string-replace.scm

(define* (string-replace-substring s substr replacement #:optional (start 0) (end (string-length s)))
       "Replace every instance of substring in s by replacement."
       (let ((substr-length (string-length substr)))
          (if (zero? substr-length)
             (error "string-replace-substring: empty substr")
             (let loop 
                 ((start start)
                   (pieces (list (substring s 0 start))))
                 (let ((idx (string-contains s substr start end)))
                   (if idx
                     (loop (+ idx substr-length)
                           (cons* replacement
                                  (substring s start idx)
                                  pieces))
                     (string-concatenate-reverse 
                                                (cons (substring s start)
                                                    pieces))))))))