overlay-hacking

(defvar my-overlays nil)

(let ((start (point-min)))
  (save-excursion
    (goto-char start)
    (while (re-search-forward "aaaa" nil t)
      (let ((overlay (make-overlay start (- (match-beginning 0) 1))))
        (add-to-list 'my-overlays overlay))
      (forward-line)
      (setq start (point)))))

(let ((o (car (last my-overlays))))
  (overlay-put o 'invisible t))

Obviously this code is partial and doesn't hide all the overlays it's created. It also needs code to restore the overlays after being done. Oh and the variable should be buffer local - yadda yadda...