apply Andy's diff (and adjust the documentation correspondingly)

closes PR 12652

collects/net/scribblings/url.scrbl

@@ -1,5 +1,6 @@
 #lang scribble/doc
 @(require "common.rkt" scribble/bnf
+          (only-in net/url url-regexp)
           (for-label net/url net/url-unit net/url-sig 
                      net/head net/uri-codec net/tcp-sig
                      (only-in net/url-connect current-https-protocol)
@@ -95,7 +96,7 @@ An HTTP connection is created as a @deftech{pure port} or a
 have been removed, so that what remains is purely the first content
 fragment. An impure port is one that still has its MIME headers.
 
-@defproc[(string->url [str string?]) url?]{
+@defproc[(string->url [str (and/c (or/c string? bytes?) url-regexp)]) url?]{
 
 Parses the URL specified by @racket[str] into a @racket[url]
 struct. The @racket[string->url] procedure uses
@@ -122,6 +123,16 @@ parsed as an absolute path, and the parsing details depend on
 
 ]}
 
+@defthing[url-regexp regexp?]{
+
+This is a regular expression based on the one in
+Appendix B of RFC 3986 for recognizing urls.
+
+This is the precise regexp:
+
+@centered{@tt{@(object-name url-regexp)}}
+                              
+}
 
 @defproc[(combine-url/relative [base url?] [relative string?]) url?]{
 

collects/net/url.rkt

@@ -433,7 +433,7 @@
 ;; `*' instead of `+' for the scheme part (it is checked later anyway, and
 ;; we don't want to parse it as a path element), and the user@host:port is
 ;; parsed here.
-(define url-rx
+(define url-regexp
   (regexp (string-append
            "^"
            "(?:"              ; / scheme-colon-opt
@@ -488,8 +488,7 @@
               [query    (if query (form-urlencoded->alist query) '())]
               [fragment (uri-decode/maybe fragment)])
          (make-url scheme user host port abs? path query fragment))))
-   (cdr (or (regexp-match url-rx str)
-            (url-error "Invalid URL string: ~e" str)))))
+   (cdr (regexp-match url-regexp str))))
 
 (define (uri-decode/maybe f) (friendly-decode/maybe f uri-decode))
 
@@ -661,7 +660,8 @@
 (provide (struct-out url) (struct-out path/param))
 
 (provide/contract
- (string->url ((or/c bytes? string?) . -> . url?))
+ [url-regexp regexp?]
+ (string->url (url-regexp . -> . url?))
  (path->url ((or/c path-string? path-for-some-system?) . -> . url?))
  (url->string (url? . -> . string?))
  (url->path (->* (url?) ((one-of/c 'unix 'windows)) path-for-some-system?))