#lang racket
(define lens
  '([AL 28] [AD 24] [AT 20] [AZ 28] [BH 22] [BE 16] [BA 20] [BR 29] [BG 22]
    [CR 21] [HR 21] [CY 28] [CZ 24] [DK 18] [DO 28] [EE 20] [FO 18] [FI 18]
    [FR 27] [GE 22] [DE 22] [GI 23] [GR 27] [GL 18] [GT 28] [HU 28] [IS 26]
    [IE 22] [IL 23] [IT 27] [KZ 20] [KW 30] [LV 21] [LB 28] [LI 21] [LT 20]
    [LU 20] [MK 19] [MT 31] [MR 27] [MU 30] [MC 27] [MD 24] [ME 22] [NL 18]
    [NO 15] [PK 24] [PS 29] [PL 28] [PT 25] [RO 24] [SM 27] [SA 24] [RS 22]
    [SK 24] [SI 19] [ES 24] [SE 24] [CH 21] [TN 24] [TR 26] [AE 23] [GB 22]
    [VG 24]))
(define (valid-iban? str)
  (define str* (regexp-replace* #px"\\s+" str ""))
  (define c (cond [(regexp-match #rx"^[A-Z][A-Z]" str*)
                   => (λ(x) (assq (string->symbol (car x)) lens))]
                  [else #f]))
  (define (letter c)
    (number->string (+ (char->integer (string-ref c 0)) -65 10)))
  (and c (= (cadr c) (string-length str*))
       (regexp-match? #rx"[A-Z0-9]" str*)
       (let* ([x (string-append (substring str* 4) (substring str* 0 4))]
              [x (string->number (regexp-replace* #rx"[A-Z]" x letter))])
         (= 1 (modulo x 97)))))

(valid-iban? "GB82 WEST 1234 5698 7654 32") ; => #t
(valid-iban? "GB82 TEST 1234 5698 7654 32") ; => #f