(define reg (make-regexp "[$][{]([[:alnum:]]+)[}]"))
(define (%xformat str alist)
  (regexp-substitute/global
   #f reg str 'pre
   (lambda (m)
     (or (assq-ref alist (string->symbol (match:substring m 1)))
         (format #f "error:~a" (match:substring m 1))))
   'post))

(display (%xformat "${invoice} from ${customer}"
                   '((customer . "client") (invoice . "facture"))))
(newline)