characters that are reserved, so only the mechanism responsible for
   generating or interpreting that component can determine whether or




 
RFC 2396                   URI Generic Syntax                August 1998


   not escaping a character will change its semantics. Likewise, a URI
   must be separated into its components before the escaped characters
   within those components can be safely decoded.

   In some cases, data that could be represented by an unreserved
   character may appear escaped; for example, some of the unreserved
   "mark" characters are automatically escaped by some systems.  If the
   given URI scheme defines a canonicalization algorithm, then
   unreserved characters may be unescaped according to that algorithm.
   For example, "%7e" is sometimes used instead of "~" in an http URL
   path, but the two are equivalent for an http URL.

   Because the percent "%" character always has the reserved purpose of
   being the escape indicator, it must be escaped as "%25" in order to
   be used as data within a URI.  Implementers should be careful not to
   escape or unescape the same string more than once, since unescaping
   an already unescaped string might lead to misinterpreting a percent
   data character as another escaped character, or vice versa in the
   case of escaping an already escaped string.

2.4.3. Excluded US-ASCII Characters

   Although they are disallowed within the URI syntax, we include here a
   description of those US-ASCII characters that have been excluded and
   the reasons for their exclusion.

   The control characters in the US-ASCII coded character set are not
   used within a URI, both because they are non-printable and because
   they are likely to be misinterpreted by some control mechanisms.

   control     = <US-ASCII coded characters 00-1F and 7F hexadecimal>

   The space character is excluded because significant spaces may
   disappear and insignificant spaces may be introduced when URI are
   transcribed or typeset or subjected to the treatment of word-
   processing programs.  Whitespace is also used to delimit URI in many
   contexts.

   space       = <US-ASCII coded character 20 hexadecimal>

   The angle-bracket "<" and ">" and double-quote (") characters are
   excluded because they are often used as the delimiters around URI in
   text documents and protocol fields.  The character "#" is excluded
   because it is used to delimit a URI from a fragment identifier in URI
   references (Section 4). The percent character "%" is excluded because
   it is used for the encoding of escaped characters.

   delims      = "<" | ">" | "#" | "%" | <">




 
RFC 2396                   URI Generic Syntax                August 1998


   Other characters are excluded because gateways and other transport
   agents are known to sometimes modify such characters, or they are
   used as delimiters.

   unwise      = "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"

   Data corresponding to excluded characters must be escaped in order to
   be properly represented within a URI.

3. URI Syntactic Components

   The URI syntax is dependent upon the scheme.  In general, absolute
   URI are written as follows:

      :<scheme-specific-part>

   An absolute URI contains the name of the scheme being used ()
   followed by a colon (":") and then a string (the <scheme-specific-
   part>) whose interpretation depends on the scheme.

   The URI syntax does not require that the scheme-specific-part have
   any general structure or set of semantics which is common among all
   URI.  However, a subset of URI do share a common syntax for
   representing hierarchical relationships within the namespace.  This
   "generic URI" syntax consists of a sequence of four main components:

      ://?

   each of which, except , may be absent from a particular URI.
   For example, some URI schemes do not allow an  component,
   and others do not use a  component.

      absoluteURI   = scheme ":" ( hier_part | opaque_part )