Racket regexp fails.

parse-irc.rkt

(define (parse s)
  (regexp-match #rx"^(?:[:](\S+) )?(\S+)(?: (?!:)(.+?))?(?: [:](.+))?$" s))

; regexp code taken from here http://mybuddymichael.com/writings/a-regular-expression-for-irc-messages.html

-> (regexp-match #rx"^(?:[:](\S+) )?(\S+)(?: (?!:)(.+?))?(?: [:](.+))?$" "PING :serve.net")
; readline-input:10:17: read: unknown escape sequence \S in string [,bt for
;   context]
#<procedure:+>
; readline-input:10:29: read: unexpected `)' [,bt for context]
; readline-input:10:31: read: unexpected `)' [,bt for context]
; ?: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; S+: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; ?:: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; ?: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; ?:: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; ?$: undefined;
;  cannot reference undefined identifier
; [,bt for context]
" "
; PING: undefined;
;  cannot reference undefined identifier
; [,bt for context]
; :serve.net: undefined;
;  cannot reference undefined identifier
; [,bt for context]