[Definition: If the entity is not internal,
it is an external entity, declared as follows:]
If the NDataDecl is present, this is a general unparsed entity; otherwise it is a parsed entity.
[Definition: The SystemLiteral is called the entity's system
identifier. It is meant to be converted to a URI reference
(as defined in [IETF RFC 3986]),
as part of the
process of dereferencing it to obtain input for the XML processor to construct the
entity's replacement text.] It is an error for a fragment identifier
(beginning with a # character) to be part of a system identifier.
Unless otherwise provided by information outside the scope of this specification
(e.g. a special XML element type defined by a particular DTD, or a processing
instruction defined by a particular application specification), relative URIs
are relative to the location of the resource within which the entity declaration
occurs. This is defined to
be the external entity containing the '<' which starts the declaration, at the
point when it is parsed as a declaration.
A URI might thus be relative to the document
entity, to the entity containing the external
DTD subset, or to some other external parameter
entity. Attempts to
retrieve the resource identified by a URI may be redirected at the parser
level (for example, in an entity resolver) or below (at the protocol level,
for example, via an HTTP Location: header). In the absence of additional
information outside the scope of this specification within the resource,
the base URI of a resource is always the URI of the actual resource returned.
In other words, it is the URI of the resource retrieved after all redirection
has occurred.
System
identifiers (and other XML strings meant to be used as URI references) may contain
characters that, according to [IETF RFC 3986],
must be escaped before a URI can be used to retrieve the referenced resource. The
characters to be escaped are the control characters #x0 to #x1F and #x7F (most of
which cannot appear in XML), space #x20, the delimiters '<' #x3C, '>' #x3E and
'"' #x22, the unwise characters '{}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and
'`' #x60, as well as all characters above #x7F. Since escaping is not always a fully
reversible process, it MUST be performed only when absolutely necessary and as late
as possible in a processing chain. In particular, neither the process of converting
a relative URI to an absolute one nor the process of passing a URI reference to a
process or software component responsible for dereferencing it SHOULD trigger escaping.
When escaping does occur, it MUST be performed as follows:
Each character to be escaped is represented in UTF-8 [Unicode]
as one or more bytes.
The resulting bytes are escaped with
the URI escaping mechanism (that is, converted to %
HH,
where HH is the hexadecimal notation of the byte value).
The original character is replaced by the resulting character sequence.
Note:
In a future edition of this specification, the XML Core Working Group intends to replace the preceding paragraph
and list of steps with a normative reference to an upcoming revision of IETF RFC 3987, which will define
"Legacy Extended IRIs (LEIRIs)". When this revision is available, it is the intent of the XML Core WG to use it to replace
language similar to the above in any future revisions of XML-related specifications under its purview.
[Definition: In addition to a system
identifier, an external identifier may include a public identifier.]
An XML processor attempting to retrieve the entity's content may use
any combination of
the public and system identifiers as well as additional information outside the
scope of this specification to try to generate an alternative URI reference.
If the processor is unable to do so, it MUST use the URI
reference specified in the system literal. Before a match is attempted,
all strings of white space in the public identifier MUST be normalized to
single space characters (#x20), and leading and trailing white space MUST
be removed.
Examples of external entity declarations:
<!ENTITY open-hatch
SYSTEM "boilerplate/OpenHatch.xml">
<!ENTITY open-hatch
PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
"boilerplate/OpenHatch.xml">
<!ENTITY hatch-pic
SYSTEM "../grafix/OpenHatch.gif"
NDATA gif >
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