[plt-scheme] Small problem with regex-replace*

I wrote a small program that scrambles text by preserving the first  
and last char of each word. There is a claim that such algorithm for  
scrambling can produce readable text, because the human brain does  
the error correction.

I was happy this was a very small program to write, however I wasn't  
able to create a regexp that only match words,
so I ended using *dummy* variable to hold the second match that is  
passed to the scramble procedure. How can I fix this? Here is the code:

(require (lib "cards.ss" "games" "cards"))

(define (shuffle-string str)
   (list->string (shuffle-list (string->list str) 7)))

(define (scramble str dummy)
   (define len (string-length str))
   (define len-1 (sub1 len))
   (string-append
    (substring str 0 1)                          ; First char
    (shuffle-string (substring str 1 len-1))     ; Shuffle mid-part
    (substring str len-1 len)))                  ; Last char

(define (scramble-text str)
   (let ([word (regexp "([a-zA-Z])+")])
     (regexp-replace* word str scramble)))

(scramble-text "In theory after scrambling this text would be still  
readable.")
---> "In theory afetr scrmlinbag tihs text would be sltil rabdelae."

Thanks,

Jaime