http|https|sftp|news|ftp|mailto|file|irc|jabber|xmpp)
:
(?:
  (?:
    # "//" authority path-abempty
    //
    (?: # userinfo
      [%(unreserved)s:]*
      @
    )?
    (?: # host
      \d+\.\d+\.\d+\.\d+ |
      [%(unreserved)s]*
    )
    (?: # port
      : \d*
    )?
    (?: / [%(unreserved)s:@]* )*
  ) | (?:
    # path-absolute
    /
    (?: [%(unreserved)s:@]+
        (?: / [%(unreserved)s:@]* )* )?
  ) | (?:
    # path-rootless
    [%(unreserved)s@]
    [%(unreserved)s:@]*
    (?: / [%(unreserved)s:@]* )*
  )
)
(?: # query
  \?
  [%(unreserved)s:@/\?\[\]]*
)?
(?: # fragment
  \#
  [%(unreserved)s:@/\?]*
)?
r4