(let ((gen-regexp
       (rx bol
           (group-n 5
                    "File \"" (group-n 1 (+ (not (any "\"\n")))) "\", "
                    "line " (group-n 2 (+ digit)) ", "
                    "characters " (group-n 3 (+ digit)) "-" (group-n 4 (+ digit))) ":\n"
           (opt (group-n 6 bol "Warning" (opt " " (+ digit)) ":")))))
  (setq compilation-error-regexp-alist `((,gen-regexp  1 2 (3 . 4) (6 . nil) 5))))