示例：字串代换

CL-USER> (defstruct buf
       vec (start -1) (used -1) (new -1) (end -1))
BUF

CL-USER> (defun bref (buf n)
       (svref (buf-vec buf)
          (mod n (length (buf-vec buf)))))
BREF

CL-USER> (defun (setf bref) (val buf n)
       (setf (svref (buf-vec buf)
            (mod n (length (buf-vec buf))))
         val))
(SETF BREF)

CL-USER> (defun new-buf (len)
       (make-buf :vec (make-array len)))
NEW-BUF

CL-USER> (defun buf-insert (x b)
       (setf (bref b (incf (buf-end b))) x))
BUF-INSERT

CL-USER> (defun buf-pop (b)
        (prog1
        (bref b (incf (buf-start b)))
          (setf (buf-used b) (buf-start b)
            (buf-new b) (buf-end b))))
BUF-POP

CL-USER> (defun buf-next (b)
       (when (< (buf-used b) (buf-new b))
         (bref b (incf (buf-used b)))))
BUF-NEXT

CL-USER> (defun buf-reset (b)
       (setf (buf-used b) (buf-start b)
         (buf-new b) (buf-end b)))
BUF-RESET

CL-USER> (defun buf-clear (b)
       (setf (buf-start b) -1 (buf-used b) -1
         (buf-new b) -1 (buf-end b) -1))
BUF-CLEAR

CL-USER> (defun buf-flush (b str)
       (do ((i (1+ (buf-used b)) (1+ i)))
           ((> i (buf-end b)))
         (princ (bref b i) str)))
BUF-FLUSH

CL-USER> (defun file-subst (old new file1 file2)
       (with-open-file (in file1 :direction :input)
         (with-open-file (out file2 :direction :output
                  :if-exists :supersede)
           (stream-subst old new in out))))
;Compiler warnings :
;   In FILE-SUBST: Undefined function STREAM-SUBST
FILE-SUBST

CL-USER> (defun stream-subst (old new in out)
       (let* ((pos 0)
          (len (length old))
          (buf (new-buf len))
          (from-buf nil))
         (do ((c (read-char in nil :eof)
             (or (setf from-buf (buf-next buf))
             (read-char in nil :eof))))
         ((eql c :eof))
           (cond ((char= c (char old pos))
              (incf pos)
              (cond ((= pos len)
                 (princ new out)
                 (setf pos 0)
                 (buf-clear buf))

                ((not from-buf)
                 (buf-insert c buf))))
             ((zerop pos)
              (princ c out)
              (when from-buf
            (buf-pop buf)
            (buf-reset buf)))
             (t
              (unless from-buf
            (buf-insert c buf))
              (princ (buf-pop buf) out)
              (buf-reset buf)
              (setf pos 0))))
         (buf-flush buf out)))
STREAM-SUBST

然后在”C:\lispbox-0.7”路径下新建文件test1,test2

test1中存储的内容：

The struggle between Liberty and Authority is the most conspicuous feature
in the portions of history with which we are earliest familiar, particularly
in that of Greece, Rome, and England.

然后在REPL中输入如下代码：

CL-USER> (file-subst " th" " z" "test1" "test2")
NIL

打开test2，其内容为：

The struggle between Liberty and Authority is ze most conspicuous feature
in ze portions of history with which we are earliest familiar, particularly
in zat of Greece, Rome, and England.