(defparameter *my-readtable* (copy-readtable))

(let ((*readtable* *my-readtable*))
  (set-dispatch-macro-character #\# #\. nil))

(defun safe-read ()
  (let ((*readtable* *my-readtable*))
    (read)))

(setf b (safe-read))
(princ b))