In emacs, run a synchronous subprocess, piping the region to it, and returning its output as a string.

itz-program-output-on-region.el

(defun itz-program-output-on-region (start end prog &rest args)
  "Run PROG with ARGS synchronously, passing it text between START END.
Return its output as a string."
  (with-output-to-string
    (let ((exit-status
           (apply 'call-process-region
                  start end prog nil standard-output nil args)))
      (cond
       ((stringp exit-status)
        (error "Program killed: %s" exit-status))
       ((/= 0 exit-status)
        (error "Program failed: %d" exit-status))
       (t nil)))))