String balanced reduction.

gistfile1.clj

(defn b-reduction
  "Reduces a string to un-balanced left and right enclosing characters."
  [s & {:keys [ls rs] :or {ls \[ rs \]}}]
  (reduce (fn [v ch]
            (if (= ch ls)
              (conj v ch)
              (if (= ch rs)
                (if (= ls (peek v))
                  (pop v)
                  (conj v ch))
                v)))
          []
          s))

Indeed! I had originally written the piece of code to only consider [ and went back and added the ls and rs!