(defun extract-integers (string)
  (loop
    :with integer
    :with end := 0
    :for start := (position-if (function digit-char-p) string :start end)
    :while start
    :do (setf (values integer end) (parse-integer string :start start :junk-allowed t))
    :collect integer))

(defun test/extract-integers ()
 (assert (equal (extract-integers "123abc456")
                '(123 456)))
 (assert (equal (extract-integers "foo")
                '()))
 (assert (equal (extract-integers "1 2 3 4")
                '(1 2 3 4)))
 (assert (equal (extract-integers "1234")
                '(1234)))
  :success)

(test/extract-integers) ; --> :success