foo.lisp

(defun parse-number (str &key (package *package*) (error nil))
  (with-standard-io-syntax
    (with-input-from-string (s str)
      (let ((*read-eval* nil)
            (*package* package))
        (let ((val
                (restart-case
                    (handler-bind
                        ((condition (lambda (c)
                                      (invoke-restart 'bad-read error))))
                      (read s))
                  (bad-read (retval)
                    retval))))
          (if (numberp val)
              val
              error))))))

;; CL-USER> (parse-number "23")
;; 23
;; CL-USER> (parse-number "23.54")
;; 23.54
;; CL-USER> (parse-number "23/23")
;; 1
;; CL-USER> (parse-number "23")
;; 23
;; CL-USER> (parse-number "23.54")
;; 23.54
;; CL-USER> (parse-number "23/2435")
;; 23/2435
;; CL-USER> (parse-number "#C(12 34)")
;; #C(12 34)
;; CL-USER> (parse-number "#.(+ 1 2 3 4)")
;; NIL
;; CL-USER> (parse-number "#.(+ 1 2 3 4)" :error :nope)
;; :NOPE