(define-module (c trigraph)
  :use-module (ice-9 regex)
  :export (replace-trigraphs))

(define rx (make-regexp "\\?\\?([=\\(\\)'!<>/-])"))

(define (proc m)
  (case (string-ref (match:substring m 2) 0)
    ((#\=) "#")
    ((#\() "[")
    ((#\)) "]")
    ((#\') "^")
    ((#\<) "{")
    ((#\>) "}")
    ((#\!) "|")
    ((#\-) "~")
    ((#\/) "\\")))

(define (replace-trigraphs string)
  (call-with-output-string
    (lambda (port)
      (regexp-substitute/global
       port rx string
       'pre proc 'post))))