Reading multiple lines of string in Common Lisp

sharp-!.lisp

(in-package #:cl-user)

(defun sharp-exclamation-mark (stream _c _arg)
  (declare (ignore _c _arg))
  (with-output-to-string (s)
    (do ((level 1)
         (prev (read-char stream) char)
         (char (read-char stream)
               (read-char stream)))
        (())
      (cond ((and (char= prev #\!) (char= char #\#))
             (setq level (1- level))
             (when (zerop level) (return (values)))
             (setq char (read-char stream)))
            ((and (char= prev #\#) (char= char #\!))
             (setq char (read-char stream))
             (setq level (1+ level)))
            (t (write-char prev s))))))

(defun enable-sharp-!-reader (&optional (rt *readtable*))
  (set-dispatch-macro-character #\# #\! #'sharp-exclamation-mark rt))

(defun disable-sharp-!-reader (&optional (rt *readtable*))
  (set-dispatch-macro-character #\# #\! nil rt))