changing delete in emacs

(defun get-back-char ()
  (buffer-substring-no-properties (- (point) 6) (point)))

(defun del-backward ()
  (interactive)
  (let ((text-back (get-back-char)))
    (do
        (message "test")
        (if (string= text-back "lambda")
            (delete-backward-char 6)
          (delete-backward-char 1)))))

(global-set-key [delete] 'del-backward)