(defn- re-clause-to-if [s [re bindings body] else]
  `(if-let [[~'_ ~@bindings] (re-find ~re ~s)]
     ~body
     ~else))

(defn- re-clauses-to-ifs [s clauses]
  (when-first [clause clauses]
	      (if (and (not (next clauses))
		       (= (count clause) 1))
		(first clause)
		(re-clause-to-if s clause (re-clauses-to-ifs s (rest clauses))))))

(defmacro cond-re [s & clauses]
  (re-clauses-to-ifs s (partition 3 3 nil clauses)))

(defn parse-flag [s]
  (cond-re s
	   #"^--([^=]+)=(.*)" [flag value] [flag value]
	   #"^--(.*)" [flag] [flag nil]))

(comment

(parse-flag "--foo")
=> ["foo" nil]

(parse-flag "--foo=bar")
=> ["foo" "bar"]

)