URI that are hierarchical in nature use the slash "/" character for
   separating hierarchical components.  For some file systems, a "/"
   character (used to denote the hierarchical structure of a URI) is the
   delimiter used to construct a file name hierarchy, and thus the URI
   path will look similar to a file pathname.  This does NOT imply that
   the resource is a file or that the URI maps to an actual filesystem
   pathname.

      hier_part     = ( net_path | abs_path ) [ "?" query ]

      net_path      = "//" authority [ abs_path ]

      abs_path      = "/"  path_segments





 
RFC 2396                   URI Generic Syntax                August 1998


   URI that do not make use of the slash "/" character for separating
   hierarchical components are considered opaque by the generic URI
   parser.

      opaque_part   = uric_no_slash *uric

      uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
                      "&" | "=" | "+" | "$" | ","

   We use the term  to refer to both the  and
   <opaque_part> constructs, since they are mutually exclusive for any
   given URI and can be parsed as a single component.

3.1. Scheme Component

   Just as there are many different methods of access to resources,
   there are a variety of schemes for identifying such resources.  The
   URI syntax consists of a sequence of components separated by reserved
   characters, with the first component defining the semantics for the
   remainder of the URI string.

   Scheme names consist of a sequence of characters beginning with a
   lower case letter and followed by any combination of lower case
   letters, digits, plus ("+"), period ("."), or hyphen ("-").  For
   resiliency, programs interpreting URI should treat upper case letters
   as equivalent to lower case in scheme names (e.g., allow "HTTP" as
   well as "http").

      scheme        = alpha *( alpha | digit | "+" | "-" | "." )

   Relative URI references are distinguished from absolute URI in that
   they do not begin with a scheme name.  Instead, the scheme is
   inherited from the base URI, as described in Section 5.2.

3.2. Authority Component

   Many URI schemes include a top hierarchical element for a naming
   authority, such that the namespace defined by the remainder of the
   URI is governed by that authority.  This authority component is
   typically defined by an Internet-based server or a scheme-specific
   registry of naming authorities.

      authority     = server | reg_name

   The authority component is preceded by a double slash "//" and is
   terminated by the next slash "/", question-mark "?", or by the end of
   the URI.  Within the authority component, the characters ";", ":",
   "@", "?", and "/" are reserved.




 
RFC 2396                   URI Generic Syntax                August 1998


   An authority component is not required for a URI scheme to make use
   of relative references.  A base URI without an authority component
   implies that any relative reference will also be without an authority
   component.

3.2.1. Registry-based Naming Authority

   The structure of a registry-based naming authority is specific to the
   URI scheme, but constrained to the allowed characters for an
   authority component.

      reg_name      = 1*( unreserved | escaped | "$" | "," |
                          ";" | ":" | "@" | "&" | "=" | "+" )

3.2.2. Server-based Naming Authority

   URL schemes that involve the direct use of an IP-based protocol to a
   specified server on the Internet use a common syntax for the server
   component of the URI's scheme-specific data:

      @: