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

closes PR 12652

original commit: fdf3fa6492

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?]{