(def explen
  {"AL" 28 "AD" 24 "AT" 20 "AZ" 28 "BE" 16 "BH" 22 "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})

(defn valid-iban? [iban]
  (let [iban (apply str (remove #{\space \tab} iban))]
    (cond
      ; Ensure upper alphanumeric input.
      (not (re-find #"^[\dA-Z]+$" iban)) false
      ; Validate country code against expected length.
      (not= (explen (subs iban 0 2)) (count iban)) false
      :else
      (let [rot   (flatten (apply conj (split-at 4 iban)))
            trans (map #(read-string (str "36r" %)) rot)]
        (= 1 (mod (bigint (apply str trans)) 97))))))

(prn (valid-iban? "GB82 WEST 1234 5698 7654 32")  ; true
     (valid-iban? "GB82 TEST 1234 5698 7654 32")) ; false