I use the same function, it requires an extra conditional
code only for regular expressions, which is a repetition of what is done
in syntax-propertize-rules level. (At syntax-propertize-rules leve, we
already know it's a beginning of a sexp comment or a possible beginning
of regular expression).

The following explains the functions implemented.

- scheme-syntax-propertize-sexp-comment

  No change from the current built-in implementation

- scheme-syntax-propertize-regexp-end

  If the posistion is already in regular expressions and not in
  comments, it searches regular expression end (/) ignoring backslash
  slash (\/).


- scheme-syntax-propertize-regexp

  This is invoked in syntax-propertize-rules. It cancels syntax class
  assignment to # of #/ if the # part is already in strings or
  comments. (Precisely, it assigns @ syntax class to # .) Otherwise it
  continues to scan the end of regular expression by calling
  scheme-syntax-propertize-regexp-end.


Thanks.


#+BEGIN_SRC
(add-hook
 'scheme-mode-hook
 (lambda ()
   (setq-local
    syntax-propertize-function
    (lambda (beg end)
      (goto-char beg)
      (scheme-syntax-propertize-sexp-comment (point) end)
      (scheme-syntax-propertize-regexp-end (point) end)
      (funcall
       (syntax-propertize-rules
        ("\\(#\\);" (1 (prog1 "< cn"
                         (scheme-syntax-propertize-sexp-comment
                          (point) end))))
        ("\\(#\\)/" (1 (prog1 "|"
                         (scheme-syntax-propertize-regexp
                          (point) end))))
         )
       (point) end)
      ))))

(defun scheme-syntax-propertize-sexp-comment (_ end)
  (let ((state (syntax-ppss)))
    (when (eq 2 (nth 7 state))
      ;; It's a sexp-comment.  Tell parse-partial-sexp where it ends.
      (condition-case nil
          (progn
            (goto-char (+ 2 (nth 8 state)))
            ;; FIXME: this doesn't handle the case where the sexp
            ;; itself contains a #; comment.
            (forward-sexp 1)
            (put-text-property (1- (point)) (point)
                               'syntax-table (string-to-syntax "> cn")))
        (scan-error (goto-char end))))))

(defun scheme-syntax-propertize-regexp-end (_ end)
  (let* ((state (syntax-ppss))
         (within-str (nth 3 state))
         (within-comm (nth 4 state))
         (start-delim-pos (nth 8 state)))
    (if (and (not within-comm)
             (and within-str
                  (string=
                   (buffer-substring-no-properties
                    start-delim-pos
                    (1+ start-delim-pos))
                   "#")))
        (let ((end-found nil))
          (while (and
                  (not end-found)
                  (re-search-forward "\\(/\\)" end t))
            (progn
              (if
                  (not (char-equal
                        (char-before (match-beginning 1))
                        ?\\ ))
                  (progn
                    (put-text-property
                     (match-beginning 1)
                     (1+ (match-beginning 1))
                     'syntax-table (string-to-syntax "|"))
                    (setq end-found t)
                    )))))
      )))

(defun scheme-syntax-propertize-regexp (_ end)
  (let* ((match-start-state (save-excursion
                              (syntax-ppss (match-beginning 1))))
         (within-str (nth 3 match-start-state))
         (within-comm (nth 4 match-start-state)))
    (if (or within-str within-comm)
          (put-text-property ;; Cancel regular expression start
           (match-beginning 1)
           (1+ (match-beginning 1))
           'syntax-table (string-to-syntax "@"))
      (scheme-syntax-propertize-regexp-end _ end)
      )))

#+END_SRC