See http://qiita.com/qube81/items/fa6ef94d3c8615b0ce64 for details.

validate-my-number.cljs

(defn
  validate-my-number [mynumber]
  (and (re-matches #"^\d{12}$" mynumber)
       (let [digits (map int mynumber)
             [xs [check_digit]] (split-at 11 digits)
             each_digit_fn (fn [num idx] (* num (if (<= idx 6) (+ idx 1) (- idx 5))))
             calculated_digit (as-> xs <>
                                (reverse <>)
                                (map each_digit_fn <> (iterate inc 1))
                                (reduce + <>)
                                (mod <> 11)
                                (if (<= <> 1) 0 (- 11 <>)))]
         (= check_digit calculated_digit))))