Extensible Markup Language (XML) 1.0
W3C Recommendation 10-February-1998
- This version:
-
http://www.w3.org/TR/1998/REC-xml-19980210
- http://www.w3.org/TR/1998/REC-xml-19980210.xml
- http://www.w3.org/TR/1998/REC-xml-19980210.html
- http://www.w3.org/TR/1998/REC-xml-19980210.pdf
- http://www.w3.org/TR/1998/REC-xml-19980210.ps
- http://www.w3.org/TR/1998/REC-xml-19980210.xml
- Latest version:
- http://www.w3.org/TR/REC-xml
- Previous version:
- http://www.w3.org/TR/PR-xml-971208
- Editors:
- Tim Bray (Textuality and Netscape) <tbray@textuality.com>
- Jean Paoli (Microsoft) <jeanpa@microsoft.com>
- C. M. Sperberg-McQueen (University of Illinois at Chicago) <cmsmcq@uic.edu>
Abstract
The Extensible Markup Language (XML) is a subset of SGML that is completely described in this document. Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.
Status of this document
This document has been reviewed by W3C Members and other interested parties and has been endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited as a normative reference from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
This document specifies a syntax created by subsetting an existing, widely used international text processing standard (Standard Generalized Markup Language, ISO 8879:1986(E) as amended and corrected) for use on the World Wide Web. It is a product of the W3C XML Activity, details of which can be found at http://www.w3.org/XML. A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.
This specification uses the term URI, which is defined by [Berners-Lee et al.], a work in progress expected to update [IETF RFC1738] and [IETF RFC1808].
The list of known errors in this specification is available at http://www.w3.org/XML/xml-19980210-errata.
Please report errors in this document to xml-editor@w3.org.
Extensible Markup Language (XML) 1.0
Table of Contents
1. Introduction1.1 Origin and Goals
1.2 Terminology
2. Documents
2.1 Well-Formed XML Documents
2.2 Characters
2.3 Common Syntactic Constructs
2.4 Character Data and Markup
2.5 Comments
2.6 Processing Instructions
2.7 CDATA Sections
2.8 Prolog and Document Type Declaration
2.9 Standalone Document Declaration
2.10 White Space Handling
2.11 End-of-Line Handling
2.12 Language Identification
3. Logical Structures
3.1 Start-Tags, End-Tags, and Empty-Element Tags
3.2 Element Type Declarations
3.2.1 Element Content
3.2.2 Mixed Content
3.3 Attribute-List Declarations
3.3.1 Attribute Types
3.3.2 Attribute Defaults
3.3.3 Attribute-Value Normalization
3.4 Conditional Sections
4. Physical Structures
4.1 Character and Entity References
4.2 Entity Declarations
4.2.1 Internal Entities
4.2.2 External Entities
4.3 Parsed Entities
4.3.1 The Text Declaration
4.3.2 Well-Formed Parsed Entities
4.3.3 Character Encoding in Entities
4.4 XML Processor Treatment of Entities and References
4.4.1 Not Recognized
4.4.2 Included
4.4.3 Included If Validating
4.4.4 Forbidden
4.4.5 Included in Literal
4.4.6 Notify
4.4.7 Bypassed
4.4.8 Included as PE
4.5 Construction of Internal Entity Replacement Text
4.6 Predefined Entities
4.7 Notation Declarations
4.8 Document Entity
5. Conformance
5.1 Validating and Non-Validating Processors
5.2 Using XML Processors
6. Notation
Appendices
A. ReferencesA.1 Normative References
A.2 Other References
B. Character Classes
C. XML and SGML (Non-Normative)
D. Expansion of Entity and Character References (Non-Normative)
E. Deterministic Content Models (Non-Normative)
F. Autodetection of Character Encodings (Non-Normative)
G. W3C XML Working Group (Non-Normative)
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.
A software module called an XML processor is used to read XML documents and provide access to their content and structure. 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
- Conforming documents and XML processors are permitted to but need not behave as described.
- must
- Conforming documents and XML processors are required to behave as described; otherwise they are in error.
- error
- 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
- 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
- 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
- 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
- A rule which applies to all well-formed XML documents. Violations of well-formedness constraints are fatal errors.
- match
- (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. At user option, processors may normalize such characters to some canonical form. 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 "Element Valid".
- for compatibility
- A feature of XML included solely to ensure that XML remains compatible with SGML.
- for interoperability
- 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
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
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.
- There is exactly one element, called the root, or document element, no part of which appears in the content of any other element. 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.
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
A parsed entity contains text, a sequence of characters, which may represent markup or character data. A character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646]. Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646. The use of "compatibility characters", as defined in section 6.8 of [Unicode], is discouraged.
| Character Range | ||||||
|
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 | ||||
|
Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic or syllabic base character possibly followed by one or more combining characters, or of an ideographic character. Full definitions of the specific characters in each class are given in "B. Character Classes".
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: The colon character within XML names is reserved for experimentation with name spaces. Its meaning is expected to be standardized at some future point, at which point those documents using the colon for experimental purposes may need to be updated. (There is no guarantee that any name-space mechanism adopted for XML will in fact use the colon as a name-space delimiter.) In practice, this means that authors should not use the colon in XML names except as part of name-space experiments, but that XML processors should 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 | ||||||||||||||||||||||||||||
|
2.4 Character Data and Markup
Text consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character references, comments, CDATA section delimiters, document type declarations, and processing instructions.
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.
They are also legal within the literal entity
value of an internal entity declaration; see
"4.3.2 Well-Formed Parsed Entities".
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 | ||||
|
2.5 Comments
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.
| Comments | ||||
|
An example of a comment:
<!-- declarations for <head> & <body> --> |
2.6 Processing Instructions
Processing instructions (PIs) allow documents to contain instructions for applications.
| Processing Instructions | ||||||||
|
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.
2.7 CDATA Sections
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
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"?> |
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. 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 | ||||||||||||||||||||||||
|
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.
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 fuller information, see "4. Physical Structures".
| Document Type Definition | ||||||||||||||||||
|
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.
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.
| 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"?> |
The system identifier
"hello.dtd" gives the URI of a DTD for the document.
The declarations can also be given locally, as in this example:
<?xml version="1.0" encoding="UTF-8" ?> |
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.
| Standalone Document Declaration | ||||||
|
In a standalone document declaration, the value "yes" indicates
that there
are no markup declarations external to the document
entity (either in the DTD external subset, or in an
external parameter entity referenced from the internal subset)
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, denoted by the nonterminal
S in this specification) 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 only
possible values are "default" and "preserve".
For example:
<!ATTLIST poem xml:space (default|preserve) '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).
To simplify the tasks of applications, wherever an external parsed entity or the literal entity value of an internal parsed entity contains either the literal two-character sequence "#xD#xA" or a standalone literal #xD, an XML processor must pass to the application the single character #xA. (This behavior can conveniently be produced by normalizing all line breaks to #xA on input, before parsing.)
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.
The values of the attribute are language identifiers as defined
by [IETF RFC 1766], "Tags for the Identification of Languages":
| Language Identification | ||||||||||||||||||||||||
|
The Langcode may be any of the following:
- a two-letter language code as defined by [ISO 639], "Codes for the representation of names of languages"
- a language identifier registered with the Internet
Assigned Numbers Authority [IANA]; these begin with the
prefix "
i-" (or "I-") - a language identifier assigned by the user, or agreed on
between parties in private use; these must begin with the
prefix "
x-" or "X-" in order to ensure that they do not conflict with names later standardized or registered with IANA
There may be any number of Subcode segments; if
the first
subcode segment exists and the Subcode consists of two
letters, then it must be a country code from
[ISO 3166], "Codes
for the representation of names of countries."
If the first
subcode consists of more than two letters, it must be
a subcode for the language in question registered with IANA,
unless the Langcode begins with the prefix
"x-" or
"X-".
It is customary to give the language code in lower case, and the country code (if any) in upper case. Note that these values, unlike other names in XML documents, are case insensitive.
For example:
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p> |
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'> |
3. Logical Structures
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
EMPTYand the element has no content. - The declaration matches
childrenand 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 nonterminalS) between each pair of child elements. - The declaration matches
Mixedand 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
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.
The Name-AttValue pairs are
referred to as
the attribute specifications of the element,
with the
Name in each pair
referred to as the attribute name and
the content of the
AttValue (the text between the
' or " delimiters)
as the attribute value.
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 (other than "<") must not contain
a <.
An example of a start-tag:
<termdef id="dt-dog" term="dog"> |
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 | ||||
|
An example of an end-tag:
</termdef> |
The text between the start-tag and end-tag is called the element's content:
| Content of Elements | ||||
|
If an element is empty, it must be represented either by a start-tag immediately followed by an end-tag or by an empty-element tag. 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 must be used, and can only be used, for elements which are
declared EMPTY.
Examples of empty elements:
<IMG align="left" |
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.
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> |
3.2.1 Element Content
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).
In this case, the
constraint includes a 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 parenthetized 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.
For interoperability,
if a parameter-entity reference appears in a
choice, seq, or
Mixed construct, its replacement text
should not be empty, 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?)> |
3.2.2 Mixed Content
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.
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)*> |
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.
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.
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, 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, as noted:
| 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.
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:
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.
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 |
3.3.3 Attribute-Value Normalization
Before the value of an attribute is passed to the application or checked for validity, the XML processor must normalize it as follows:
- a character reference is processed by appending the referenced character to the attribute value
- an entity reference is processed by recursively processing the replacement text of the entity
- a whitespace character (#x20, #xD, #xA, #x9) is processed by appending #x20 to the normalized value, except that only a single #x20 is appended for a "#xD#xA" sequence that is part of an external parsed entity or the literal entity value of an internal parsed entity
- other characters are processed by appending them to the normalized value
If the declared value 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.
All attributes for which no declaration has been read should be treated
by a non-validating parser as if declared
CDATA.
3.4 Conditional Sections
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 | ||||||||||||||||||||
|
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.
Note that for reliable parsing, the contents of even ignored
conditional sections must be read in order to
detect nested conditional sections and ensure that the end of the
outermost (ignored) conditional section is properly detected.
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.
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' > |
4. Physical Structures
An XML document may consist of one or many storage units. These are called entities; they all have content and are all (except for the document entity, see below, and the external DTD subset) identified by 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. A parsed entity's contents are referred to as its replacement text; this text is considered an integral part of the document.
An unparsed entity is a resource whose contents may or may not be text, and if text, may not be 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.
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. 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
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.
&#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.
An entity
reference refers to the content of a named entity.
References to
parsed general entities
use ampersand (&) and semicolon (;) as
delimiters.
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'",
the Name given in the entity reference must
match that in an
entity declaration, except that
well-formed documents need not declare
any of the following entities: amp,
lt,
gt,
apos,
quot.
The declaration of a parameter entity must precede any reference to it.
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
reference to it which appears in a default value in an attribute-list
declaration.
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. |
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... --> |
4.2 Entity Declarations
| 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.
4.2.1 Internal Entities
If
the entity definition is an
EntityValue,
the defined entity is called an internal entity.
There is no separate physical
storage object, and the content of the entity is given in the
declaration.
Note that some processing of entity and character references in the
literal entity value may be required to
produce the correct replacement
text: see "4.5 Construction of Internal Entity Replacement Text".
An internal entity is a parsed entity.
Example of an internal entity declaration:
<!ENTITY Pub-Status "This is a pre-release of the |
4.2.2 External Entities
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.
The
SystemLiteral
is called the entity's system identifier. It is a URI,
which may be used to retrieve the entity.
Note that the hash mark (#) and fragment identifier
frequently used with URIs are not, formally, part of the URI itself;
an XML processor may signal an error if a fragment identifier is
given as 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.
An XML processor should handle a non-ASCII character in a URI by representing the character in UTF-8 as one or more bytes, and then escaping these bytes with the URI escaping mechanism (i.e., by converting each byte to %HH, where HH is the hexadecimal notation of the byte value).
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 URI. If the processor is unable to do so, it must use the URI 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 |
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.
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 either UTF-8 or UTF-16.
Entities encoded in UTF-16 must begin with the Byte Order Mark described by ISO/IEC 10646 Annex E and Unicode Appendix B (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. Parsed entities which are stored in an encoding other than UTF-8 or UTF-16 must begin with a text declaration containing an encoding declaration:
| Encoding Declaration | ||||||||||
|
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. XML
processors may recognize other encodings; it is recommended that
character encodings registered (as charsets)
with the Internet Assigned Numbers
Authority [IANA], other than those just listed, should be
referred to
using their registered names.
Note that these registered names are defined to be
case-insensitive, so processors wishing to match against them
should do so in a case-insensitive
way.
In the absence of i