(defun extract-integers (string)
  (loop
    :with integer
    :with end
    :for start := (position-if (function digit-char-p) string)
      :then  (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))

(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)))