Clojure program that converts roman numeral to decimal.

core.clj

(ns roman.core
  (:require [clojure.string :as str]))

(def grammar '("^M{0,4}" "^CM|^CD|^D?C{0,3}" "^XC|^XL|^L?X{0,3}" "^IX|^IV|^V?I{0,3}"))
(def values {:M 1000 :D 500 :C 100 :L 50 :X 10 :V 5 :I 1})

(defn sum-tokens [tokens]
  (reduce
   #(if (< %1 %2) (- %2 %1) (+ %1 %2))
   (map #((keyword (str %)) values) (seq tokens))))

(defn process-rule [grule roman]
  (if (not-empty grule)
    (let [patt (re-pattern (first grule))
          match (re-find patt roman)]
        (+ (if (not-empty match)
             (sum-tokens match) 0)
           (process-rule (next grule) (str/replace roman patt ""))))
    (if (empty? roman) 0
      (throw (ex-info (str "Invalid roman numeral: " roman) {})))))

(defn -main
  "Main function that converts roman numeral to decimal."
  [roman]
  (println "Convert " roman)
  (println "Result  " (try (process-rule grammar (str/upper-case roman))
                           (catch Exception e (.getMessage e)))))