core.clj

(def input ", \t, , , UTF-8;q=0.8,shift_JIS;q=0.4,a,b")

(def rfc-7231-re #"(?x)
                   (?:
                      \s* # OWS
                      (?<charset>\*|[\p{Alnum}!\#\$\%\&'\*\+\-\.\^_\`\|\~]+)
                      (?i:
                        ;q=
                        (?<weight>0\.\d{1,3}|1\.0{1,3})
                      )?
                   )+?,?")

(defn parse-rfc7231-accept-charset [s]
  (->> s
       (re-seq rfc-7231-re)
       (keep (fn [m]
               (when-let [[charset weight] (next m)]
                 (vector charset (when weight (Float/parseFloat weight))))))
       (into {})))

(comment

  (parse-rfc7231-accept-charset input)
  ;; {"UTF-8" 0.8, "shift_JIS" 0.4, "a" nil, "b" nil}

  (criterium.core/quick-bench
   (parse-rfc7231-accept-charset input)))