(defun run-bf (string)
  (let ((mem (make-vector 30000 0))
        (p 0)
        (i 0) (l (length string)) (return 0))
    (while (< i l)
      (let ((c (aref string i)))
        (cond
         ((= c ?+) (incf (aref mem p)))
         ((= c ?-) (decf (aref mem p)))
         ((= c ?>) (incf p))
         ((= c ?<) (decf p))
         ((= c ?.) (insert (aref mem p)))
         ((= c ?,) (progn (char-after (point)) (forward-char 1)))
         ((= c ?\[) (if (equal (aref mem p) 0)
                        (setq i (string-match "\\]" string i))
                      (setq return i)))
         ((= c ?\]) (setq i (1- return))))
        (incf i)))))

;; And here a test-case to demonstrate that it actually works:

(run-bf "++++++++++++++++++++++++++++++++[>+>+<<-]>>+++++++++++++++++++++++++<<++++++++++[>>.-<.<-]")

;; Which should output 9 8 7 6 5 4 3 2 1 0 in the current buffer.
;; Bugs: The matching bracket rules is not implemented, so nested
;; loops will fail.