(defun mainf(str)
  (if (not (search "begin" str))
          (if (not (search "end;" str))
            nil
            (cons (part-to-end str) (mainf (subs-end str)))
          )
         ;--------------------------------------------
         (if (next-begin str) 
           (if (> (length (part-to-begin str))  0 )
             (list (part-to-begin str) (list :begin (mainf (subs-begin str))))
             (list :begin (mainf (subs-begin str)))
           )
           (list (part-to-end str))
         )
         ;--------------------------------------------
  )
)

(defun subs-begin(str)
  (subseq str (+ (search "begin" str)  5 ) (length str)))

(defun subs-end(str)
  (subseq str (+ (search "end;" str)  4 ) (length str))) 

(defun next-begin(str)
 (let ((b nil)
       (e nil))
  (setf b (search "begin" str))
  (setf e (search "end" str))
  (if (eq b nil)
      (setf b - 1 ))
  (if (eq e nil)
      (setf e - 1 ))
  (if (> e b)
   t
   nil)))

(defun part-to-begin(str)
  (subseq str  0  (search "begin" str)))

(defun part-to-end(str)
  (subseq str  0  (search "end;" str)))

(study:mainf "begin d1 begin d2 begin d3 end; d4 end; d5 end;")

(:BEGIN (" d1 " (:BEGIN (" d2 " (:BEGIN (" d3 " " d4 " " d5 "))))))

(:BEGIN (" d1 " (:BEGIN (" d2 " (:BEGIN (" d3 ") " d4 ") " d5 "))))

(defun parse (text)
   ""
   (loop
      :for el = (read text nil nil)
      :while el
      :append (cond ((string-equal el :begin)
		     (list :begin (parse text)))
		    ((string-equal el :|END.|)
		     nil)
		    ((null el)
		     nil)
		    (t (list el)))))

CL-USER> (parse "begin d1 begin d2 end; begin d3 end; d5 end;")

There is no applicable method for the generic function
  #<STANDARD-GENERIC-FUNCTION STREAM-READ-CHAR ( 1 )>
when called with arguments
  ("begin d1 begin d2 end; begin d3 end; d5 end;").
   [Condition of type SIMPLE-ERROR]

Restarts:
  0 : [ABORT] Abort
  1 : [CLOSE-CONNECTION] Close SLIME connection
  2 : [ABORT] Exit debugger, returning to top level.

 (parse "begin d1 begin d2 begin d3 end; d4 end; d5 end;")