New Scheme string routines

[Charles Day]

To support multi-byte account separators in the QIF importer I have had to
write several new Scheme string manipulation routines. They are pretty
useful variations on some of the standard routines. I have them working in
the QIF importer, but is there a better place to stick them so that they can
be used from other modules, such as reports?  Perhaps in main.scm?

-Charles

P.S. For example, here are a three of the Scheme procedures I have written.
There are a few more, but this will give you a good idea. If you happen to
see any room for performance improvement, let me know.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;  string-rcontains
;;
;;  Like string-contains, but searches from the right.
;;
;;  Example: (string-rcontains "foobarfoobarf" "bar") returns 9.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (string-rcontains s1 s2)
  (let ((s2len (string-length s2)))
    (let loop ((i (string-contains s1 s2))
               (retval #f))
      (if i
          (loop (string-contains s1 s2 (+ i s2len)) i)
          retval))))


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;  substring-count
;;
;;  Similar to string-count, but searches for a substring rather
;;  than a single character.
;;
;;  Example: (substring-count "foobarfoobarf" "bar") returns 2.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (substring-count s1 s2)
  (let ((s2len (string-length s2)))
    (let loop ((i (string-contains s1 s2))
               (retval 0))
      (if i
          (loop (string-contains s1 s2 (+ i s2len)) (+ 1 retval))
          retval))))


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;  substring-split
;;
;;  Similar to string-split, but the delimiter is a string
;;  rather than a single character.
;;
;;  Example: (substring-split "foobarfoobarf" "bar") returns
;;           ("foo" "foo" "f")
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (substring-split s1 s2)
  (let ((i (string-contains s1 s2)))
    (if i
        (cons (substring s1 0 i)
              (substring-split (substring s1 (+ i (string-length s2))) s2))
        (list s1))))