(defpackage #:custom-string-reader
  (:use #:common-lisp)
  (:local-nicknames (#:nr #:named-readtables))
  (:export #:custom-string-reader))

(in-package #:custom-string-reader)

(defun custom-string-reader (stream char)
  (declare (ignore char))
  (loop with result = (make-array 0 :element-type 'character
                                    :fill-pointer 0
                                    :adjustable t)
        for char = (read-char stream t nil t)
        do (case char
             ((#\|) (vector-push-extend (read-char stream t nil t) result))
             ((#\") (return result))
             (t (vector-push-extend char result)))))

(nr:defreadtable custom-string-reader
  (:merge :standard)
  (:macro-char #\" #'custom-string-reader t))



CL-USER> (named-readtables:in-readtable custom-string-reader:custom-string-reader)
#<NAMED-READTABLE CUSTOM-STRING-READER:CUSTOM-STRING-READER {10021AC5A3}>
CL-USER> "foo|\bar"
"foo\\bar"