1 Introduction
Extensible
Markup Language, abbreviated XML, describes a class of data objects called XML documents and partially describes
the behavior of computer programs which process them. XML is an application
profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming
SGML documents.
XML documents are made up of storage units called entities, which contain either parsed
or unparsed data. Parsed data is made up of
characters, some of which form
character data, and some of which form
markup. Markup encodes a description of the document's storage layout
and logical structure. XML provides a mechanism to impose constraints on the
storage layout and logical structure.
[
Definition: A software module called an XML processor is used to
read XML documents and provide access to their content and structure.] [Definition: It is assumed that an XML
processor is doing its work on behalf of another module, called the application
.] This specification describes the required behavior of an XML processor
in terms of how it must read XML data and the information it must provide
to the application.
1.1
Origin and Goals
XML was developed by an XML Working Group (originally
known as the SGML Editorial Review Board) formed under the auspices of the
World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun
Microsystems with the active participation of an XML Special Interest Group
(previously known as the SGML Working Group) also organized by the W3C. The
membership of the XML Working Group is given in an appendix. Dan Connolly
served as the WG's contact with the W3C.
The design goals for XML are:
XML shall be straightforwardly usable over the Internet.
XML shall support a wide variety of applications.
XML shall be compatible with SGML.
It shall be easy to write programs which process XML documents.
The number of optional features in XML is to be kept to the absolute
minimum, ideally zero.
XML documents should be human-legible and reasonably clear.
The XML design should be prepared quickly.
The design of XML shall be formal and concise.
XML documents shall be easy to create.
Terseness in XML markup is of minimal importance.
This specification, together with associated standards (Unicode and
ISO/IEC 10646 for characters, Internet RFC 1766 for language identification
tags, ISO 639 for language name codes, and ISO 3166 for country name codes),
provides all the information necessary to understand XML Version 1.0 and construct
computer programs to process it.
This version of the XML specification
may be distributed freely, as long as all text and legal notices remain intact.
1.2 Terminology
The terminology used to describe XML documents is defined in the
body of this specification. The terms defined in the following list are used
in building those definitions and in describing the actions of an XML processor:
- may
[Definition: Conforming documents
and XML processors are permitted to but need not behave as described.]
- must
[Definition: Conforming documents
and XML processors are required to behave as described; otherwise they are
in error. ]
- error
[Definition: A violation of the
rules of this specification; results are undefined. Conforming software may
detect and report an error and may recover from it.]
- fatal error
[Definition: An error which
a conforming XML processor
must detect and report to the application. After encountering a fatal error,
the processor may continue processing the data to search for further errors
and may report such errors to the application. In order to support correction
of errors, the processor may make unprocessed data from the document (with
intermingled character data and markup) available to the application. Once
a fatal error is detected, however, the processor must not continue normal
processing (i.e., it must not continue to pass character data and information
about the document's logical structure to the application in the normal way).]
- at user option
[Definition: Conforming
software may or must (depending on the modal verb in the sentence) behave
as described; if it does, it must provide users a means to enable or disable
the behavior described.]
- validity constraint
[Definition: A rule
which applies to all valid XML documents.
Violations of validity constraints are errors; they must, at user option,
be reported by validating
XML processors.]
- well-formedness constraint
[Definition:
A rule which applies to all
well-formed XML documents. Violations of well-formedness constraints are fatal errors.]
- match
[Definition: (Of strings or names:)
Two strings or names being compared must be identical. Characters with multiple
possible representations in ISO/IEC 10646 (e.g. characters with both precomposed
and base+diacritic forms) match only if they have the same representation
in both strings. No case folding is performed. (Of strings and rules in the
grammar:) A string matches a grammatical production if it belongs to the language
generated by that production. (Of content and content models:) An element
matches its declaration when it conforms in the fashion described in the constraint [VC: Element Valid].]
- for compatibility
[Definition:
[E87]Marks a sentence describing a feature of XML included solely to ensure
that XML remains compatible with SGML.]
- for interoperability
[Definition: [E87]Marks a sentence
describing a non-binding recommendation included to increase the chances that
XML documents can be processed by the existing installed base of SGML processors
which predate the WebSGML Adaptations Annex to ISO 8879.]
2
Documents
[Definition:
A data object is an XML document if it is
well-formed, as defined in this specification. A well-formed XML document
may in addition be valid if it meets
certain further constraints.]
Each XML document has both a logical
and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in
the document. A document begins in a "root" or document entity. Logically, the document is composed
of declarations, elements, comments, character references, and processing
instructions, all of which are indicated in the document by explicit markup.
The logical and physical structures must nest properly, as described in 4.3.2 Well-Formed Parsed Entities.
2.1 Well-Formed XML Documents
[Definition: A textual
object is a well-formed XML document if:]
Taken as a whole, it matches the production labeled
document.
It meets all the well-formedness constraints given in this specification.
Each of the parsed entities
which is referenced directly or indirectly within the document is well-formed.
Document
Matching the document production
implies that:
It contains one or more elements
.
[Definition: There is exactly
one element, called the root, or document element, no part of which
appears in the content of any other
element.] [E17]For
all other elements, if the start-tag
is in the content of another element, the
end-tag is in the content of the same element. More simply stated, the
elements, delimited by start- and end-tags, nest properly within each other.
[Definition: As
a consequence of this, for each non-root element C
in the document,
there is one other element P
in the document such that
C
is in the content of P
, but is not in the content of
any other element that is in the content of P
. P
is referred to as the parent of C
, and C
as a child of P
.]
2.2
Characters
[Definition: A parsed
entity contains text, a sequence of
characters, which may represent markup or character data.] [Definition: A character is an atomic unit of
text as specified by ISO/IEC 10646 [ISO/IEC 10646] [E67](see also [ISO/IEC 10646-2000]). Legal characters are tab,
carriage return, line feed, and the legal characters of Unicode and ISO/IEC
10646. [E69]The
versions of these standards cited in A.1 Normative
References were current at the time this document was prepared. New
characters may be added to these standards by amendments or new editions.
Consequently, XML processors must accept any character in the range specified
for Char. The use of "compatibility characters", as
defined in section 6.8 of [Unicode]
[E67](see also D21 in section 3.6 of [Unicode3]),
is discouraged.]
Character Range
[2] |
Char | ::= |
#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
| /* any Unicode character, excluding the surrogate blocks,
FFFE, and FFFF. */ |
The mechanism for encoding character code points into
bit patterns may vary from entity to entity. All XML processors must accept
the UTF-8 and UTF-16 encodings of 10646; the mechanisms for signaling which
of the two is in use, or for bringing other encodings into play, are discussed
later, in 4.3.3 Character Encoding in Entities
.
2.3
Common Syntactic Constructs
This section defines some symbols used
widely in the grammar.
S (white space) consists
of one or more space (#x20) characters, carriage returns, line feeds, or tabs.
White Space
[3] |
S | ::= | (#x20
| #x9 | #xD | #xA)+ |
Characters are classified for convenience as letters,
digits, or other characters.
[E30]A letter consists of an alphabetic or syllabic base character or
an ideographic character. Full definitions of the specific characters in each
class are given in B Character Classes.
[Definition: A Name is a
token beginning with a letter or one of a few punctuation characters, and
continuing with letters, digits, hyphens, underscores, colons, or full stops,
together known as name characters.] Names beginning with the string "
xml
", or any string which would match (('X'|'x') ('M'|'m') ('L'|'l'))
, are reserved for standardization in this or future versions of this
specification.
Note:
[E98]The Namespaces
in XML Recommendation [XML Names] assigns a meaning
to names containing colon characters. Therefore, authors should not use the
colon in XML names except for namespace purposes, but XML processors must
accept the colon as a name character.
An
Nmtoken (name token) is any mixture of name characters.
Names
and Tokens
Literal data is any quoted string not containing the quotation
mark used as a delimiter for that string. Literals are used for specifying
the content of internal entities (EntityValue),
the values of attributes (AttValue), and external
identifiers (SystemLiteral). Note that a SystemLiteral can be parsed without scanning
for markup.
Literals
[9]
| EntityValue | ::=
| '"' ([^%&"] | PEReference
| Reference)* '"' |
| | | | "'" ([^%&']
| PEReference | Reference
)* "'" |
[10]
| AttValue | ::=
| '"' ([^<&"] | Reference)*
'"' |
| | | | "'" ([^<&']
| Reference)* "'" |
[11]
| SystemLiteral | ::=
| ('"' [^"]* '"') | ("'" [^']* "'") |
[12]
| PubidLiteral | ::=
| '"' PubidChar* '"' | "'" (
PubidChar - "'")* "'" |
[13]
| PubidChar | ::=
| #x20 | #xD | #xA | [a-zA-Z0-9] | [-'()+,./:=?;!*#@$_%]
|
Note:
[E72]Although the EntityValue production allows the definition of
an entity consisting of a single explicit <
in the literal
(e.g., <!ENTITY mylt "<">
), it is strongly advised to avoid
this practice since any reference to that entity will cause a well-formedness
error.
2.4 Character
Data and Markup
Text consists
of intermingled character data
and markup. [Definition: Markup
takes the form of start-tags, end-tags,
empty-element tags, entity
references, character
references, comments, CDATA section delimiters, document type declarations, processing instructions,
[E89]XML declarations,
text declarations, and any white space that is at the top level of the
document entity (that is, outside the document element and not inside any
other markup).]
[Definition
: All text that is not markup constitutes the character data of
the document.]
The ampersand character (&) and the left angle bracket
(<) may appear in their literal form only when used as markup
delimiters, or within a comment,
a processing instruction,
or a CDATA section. If
they are needed elsewhere, they must be
escaped using either
numeric character references or the strings "&
" and
"<
" respectively. The right angle bracket (>) may be represented
using the string ">
", and must, for compatibility, be escaped using ">
" or a character reference when it appears in the string "]]>
" in content, when that string is not marking the end of a CDATA section.
In the content of elements,
character data is any string of characters which does not contain the start-delimiter
of any markup. In a CDATA section, character data is any string of characters
not including the CDATA-section-close delimiter, "]]>
".
To allow attribute values to contain both single and double quotes, the apostrophe
or single-quote character (') may be represented as "'
",
and the double-quote character (") as ""
".
Character Data
[14]
| CharData | ::=
| [^<&]* - ([^<&]* ']]>' [^<&]*) |
2.5
Comments
[Definition:
Comments may appear anywhere in a document outside other markup; in addition, they may appear within the document
type declaration at places allowed by the grammar. They are not part of the
document's character data;
an XML processor may, but need not, make it possible for an application to
retrieve the text of comments.
For compatibility, the string "--
" (double-hyphen) must not
occur within comments.]
[E63]Parameter entity references are not recognized within comments.
Comments
[15]
| Comment | ::=
| '<!--' ((Char - '-') | ('-' (
Char - '-')))* '-->' |
An example of a comment:
<!-- declarations for <head> & <body> --> |
[E27]Note that the grammar does not allow a comment ending in --->
. The following example is not well-formed.
2.6 Processing
Instructions
[Definition
: Processing instructions (PIs) allow documents to contain instructions
for applications.]
Processing Instructions
[16] |
PI | ::= |
'<?' PITarget (S (
Char* - (Char* '?>' Char*)))?
'?>' |
[17]
| PITarget | ::=
| Name - (('X' | 'x') ('M' | 'm') ('L'
| 'l')) |
PIs are not part of the document's character data, but must be passed through to the
application. The PI begins with a target (PITarget
) used to identify the application to which the instruction is directed.
The target names "XML
", "xml
", and so on are reserved
for standardization in this or future versions of this specification. The
XML Notation mechanism may be
used for formal declaration of PI targets.
[E63]Parameter entity references are not recognized within processing
instructions.
2.7
CDATA Sections
[Definition
: CDATA sections may occur anywhere character data may occur; they
are used to escape blocks of text containing characters which would otherwise
be recognized as markup. CDATA sections begin with the string "<![CDATA[
" and end with the string "]]>
":]
CDATA Sections
Within a CDATA section, only the
CDEnd string is recognized as markup, so that left angle brackets and
ampersands may occur in their literal form; they need not (and cannot) be
escaped using "<
" and "&
". CDATA
sections cannot nest.
An example of a CDATA section, in which "
<greeting>
" and "</greeting>
" are recognized as character data, not markup:
<![CDATA[<greeting>Hello, world!</greeting>]]>
|
2.8
Prolog and Document Type Declaration
[
Definition: XML documents may, and should, begin with an XML declaration
which specifies the version of XML being used.] For example, the following
is a complete XML document,
well-formed but not valid:
<?xml version="1.0"?> <greeting>Hello, world!</greeting>
|
and so is this:
<greeting>Hello, world!</greeting> |
The version number "1.0
" should be used to
indicate conformance to this version of this specification; it is an error
for a document to use the value "1.0
" if it does not conform
to this version of this specification. It is the intent of the XML working
group to give later versions of this specification numbers other than "
1.0
", but this intent does not indicate a commitment to produce any
future versions of XML, nor if any are produced, to use any particular numbering
scheme. Since future versions are not ruled out, this construct is provided
as a means to allow the possibility of automatic version recognition, should
it become necessary. Processors may signal an error if they receive documents
labeled with versions they do not support.
The function of the markup
in an XML document is to describe its storage and logical structure and to
associate attribute-value pairs with its logical structures. XML provides
a mechanism, the
document type declaration, to define constraints on the logical structure
and to support the use of predefined storage units. [
Definition: An XML document is valid if it has an associated document
type declaration and if the document complies with the constraints expressed
in it.]
The document type declaration must appear before the first element in the document.
Prolog
[
Definition: The XML document type declaration contains or points
to markup declarations
that provide a grammar for a class of documents. This grammar is known
as a document type definition, or DTD. The document type declaration
can point to an external subset (a special kind of external entity) containing markup declarations, or
can contain the markup declarations directly in an internal subset, or can
do both. The DTD for a document consists of both subsets taken together.]
[Definition:
A markup declaration is an
element type declaration, an
attribute-list declaration, an
entity declaration, or a
notation declaration.] These declarations may be contained in whole or
in part within parameter entities
, as described in the well-formedness and validity constraints below.
For [E14]further
information, see 4 Physical Structures.
Document Type Definition
[E82]Note that it is possible to construct a well-formed document containing
a doctypedecl that neither points to an external
subset nor contains an internal subset.
The markup declarations may
be made up in whole or in part of the
replacement text of parameter
entities. The productions later in this specification for individual nonterminals
(elementdecl, AttlistDecl
, and so on) describe the declarations after all the parameter
entities have been included.
[E75]Parameter entity
references are recognized anywhere in the DTD (internal and external subsets
and external parameter entities), except in literals, processing instructions,
comments, and the contents of ignored conditional sections (see
3.4 Conditional Sections). They are also recognized in entity value
literals. The use of parameter entities in the internal subset is restricted
as described below.
Validity constraint: Root Element Type
The
Name in the document type declaration must match the element type of the root element.
Validity constraint: Proper
Declaration/PE Nesting
Parameter-entity replacement text must be properly nested with markup
declarations. That is to say, if either the first character or the last character
of a markup declaration (markupdecl above) is
contained in the replacement text for a parameter-entity reference, both must be contained in
the same replacement text.
Well-formedness constraint: PEs in Internal
Subset
In the internal DTD subset, parameter-entity references can occur only where markup
declarations can occur, not within markup declarations. (This does not apply
to references that occur in external parameter entities or to the external
subset.)
Like the internal subset, the external subset and any
external parameter entities referred to in the DTD must consist of a series
of complete markup declarations of the types allowed by the non-terminal symbol markupdecl, interspersed with white space or parameter-entity references
. However, portions of the contents of the external subset or of external
parameter entities may conditionally be ignored by using the conditional section construct; this is not allowed
in the internal subset.
[E13]The external subset and any external parameter entities referred
to in the DTD must match the production for extPE.
See 4.3.2 Well-Formed Parsed Entities.
External Subset
The external subset and external parameter entities also
differ from the internal subset in that in them, parameter-entity references are permitted within
markup declarations, not only between markup declarations.
An example of an XML document with a document type declaration:
<?xml version="1.0"?> <!DOCTYPE greeting SYSTEM "hello.dtd"> <greeting>Hello, world!</greeting>
|
The system
identifier "hello.dtd
" gives the
[E78]address (a URI reference) of a DTD for the document.
The declarations
can also be given locally, as in this example:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE greeting [
<!ELEMENT greeting (#PCDATA)>
]>
<greeting>Hello, world!</greeting> |
If both the external and internal subsets are used, the
internal subset is considered to occur before the external subset. This has
the effect that entity and attribute-list declarations in the internal subset
take precedence over those in the external subset.
2.9 Standalone Document Declaration
Markup declarations
can affect the content of the document, as passed from an XML processor to an application; examples are attribute
defaults and entity declarations. The standalone document declaration, which
may appear as a component of the XML declaration, signals whether or not there
are such declarations which appear external to the document entity
[E64] or in parameter entities. [Definition: An external markup declaration
is defined as a markup declaration occurring in the external subset or in
a parameter entity (external or internal, the latter being included because
non-validating processors are not required to read them).]
Standalone
Document Declaration
In a standalone document declaration, the value "
yes
" indicates that there are no
[E64]external
markup declarations which affect the information passed from the XML processor
to the application. The value "no
" indicates that there are or
may be such external markup declarations. Note that the standalone document
declaration only denotes the presence of external declarations; the
presence, in a document, of references to external entities, when
those entities are internally declared, does not change its standalone status.
If there are no external markup declarations, the standalone document
declaration has no meaning. If there are external markup declarations but
there is no standalone document declaration, the value "no
" is
assumed.
Any XML document for which standalone="no"
holds
can be converted algorithmically to a standalone document, which may be desirable
for some network delivery applications.
Validity constraint: Standalone Document Declaration
The standalone document declaration must have the value "
no
" if any external markup declarations contain declarations of:
attributes with default
values, if elements to which these attributes apply appear in the document
without specifications of values for these attributes, or
entities (other than amp
, lt
, gt
,
apos
, quot
), if
references to those entities appear in the document, or
attributes with values subject to normalization
, where the attribute appears in the document with a value which
will change as a result of normalization, or
element types with
element content, if white space occurs directly within any instance of
those types.
An example XML declaration with a standalone document declaration:
<?xml version="1.0" standalone='yes'?> |
2.10
White Space Handling
In editing XML documents, it is often convenient
to use "white space" (spaces, tabs, and blank lines) to set apart the markup
for greater readability. Such white space is typically not intended for inclusion
in the delivered version of the document. On the other hand, "significant"
white space that should be preserved in the delivered version is common, for
example in poetry and source code.
An
XML processor must always pass all characters in a document that are not
markup through to the application. A
validating XML processor must also inform the application which of these
characters constitute white space appearing in element content.
A special attribute named xml:space may be attached to an
element to signal an intention that in that element, white space should be
preserved by applications. In valid documents, this attribute, like any other,
must be declared
if it is used. When declared, it must be given as an enumerated type whose
[E81]values are one or both of "default
" and "preserve
". For example:
<!ATTLIST poem xml:space (default|preserve) 'preserve'>
<!-- [E81]-->
<!ATTLIST pre xml:space (preserve) #FIXED 'preserve'> |
The value "default
" signals that applications'
default white-space processing modes are acceptable for this element; the
value "preserve
" indicates the intent that applications preserve
all the white space. This declared intent is considered to apply to all elements
within the content of the element where it is specified, unless overriden
with another instance of the xml:space attribute.
The root element of any document is considered to have signaled
no intentions as regards application space handling, unless it provides a
value for this attribute or the attribute is declared with a default value.
2.11 End-of-Line
Handling
XML parsed entities
are often stored in computer files which, for editing convenience, are
organized into lines. These lines are typically separated by some combination
of the characters carriage-return (#xD) and line-feed (#xA).
[E86]To simplify the tasks of applications
, an XML processor must
normalize line breaks in parsed entities to #xA either by translating the
two-character sequence #xD #xA and any #xD that is not followed by #xA to
#xA on input before parsing, or by using some other method such that the characters
passed to the application are the same as if it did this translation.
2.12 Language Identification
In document processing, it is often useful to identify the natural or formal
language in which the content is written. A special
attribute named xml:lang may be inserted in documents to specify
the language used in the contents and attribute values of any element in an
XML document. In valid documents, this attribute, like any other, must be declared if it is
used. [E73]The
values of the attribute are language identifiers as defined by
[IETF RFC 1766], Tags for the Identification of Languages,
or its successor on the IETF Standards Track.
[E73](Productions 33 through 38 have been removed.)
For example:
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p>
<p xml:lang="en-GB">What colour is it?</p>
<p xml:lang="en-US">What color is it?</p>
<sp who="Faust" desc='leise' xml:lang="de">
<l>Habe nun, ach! Philosophie,</l>
<l>Juristerei, und Medizin</l>
<l>und leider auch Theologie</l>
<l>durchaus studiert mit heißem Bemüh'n.</l>
</sp> |
The intent declared with xml:lang is considered
to apply to all attributes and content of the element where it is specified,
unless overridden with an instance of xml:lang on another element within
that content.
A simple declaration for xml:lang might take
the form
xml:lang NMTOKEN #IMPLIED |
but specific default values may also be given, if appropriate.
In a collection of French poems for English students, with glosses and notes
in English, the xml:lang attribute might be declared this way:
<!ATTLIST poem xml:lang NMTOKEN 'fr'>
<!ATTLIST gloss xml:lang NMTOKEN 'en'>
<!ATTLIST note xml:lang NMTOKEN 'en'> |
3
Logical Structures
[Definition
: Each XML document contains
one or more elements, the boundaries of which are either delimited
by start-tags and end-tags, or, for empty
elements, by an empty-element
tag. Each element has a type, identified by name, sometimes called its
"generic identifier" (GI), and may have a set of attribute specifications.]
Each attribute specification has a
name and a value.
Element
This specification does not constrain the semantics, use,
or (beyond syntax) names of the element types and attributes, except that
names beginning with a match to (('X'|'x')('M'|'m')('L'|'l'))
are reserved for standardization in this or future versions of this specification.
Well-formedness
constraint: Element Type Match
The Name
in an element's end-tag must match the element type in the start-tag.
Validity
constraint: Element Valid
An element is valid if there is a declaration
matching elementdecl where the
Name matches the element type, and one of the following holds:
The declaration matches EMPTY and the element has no content.
The declaration matches children and the
sequence of child elements
belongs to the language generated by the regular expression in the content
model, with optional white space (characters matching the nonterminal
S) between [E59]
the start-tag and the first child element, between child elements, or
between the last child element and the end-tag. Note that a CDATA section
containing only white space does not match the nonterminal
S, and hence cannot appear in these positions.
The declaration matches Mixed and the content
consists of character data
and child elements whose
types match names in the content model.
The declaration matches ANY, and the types of any child elements have been declared.
3.1 Start-Tags,
End-Tags, and Empty-Element Tags
[
Definition: The beginning of every non-empty XML element is marked by
a start-tag.]
Start-tag
The Name in the start- and end-tags
gives the element's type. [Definition
: The Name-AttValue
pairs are referred to as the attribute specifications of the element],
[Definition: with the Name in each pair referred to as the attribute name
] and [Definition: the
content of the AttValue (the text between the
'
or "
delimiters) as the attribute value.]
[E46]Note that the order of attribute specifications in a start-tag or
empty-element tag is not significant.
Well-formedness constraint: Unique Att Spec
No attribute name may appear more than once in the same start-tag or empty-element
tag.
Validity constraint: Attribute Value Type
The attribute must have
been declared; the value must be of the type declared for it. (For attribute
types, see 3.3 Attribute-List Declarations.)
Well-formedness constraint: No External Entity References
Attribute
values cannot contain direct or indirect entity references to external entities.
Well-formedness constraint: No <
in Attribute Values
The replacement text of
any entity referred to directly or indirectly in an attribute value must not
contain a <
.
An example of a start-tag:
<termdef id="dt-dog" term="dog"> |
[Definition: The
end of every element that begins with a start-tag must be marked by an
end-tag containing a name that echoes the element's type as given in the
start-tag:]
End-tag
[42] |
ETag | ::= |
'</' Name S? '>' |
An example of an end-tag:
[Definition:
The text between the start-tag and end-tag
is called the element's content:]
Content of Elements
[Definition:
[E97]An element with no content is said to be empty.] The representation
of an empty element is either a start-tag immediately followed by an end-tag,
or an empty-element tag. [Definition
: An empty-element tag takes a special form:]
Tags for
Empty Elements
Empty-element tags may be used for any element which has
no content, whether or not it is declared using the keyword EMPTY. For interoperability,
the empty-element tag
[E45]should be used, and should only be used, for elements which are declared
EMPTY.
Examples of empty elements:
<IMG align="left"
src="http://www.w3.org/Icons/WWW/w3c_home" />
<br></br>
<br/> |
3.2 Element
Type Declarations
The element
structure of an XML document
may, for validation purposes,
be constrained using element type and attribute-list declarations. An element
type declaration constrains the element's
content.
Element type declarations often constrain which element
types can appear as children
of the element. At user option, an XML processor may issue a warning
when a declaration mentions an element type for which no declaration is provided,
but this is not an error.
[
Definition: An element type declaration takes the form:]
Element Type Declaration
where the Name gives the element
type being declared.
Validity constraint: Unique Element Type Declaration
No element
type may be declared more than once.
Examples of element type
declarations:
<!ELEMENT br EMPTY>
<!ELEMENT p (#PCDATA|emph)* >
<!ELEMENT %name.para; %content.para; >
<!ELEMENT container ANY> |
3.2.1
Element Content
[
Definition: An element type has
element content when elements of that type must contain only child elements (no character data), optionally
separated by white space (characters matching the nonterminal
S).][Definition:
In this case, the constraint includes a
[E55]content model, a simple grammar governing the allowed types
of the child elements and the order in which they are allowed to appear.]
The grammar is built on content particles (cps), which
consist of names, choice lists of content particles, or sequence lists of
content particles:
Element-content Models
where each Name is the type of
an element which may appear as a
child. Any content particle in a choice list may appear in the element content at the location where the choice
list appears in the grammar; content particles occurring in a sequence list
must each appear in the
element content in the order given in the list. The optional character
following a name or list governs whether the element or the content particles
in the list may occur one or more (+
), zero or more (*
), or zero or one times (?
). The absence of such an operator
means that the element or content particle must appear exactly once. This
syntax and meaning are identical to those used in the productions in this
specification.
The content of an element matches a content model if
and only if it is possible to trace out a path through the content model,
obeying the sequence, choice, and repetition operators and matching each element
in the content against an element type in the content model. For compatibility, it is an error if an element in the
document can match more than one occurrence of an element type in the content
model. For more information, see E Deterministic
Content Models.
Validity constraint: Proper Group/PE Nesting
Parameter-entity replacement
text must be properly nested with
[E11]parenthesized groups. That is to say, if either of the opening or
closing parentheses in a choice,
seq, or Mixed construct is contained in the replacement
text for a parameter
entity, both must be contained in the same replacement text.
[E19]For interoperability, if a parameter-entity reference
appears in a choice, seq, or Mixed construct, its replacement text should contain
at least one non-blank character, and neither the first nor last non-blank
character of the replacement text should be a connector (|
or
,
).
Examples of element-content models:
<!ELEMENT spec (front, body, back?)>
<!ELEMENT div1 (head, (p | list | note)*, div2*)>
<!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*> |
3.2.2
Mixed Content
[Definition
: An element type has mixed
content when elements of that type may contain character data, optionally
interspersed with child
elements.] In this case, the types of the child elements may be constrained,
but not their order or their number of occurrences:
Mixed-content
Declaration
where the Names give the types
of elements that may appear as children.
[E10]The keyword #PCDATA derives historically from the term "parsed
character data."
Validity constraint: No Duplicate Types
The same name must not
appear more than once in a single mixed-content declaration.
Examples of mixed content declarations:
<!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
<!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* >
<!ELEMENT b (#PCDATA)> |
3.3
Attribute-List Declarations
Attributes are used to associate name-value pairs with elements. Attribute specifications may appear only
within start-tags and empty-element tags; thus, the productions used to recognize
them appear in 3.1 Start-Tags, End-Tags, and Empty-Element
Tags. Attribute-list declarations may be used:
To define the set of attributes pertaining to a given element type.
To establish type constraints for these attributes.
To provide default
values for attributes.
[Definition
: Attribute-list declarations specify the name, data type, and
default value (if any) of each attribute associated with a given element type:]
Attribute-list Declaration
The Name in the
AttlistDecl rule is the type of an element. At user option, an XML processor
may issue a warning if attributes are declared for an element type not itself
declared, but this is not an error. The Name in the AttDef rule is the name of the attribute.
When
more than one AttlistDecl is provided for a
given element type, the contents of all those provided are merged. When more
than one definition is provided for the same attribute of a given element
type, the first declaration is binding and later declarations are ignored. [E9]For interoperability, writers of DTDs may choose to
provide at most one attribute-list declaration for a given element type, at
most one attribute definition for a given attribute name in an attribute-list
declaration, and at least one attribute definition in each attribute-list
declaration. For interoperability, an XML processor may at user option issue
a warning when more than one attribute-list declaration is provided for a
given element type, or more than one attribute definition is provided for
a given attribute, but this is not an error.
3.3.1 Attribute Types
XML attribute
types are of three kinds: a string type, a set of tokenized types, and enumerated
types. The string type may take any literal string as a value; the tokenized
types have varying lexical and semantic constraints
[E8]. The validity constraints noted in the grammar are applied after
the attribute value has been normalized as described in
3.3 Attribute-List Declarations.
Attribute Types
Validity constraint: ID
Values of type ID must match the Name production. A name must not appear more than once
in an XML document as a value of this type; i.e., ID values must uniquely
identify the elements which bear them.
Validity constraint: One ID
per Element Type
No element type may have more than one ID attribute
specified.
Validity constraint: ID Attribute Default
An ID attribute must
have a declared default of #IMPLIED or #REQUIRED.
Validity constraint:
IDREF
Values of type IDREF must match the
Name production, and values of type IDREFS must match
Names; each Name must match the value of an ID
attribute on some element in the XML document; i.e. IDREF values must
match the value of some ID attribute.
Validity constraint: Entity Name
Values of type ENTITY must match the Name production,
values of type ENTITIES must match Names; each Name must match the name of an unparsed entity declared in the DTD.
Validity constraint: Name Token
Values of type
NMTOKEN must match the Nmtoken production; values
of type NMTOKENS must match Nmtokens
.
[
Definition: Enumerated attributes can take one of a list of values
provided in the declaration]. There are two kinds of enumerated types:
Enumerated Attribute Types
A NOTATION attribute identifies a notation, declared in the DTD with associated system
and/or public identifiers, to be used in interpreting the element to which
the attribute is attached.
Validity constraint: Notation Attributes
Values of this type must match one of the notation
names included in the declaration; all notation names in the declaration
must be declared.
Validity constraint:
[E7]One Notation Per Element Type
No element type may have
more than one NOTATION attribute specified.
Validity constraint:
[E68]Non-Empty Element
For compatibility, an attribute of type NOTATION must not be declared
on an element declared EMPTY.
Validity constraint: Enumeration
Values of this type must match one of the Nmtoken
tokens in the declaration.
For interoperability, the same Nmtoken should
not occur more than once in the enumerated attribute types of a single element
type.
3.3.2
Attribute Defaults
An
attribute declaration provides information on whether the attribute's
presence is required, and if not, how an XML processor should react if a declared
attribute is absent in a document.
Attribute Defaults
In an attribute declaration, #REQUIRED means that
the attribute must always be provided, #IMPLIED that no default value
is provided. [Definition:
If the declaration is neither #REQUIRED nor #IMPLIED, then the AttValue value contains the declared default
value; the #FIXED keyword states that the attribute must always have
the default value. If a default value is declared, when an XML processor encounters
an omitted attribute, it is to behave as though the attribute were present
with the declared default value.]
Validity constraint: Required Attribute
If the default declaration is the keyword #REQUIRED, then the attribute
must be specified for all elements of the type in the attribute-list declaration.
Validity constraint: Attribute Default Legal
The declared default
value must meet the lexical constraints of the declared attribute type.
Validity constraint:
Fixed Attribute Default
If an attribute has a default value declared
with the #FIXED keyword, instances of that attribute must match the
default value.
Examples of attribute-list declarations:
<!ATTLIST termdef
id ID #REQUIRED
name CDATA #IMPLIED>
<!ATTLIST list
type (bullets|ordered|glossary) "ordered">
<!ATTLIST form
method CDATA #FIXED "POST"> |
3.3.3 [E70]Attribute-Value
Normalization
Before the value of an attribute is passed to the application
or checked for validity, the XML processor must normalize the attribute value
by applying the algorithm below, or by using some other method such that the
value passed to the application is the same as that produced by the algorithm.
All line breaks must have been normalized on input to #xA as described
in 2.11 End-of-Line Handling, so the rest
of this algorithm operates on text normalized in this way.
Begin with a normalized value consisting of the empty string.
For each character, entity reference, or character reference in the
unnormalized attribute value, beginning with the first and continuing to the
last, do the following:
For a character reference, append the referenced character to the normalized
value.
For an entity reference, recursively apply step 3 of this algorithm
to the replacement text of the entity.
For a white space character (#x20, #xD, #xA, #x9), append a space character
(#x20) to the normalized value.
For another character, append the character to the normalized value.
If the attribute type is not CDATA, then the XML processor must further
process the normalized attribute value by discarding any leading and trailing
space (#x20) characters, and by replacing sequences of space (#x20) characters
by a single space (#x20) character.
Note that if the unnormalized attribute
value contains a character reference to a white space character other than
space (#x20), the normalized value contains the referenced character itself
(#xD, #xA or #x9). This contrasts with the case where the unnormalized value
contains a white space character (not a reference), which is replaced with
a space character (#x20) in the normalized value and also contrasts with the
case where the unnormalized value contains an entity reference whose replacement
text contains a white space character; being recursively processed, the white
space character is replaced with a space character (#x20) in the normalized
value.
All attributes for which no declaration has been read should
be treated by a non-validating
[E95]processor as if declared CDATA.
Following are examples
of attribute normalization. Given the following declarations:
<!ENTITY d "
">
<!ENTITY a "
">
<!ENTITY da "
"> |
the attribute specifications in the left column below
would be normalized to the character sequences of the middle column if the
attribute a
is declared NMTOKENS and to those of the right
columns if a
is declared CDATA.
Attribute specification | a is NMTOKENS | a is CDATA |
| | |
| |
#x20 #x20 A #x20 #x20 B #x20 #x20 |
|
a=
"

A

B
" |
|
#xD #xD A #xA #xA B #xD #xA |
|
#xD #xD A #xA #xA B #xD #xD |
|
Note that the last example is invalid (but well-formed)
if a
is declared to be of type NMTOKENS.
3.4 Conditional Sections
[Definition
: Conditional sections are portions of the document type declaration external subset which are
included in, or excluded from, the logical structure of the DTD based on the
keyword which governs them.]
Conditional Section
Validity constraint:
[E90]Proper Conditional Section/PE Nesting
If any of the "
<![
", "[
", or "]]>
” of a conditional
section is contained in the replacement text for a parameter-entity reference,
all of them must be contained in the same replacement text.
Like the internal and external DTD subsets, a conditional section may contain
one or more complete declarations, comments, processing instructions, or nested
conditional sections, intermingled with white space.
If the keyword
of the conditional section is INCLUDE, then the contents of the conditional
section are part of the DTD. If the keyword of the conditional section is
IGNORE, then the contents of the conditional section are not logically
part of the DTD. If a conditional section with a keyword of INCLUDE
occurs within a larger conditional section with a keyword of IGNORE,
both the outer and the inner conditional sections are ignored.
[E90]The contents of an ignored conditional section are parsed by ignoring
all characters after the "[
" following the keyword, except conditional
section starts "<![
" and ends "]]>
", until
the matching conditional section end is found. Parameter entity references
are not recognized in this process.
If the keyword of the conditional
section is a parameter-entity reference, the parameter entity must be replaced
by its content before the processor decides whether to include or ignore the
conditional section.
An example:
<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >
<![%draft;[
<!ELEMENT book (comments*, title, body, supplements?)>
]]>
<![%final;[
<!ELEMENT book (title, body, supplements?)>
]]> |
4
Physical Structures
[Definition
: An XML document may consist of one or many storage units.
[E6]These are called entities; they all have content and
are all (except for the document
entity and the
external DTD subset) identified by entity name.]Each XML document
has one entity called the document
entity, which serves as the starting point for the XML processor and may contain the whole document.
Entities may be either parsed or unparsed. [Definition: A parsed entity's contents are
referred to as its replacement
text; this text is considered an integral
part of the document.]
[
Definition: An unparsed entity is a resource whose contents may
or may not be text, and if text,
[E25]may be other than XML. Each unparsed entity has an associated notation, identified by name. Beyond
a requirement that an XML processor make the identifiers for the entity and
notation available to the application, XML places no constraints on the contents
of unparsed entities.]
Parsed entities are invoked by name using entity
references; unparsed entities by name, given in the value of ENTITY
or ENTITIES attributes.
[
Definition: General entities are entities for use within the document
content. In this specification, general entities are sometimes referred to
with the unqualified term entity when this leads to no ambiguity.]
[Definition:
[E53]Parameter entities are parsed entities for use within the
DTD.] These two types of entities use different forms of reference and are
recognized in different contexts. Furthermore, they occupy different namespaces;
a parameter entity and a general entity with the same name are two distinct
entities.
4.1 Character
and Entity References
[
Definition: A character reference refers to a specific character
in the ISO/IEC 10646 character set, for example one not directly accessible
from available input devices.]
Character Reference
Well-formedness constraint: Legal Character
Characters referred
to using character references must match the production for
Char.
If the character reference begins with "&#x
", the digits and letters up to the terminating ;
provide
a hexadecimal representation of the character's code point in ISO/IEC 10646.
If it begins just with "&#
", the digits up to the terminating
;
provide a decimal representation of the character's code point.
[Definition: An entity
reference refers to the content of a named entity.] [Definition: References to parsed general entities use
ampersand (&
) and semicolon (;
) as delimiters.]
[Definition:
Parameter-entity references use percent-sign (%
) and semicolon
(;
) as delimiters.]
Entity Reference
Well-formedness constraint: Entity Declared
In a document without
any DTD, a document with only an internal DTD subset which contains no parameter
entity references, or a document with "standalone='yes'
", [E34]for an entity
reference that does not occur within the external subset or a parameter entity,
the Name given in the entity reference must match that in an entity
declaration that does not occur within the external subset or a
parameter entity, except that well-formed documents need not declare any of
the following entities: amp
, lt
, gt
,
apos
, quot
. Similarly, the declaration of a general entity
must precede any reference to it which appears in a default value in an attribute-list
declaration.
Note that if entities are declared in the external subset
or in external parameter entities, a non-validating processor is
not obligated to read and process their declarations; for such
documents, the rule that an entity must be declared is a well-formedness constraint
only if standalone='yes'.
Validity
constraint: Entity Declared
In a document with an external subset
or external parameter entities with "standalone='no'
", the Name given in the entity reference must match that in an entity
declaration. For interoperability, valid documents should declare
the entities amp
, lt
, gt
, apos
, quot
, in the form specified in
4.6 Predefined Entities. The declaration of a parameter entity must
precede any reference to it. Similarly, the declaration of a general entity
must precede any
[E92]attribute-list declaration containing a default value with a direct
or indirect reference to that general entity.
Well-formedness constraint: Parsed
Entity
An entity reference must not contain the name of an unparsed entity. Unparsed entities may be referred
to only in attribute values
declared to be of type ENTITY or ENTITIES.
Well-formedness
constraint: No Recursion
A parsed entity must not contain a recursive
reference to itself, either directly or indirectly.
Well-formedness constraint: In DTD
Parameter-entity references may only appear in the DTD.
Examples of character and entity
references:
Type <key>less-than</key> (<) to save options.
This document was prepared on &docdate; and
is classified &security-level;. |
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... -->
<!ENTITY % ISOLat2
SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >
<!-- ... now reference it. -->
%ISOLat2; |
4.2
Entity Declarations
[
Definition: Entities are declared thus:]
Entity Declaration
The Name identifies the entity
in an entity reference or,
in the case of an unparsed entity, in the value of an ENTITY or
ENTITIES attribute. If the same entity is declared more than once, the
first declaration encountered is binding; at user option, an XML processor
may issue a warning if entities are declared multiple times.
4.2.2
External Entities
[Definition
: If the entity is not internal, it is an external entity, declared
as follows:]
External Entity Declaration
If the NDataDecl is present,
this is a general unparsed
entity; otherwise it is a parsed entity.
Validity constraint: Notation
Declared
The Name must match the declared
name of a notation.
[Definition: The
SystemLiteral is called the entity's system identifier. It is a [E88]URI reference[E66] (as defined
in [IETF RFC 2396], updated by
[IETF RFC 2732]),
[E76]meant to be dereferenced 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. 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.
[E78]URI references require encoding and escaping of certain characters.
The disallowed characters include all non-ASCII characters, plus the excluded
characters listed in Section 2.4 of [IETF RFC 2396],
except for the number sign (#
) and percent sign (%
)
characters and the square bracket characters re-allowed in
[IETF RFC 2732]. Disallowed characters must be escaped as follows:
Each disallowed character is converted to UTF-8
[IETF RFC 2279] as one or more bytes.
Any octets corresponding to a disallowed character 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.
[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 the public identifier to try to generate an alternative [E88]URI reference.
If the processor is unable to do so, it must use the
[E88]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 "http://www.textuality.com/boilerplate/OpenHatch.xml">
<!ENTITY open-hatch
PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
"http://www.textuality.com/boilerplate/OpenHatch.xml">
<!ENTITY hatch-pic
SYSTEM "../grafix/OpenHatch.gif"
NDATA gif > |
4.3
Parsed Entities
4.3.1
The Text Declaration
External parsed entities may each begin with
a text declaration.
Text Declaration
The text declaration must be provided literally, not by
reference to a parsed entity. No text declaration may appear at any position
other than the beginning of an external parsed entity.
[E94]The text declaration in an external parsed entity is not considered
part of its replacement text
.
4.3.2 Well-Formed
Parsed Entities
The document entity is well-formed if it matches the
production labeled document. An external general
parsed entity is well-formed if it matches the production labeled
extParsedEnt. An external parameter entity is well-formed if it matches
the production labeled extPE.
Well-Formed
External Parsed Entity
An internal general parsed entity is well-formed if its
replacement text matches the production labeled content
. All internal parameter entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and physical
structures in an XML document are properly nested; no start-tag, end-tag
, empty-element tag, element, comment
, processing instruction
, character reference
, or entity reference
can begin in one entity and end in another.
4.3.3 Character Encoding in Entities
Each
external parsed entity in an XML document may use a different encoding for
its characters. All XML processors must be able to read entities in
[E56]both the UTF-8 and UTF-16 encodings.
[E77]The terms "UTF-8" and "UTF-16" in this specification do not apply
to character encodings with any other labels, even if the encodings or labels
are very similar to UTF-8 or UTF-16.
Entities encoded in UTF-16 must
begin with the Byte Order Mark described by
[E67]Annex F of [ISO/IEC 10646], Annex H of [ISO/IEC 10646-2000], section 2.4 of
[Unicode], and section 2.7 of [Unicode3] (the
ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature,
not part of either the markup or the character data of the XML document. XML
processors must be able to use this character to differentiate between UTF-8
and UTF-16 encoded documents.
Although an XML processor is required
to read only entities in the UTF-8 and UTF-16 encodings, it is recognized
that other encodings are used around the world, and it may be desired for
XML processors to read entities that use them.
[E47]In the absence of external character encoding information (such as
MIME headers), parsed entities which are stored in an encoding other than
UTF-8 or UTF-16 must begin with a text declaration (see
4.3.1 The Text Declaration) containing an encoding declaration:
Encoding Declaration
[80]
| EncodingDecl | ::=
| S 'encoding' Eq ('"' EncName '"' | "'" EncName
"'" ) |
[81]
| EncName | ::=
| [A-Za-z] ([A-Za-z0-9._] | '-')* | /* Encoding
name contains only Latin characters */ |
In the document
entity, the encoding declaration is part of the XML declaration. The EncName
is the name of the encoding used.
In an encoding declaration,
the values "UTF-8
", "UTF-16
", "ISO-10646-UCS-2
", and "ISO-10646-UCS-4
" should be used for the various
encodings and transformations of Unicode / ISO/IEC 10646, the values "
ISO-8859-1
", "ISO-8859-2
", ... "ISO-8859-9
"
should be used for the parts of ISO 8859, and the values "ISO-2022-JP
", "Shift_JIS
", and "EUC-JP
" should be used
for the various encoded forms of JIS X-0208-1997.
[E57]It is recommended that character encodings registered (as charset
s) with the Internet Assigned Numbers Authority
[E58][IANA-CHARSETS], other than those just listed,
should be referred to using their registered names; other encodings should
use names starting with an "x-" prefix. XML processors should match character
encoding names in a case-insensitive way and should either interpret an IANA-registered
name as the encoding registered at IANA for that name or treat it as unknown
(processors are, of course, not required to support all IANA-registered encodings).
In the absence of information provided by an external transport protocol
(e.g. HTTP or MIME), it is an error
for an entity including an encoding declaration to be presented to the XML
processor in an encoding other than that named in the declaration, or for
an entity which begins with neither a Byte Order Mark nor an encoding declaration
to use an encoding other than UTF-8. Note that since ASCII is a subset of
UTF-8, ordinary ASCII entities do not strictly need an encoding declaration.
[E5]It
is [E36]a fatal
error for a TextDecl to occur other than at the
beginning of an external entity.
It is a
fatal error when an XML processor encounters an entity with an encoding
that it is unable to process.
[E79]It is a fatal error if an XML entity is determined (via default,
encoding declaration, or higher-level protocol) to be in a certain encoding
but contains octet sequences that are not legal in that encoding. It is also
a fatal error if an XML entity contains no encoding declaration and its content
is not legal UTF-8 or UTF-16.
Examples of
[E23]text declarations containing encoding declarations:
<?xml encoding='UTF-8'?>
<?xml encoding='EUC-JP'?> |
4.4
XML Processor Treatment of Entities and References
The table below
summarizes the contexts in which character references, entity references,
and invocations of unparsed entities might appear and the required behavior
of an XML processor in each
case. The labels in the leftmost column describe the recognition context:
- Reference in Content
as a reference anywhere after the
start-tag and before the end-tag
of an element; corresponds to the nonterminal content
.
- Reference in Attribute Value
as a reference within either the value of an attribute in a start-tag, or a default value in an attribute declaration; corresponds to the nonterminal AttValue.
- Occurs as Attribute Value
as a Name, not a reference, appearing either
as the value of an attribute which has been declared as type ENTITY,
or as one of the space-separated tokens in the value of an attribute which
has been declared as type ENTITIES.
- Reference in Entity Value
as a reference within a parameter or internal entity's literal entity value in the entity's declaration;
corresponds to the nonterminal EntityValue.
- Reference in DTD
[E90]as
a reference within either the internal or external subsets of the DTD, but outside of an EntityValue
, AttValue, PI,
Comment, SystemLiteral,
PubidLiteral, or the contents of an ignored conditional section (see 3.4 Conditional Sections).
.
4.4.1
Not Recognized
Outside the DTD, the %
character has no
special significance; thus, what would be parameter entity references in the
DTD are not recognized as markup in content. Similarly,
the names of unparsed entities are not recognized except when they appear
in the value of an appropriately declared attribute.
4.4.2 Included
[
Definition: An entity is included when its replacement text is retrieved and processed, in place
of the reference itself, as though it were part of the document at the location
the reference was recognized.] The replacement text may contain both character data and (except for parameter entities) markup, which must be recognized in the
usual way. (The string "AT&T;
" expands to "AT&T;
" and the remaining ampersand is not recognized as an entity-reference
delimiter.) A character reference is included when the indicated character
is processed in place of the reference itself.
4.4.3 Included If Validating
When an XML
processor recognizes a reference to a parsed entity, in order to validate the document, the processor must include its replacement text. If the entity is external,
and the processor is not attempting to validate the XML document, the processor may, but need not, include the entity's replacement
text. If a non-validating
[E95]processor does not include the replacement text, it must inform
the application that it recognized, but did not read, the entity.
This rule is based on the recognition that the automatic inclusion provided
by the SGML and XML entity mechanism, primarily designed to support modularity
in authoring, is not necessarily appropriate for other applications, in particular
document browsing. Browsers, for example, when encountering an external parsed
entity reference, might choose to provide a visual indication of the entity's
presence and retrieve it for display only on demand.
4.4.4 Forbidden
The following are forbidden,
and constitute fatal errors:
the appearance of a reference to an
unparsed entity.
the appearance of any character or general-entity reference in the
DTD except within an EntityValue or
AttValue.
a reference to an external entity in an attribute value.
4.4.5 Included in
Literal
When an entity
reference appears in an attribute value, or a parameter entity reference
appears in a literal entity value, its
replacement text is processed in place of the reference itself as though
it were part of the document at the location the reference was recognized,
except that a single or double quote character in the replacement text is
always treated as a normal data character and will not terminate the literal.
For example, this is well-formed:
<!--
[E4] -->
<!ENTITY % YN '"Yes"' >
<!ENTITY WhatHeSaid "He said %YN;" > |
while this is not:
<!ENTITY EndAttr "27'" >
<element attribute='a-&EndAttr;> |
4.4.6 Notify
When the name of an
unparsed entity appears as a token in the value of an attribute of declared
type ENTITY or ENTITIES, a validating processor must inform
the application of the system
and public (if any)
identifiers for both the entity and its associated
notation.
4.4.7 Bypassed
When a general entity reference appears in the
EntityValue in an entity declaration, it is bypassed and left as is.
4.4.8 Included as PE
Just as
with external parsed entities, parameter entities need only be
included if validating. When a parameter-entity reference is recognized
in the DTD and included, its
replacement text is enlarged by the attachment of one leading and one
following space (#x20) character; the intent is to constrain the replacement
text of parameter entities to contain an integral number of grammatical tokens
in the DTD. [E96]
This behavior does not apply to parameter entity references within entity
values; these are described in 4.4.5 Included in Literal
.
4.5
Construction of Internal Entity Replacement Text
In discussing the
treatment of internal entities, it is useful to distinguish two forms of the
entity's value. [Definition
: The literal entity value is the quoted string actually present
in the entity declaration, corresponding to the non-terminal
EntityValue.] [Definition
: The replacement text is the content of the entity, after replacement
of character references and parameter-entity references.]
The literal
entity value as given in an internal entity declaration (
EntityValue) may contain character, parameter-entity, and general-entity
references. Such references must be contained entirely within the literal
entity value. The actual replacement text that is
included as described above must contain the replacement text
of any parameter entities referred to, and must contain the character referred
to, in place of any character references in the literal entity value; however,
general-entity references must be left as-is, unexpanded. For example, given
the following declarations:
<!ENTITY % pub "Éditions Gallimard" >
<!ENTITY rights "All rights reserved" >
<!ENTITY book "La Peste: Albert Camus,
© 1947 %pub;. &rights;" > |
then the replacement text for the entity "book
"
is:
La Peste: Albert Camus,
© 1947 Éditions Gallimard. &rights; |
The general-entity reference "&rights;
"
would be expanded should the reference "&book;
" appear in
the document's content or an attribute value.
These simple rules may
have complex interactions; for a detailed discussion of a difficult example,
see D Expansion of Entity and Character References
.
4.6
Predefined Entities
[Definition
: Entity and character references can both be used to escape the
left angle bracket, ampersand, and other delimiters. A set of general entities
(amp
, lt
, gt
, apos
,
quot
) is specified for this purpose. Numeric character references may
also be used; they are expanded immediately when recognized and must be treated
as character data, so the numeric character references "<
"
and "&
" may be used to escape <
and &
when they occur in character data.]
All XML processors must recognize
these entities whether they are declared or not. For interoperability, valid XML documents should declare
these entities, like any others, before using them.
[E80]If the entities lt
or amp
are declared,
they must be declared as internal entities whose replacement text is a character
reference to the character being escaped; the double escaping is required
for these entities so that references to them produce a well-formed result.
If the entities gt
, apos
, or quot
are
declared, they must be declared as internal entities whose replacement text
is the single character being escaped (or a character reference to that character;
the double escaping here is unnecessary but harmless). For example:
<!ENTITY lt "&#60;">
<!ENTITY gt ">">
<!ENTITY amp "&#38;">
<!ENTITY apos "'">
<!ENTITY quot """> |
4.7
Notation Declarations
[Definition
: Notations identify by name the format of unparsed entities, the format of elements which bear
a notation attribute, or the application to which a processing instruction is addressed.]
[Definition: Notation declarations provide a
name for the notation, for use in entity and attribute-list declarations and
in attribute specifications, and an external identifier for the notation which
may allow an XML processor or its client application to locate a helper application
capable of processing data in the given notation.]
Notation Declarations
Validity constraint:
[E22]Unique Notation Name
Only one notation declaration can
declare a given Name .
XML processors
must provide applications with the name and external identifier(s) of any
notation declared and referred to in an attribute value, attribute definition,
or entity declaration. They may additionally resolve the external identifier
into the system identifier,
file name, or other information needed to allow the application to call a
processor for data in the notation described. (It is not an error, however,
for XML documents to declare and refer to notations for which notation-specific
applications are not available on the system where the XML processor or application
is running.)
4.8
Document Entity
[Definition
: The document entity serves as the root of the entity tree and
a starting-point for an XML processor
.] This specification does not specify how the document entity is to be
located by an XML processor; unlike other entities, the document entity has
no name and might well appear on a processor input stream without any identification
at all.