Mimics the GNU Emacs function with the same name:

   (define (replace-regexp-in-string regexp replacement string)
     ;; Replace all occurrences of REGEXP with REPLACEMENT in STRING.
     ;; If REPLACEMENT is a procedure, it is applied to the match
     ;; structure for the given match and should return a string to be
     ;; used in the result.
     ;; Example:
     ;;    (replace-regexp-in-string (rx "foo") "bar" "foo foobar abc foofoo")
     ;;       => "bar barbar abc barbar"
     (regexp-substitute/global #f regexp string 'pre replacement 'post))

This one is slightly more complex.

  (define (replace-regexp-in-string regexp rep string)
    (cond
     ((string? rep)
      (regexp-substitute/global #f regexp string 'pre rep   'post))
     ((pair? rep)
      (apply regexp-substitute/global #f regexp string 'pre (append rep (list 'post))))
     (else
      (error '&contract replace-regexp-in-string regexp rep string))))

Examples:

  > (replace-regexp-in-string (rx " ")
                              "-"
                              "foo bar")
  "foo-bar"
  > (replace-regexp-in-string (rx (+ alphanumeric))
                              '("(" 0 ")")
                              "foo bar")
  "(foo) (bar)"