;; Usage: (split s)
;;
;; Returns: String s split into words, in lowercase without non-alphabetic
;;          characters.
;;
;; Example:
;;   (split "This is a string!!")
;; Returns:
;;   ("this" "is" "a" "string")

(defun rsplit (list &optional (splitted ()) (cur ()))
  (if (null list) (if cur (cons cur splitted) splitted)
    (let ((ch (car list)))
      (if (alpha-char-p ch)
	  (rsplit (cdr list) splitted (cons ch cur))
	(if cur (rsplit (cdr list) (cons cur splitted))
	  (rsplit (cdr list) splitted))))))

(defun split (s) 
  (mapcar #'(lambda (x) (map 'string #'char-downcase x)) 
	  (rsplit (nreverse (coerce s 'list)))))