. Uniform Resource Locators in XBNF
This is an XBNF(eXtreme Bachus-Naur-Form) description of
the Uniform Resource Locator syntax.

Each definition has the traditional separator with two collons and an
equal sign, but is terminated by a period (.).
A vertical  line  |   indicates alternatives,
and O(brackets) indicate optional
parts.  N(brackets) indicate a number
(one or more) occurences.
#(brakets) is zero or more.
Spaces are represented by
the word "space", and the vertical
line character by "vline".   Single
letters stand for single letters.
All words of more than one letter
below are entities described somewhere
in this description.  Characters are shown in double quotation marks.
And quoation marks as "quotes".

The current IETF URI working group
preference  is for the prefixedurl
production. (Nov 1993. July 93: url).
The "national" and "punctuation"
characters do not appear in any productions
and therefore may not appear in URLs.
The "afsaddress" is left in as historical
note, but is not "a" url production

prefixedurl ::= "url:" $url.
url ::= $httpaddress  |  $ftpaddress  |  $newsaddress  |  $nntpaddress  |  $prosperoaddress  |  $telnetaddress   |  $gopheraddress  |  $waisaddress  |  $mailtoaddress   |  $midaddress  |  $cidaddress.
scheme ::= $ialpha.
httpaddress ::= "http:"   "//" $hostport O(  "/" $path ) O( "?" $search )  $ftpaddress ::= "ftp://" $login "/" $path O(  $ftptype ).
afsaddress ::= "afs://" $cellname "/" $path.
newsaddress ::= "news:" $group_art.
nntpaddress ::= "nntp:" $group "/"  N($digit).
midaddress ::= "mid"  ":"  addr_spec.
cidaddress ::= "cid:" content_identifier.
mailtoaddress ::= "mailto:" N($xalpha) "@" $hostname.
waisaddress ::= $waisindex  |  $waisdoc.
waisindex ::= "wais://" $hostport "/" $database O( "?" $search ).
waisdoc ::= "wais://" $hostport "/" $database "/" $wtype  "/" $wpath.
wpath ::= N($digit) "=" $path ";"  O( $wpath ).
group_art ::= "*"  |  $group  |  $article.
group ::= $ialpha O( "." $group ).
article ::= N($xalpha) "@" $host.
database ::= N($xalpha).
wtype ::= N($xalpha).
prosperoaddress ::= $prosperolink.
prosperolink ::= "prospero://" $hostport "/" $hsoname O( "%"  "00" $version
O( N(attribute) ) ).
hsoname ::= $path.
version ::= N($digit).
attribute ::= N(alphanum).
telnetaddress ::= "telnet://" $login.
gopheraddress ::= "gopher://" $hostport O(/ $gtype  O( $gcommand ) ).
login ::= O( $user O( ":" $password ")"@")" $hostport.
hostport ::= $host O( ":" $port ).
host ::= $hostname  |  $hostnumber..
ftptype ::= "A" $formcode  |  "E" $formcode  |  "I"  |  "L" N($digit).
formcode ::= "N"  |  "T"  |  "C".
cellname ::= $hostname.
hostname ::= $ialpha O(  "."  $hostname ).
hostnumber ::= N($digit) "." N($digit) "." N($digit) "." N($digit).
port ::= N($digit).
gcommand ::= $path.
path ::= $void  |   $segment  O(  "/" $path ).
segment ::= N($xpalpha).
search ::= N($xalpha) O(   $search ).
user ::= $alphanum2 O( $user ).
password ::= $alphanum2 O( $password ).
fragmentid ::= N($xalpha).
gtype ::= $xalpha.
alphanum2 ::=alpha  |  $digit  | "-" | "_"    |  "."  | "+".
xalpha ::= $alpha  |  $digit  |  $safe  |  $extra  |  $escape.
xpalpha ::= $xalpha  |  v$oid.
ialpha ::= $alpha O( N($xalpha) ).
safe ::= "$"  |  "-"  |     |  "@"  |  .  |  "&"   |     |  "-".
extra ::="!"  |  "*"  |   quotes  |   "'"  |  "("  |  ")"   |  ",".
reserved ::=  "="   |   ";"   |   "/"   |   "#"   |  ?  |   ":"  |  space.
escape ::= "%" $hex $hex.
hex ::= $digit  |  "a".."f"  |  "A".."F".
national ::= "{"  |  "}"  |  vline  |  "["  |  "]"  |  "\\"  |  "^"  |  "~".
punctuation ::= "<"  |  ">".
alphanum ::= $alpha  |  $digit
void ::=""

(end of URL BNF)