Extensible Markup Language (XML) 1.0 (Second Edition)
Extensible Markup Language (XML)
1.0 (Second Edition)
W3C Recommendation 6 October 2000
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Editors:
Tim Bray, Textuality and Netscape
Jean Paoli, Microsoft
C. M. Sperberg-McQueen, University of Illinois at Chicago and Text Encoding
Initiative
Eve Maler, Sun Microsystems, Inc.
- Second Edition
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MIT
INRIA
Keio
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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
The English version of this specification is the only normative version.
However, for translations of this document, see
A list of current W3C Recommendations and other technical documents can be
found at
This
second edition is
not
a new version of XML (first published 10 February 1998); it merely incorporates
the changes dictated by the first-edition errata (available at
) as a convenience to readers. The errata list for this second edition is
available at
Please
report errors in this document to
xml-editor@w3.org
archives
are available.
Note:
C. M. Sperberg-McQueen's
affiliation has changed since the publication of the first edition. He is
now at the World Wide Web Consortium, and can be contacted at
cmsmcq@w3.org
Table of Contents
Introduction
1.1
Origin and Goals
1.2
Terminology
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
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
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
Conformance
5.1
Validating
and Non-Validating Processors
5.2
Using XML Processors
Notation
Appendices
References
A.1
Normative References
A.2
Other References
Character
Classes
XML and
SGML
(Non-Normative)
Expansion
of Entity and Character References
(Non-Normative)
Deterministic Content Models
(Non-Normative)
Autodetection of Character Encodings
(Non-Normative)
F.1
Detection
Without External Encoding Information
F.2
Priorities in the Presence of External
Encoding Information
W3C
XML Working Group
(Non-Normative)
W3C
XML Core Group
(Non-Normative)
Production
Notes
(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.
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
: Marks
a sentence describing a feature of XML included solely to ensure that XML
remains compatible with SGML.]
for interoperability
Definition
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.]
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
[1]
document
::=
prolog
element
Misc
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.] 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
in the document,
there is one other element
in the document such that
is in the content of
, but is not in the content of any other
element that is in the content of
is referred
to as the
parent
of
, and
as a
child
of
.]
2.2
Characters
Definition
: A parsed
entity contains
text
, a sequence of
characters
which may represent markup or character data.] [
Definition
character
is an atomic unit of text as specified by ISO/IEC 10646
[ISO/IEC 10646]
(see also
[ISO/IEC
10646-2000]
). Legal characters are tab, carriage return, line feed, and
the legal characters of Unicode and ISO/IEC 10646. 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]
(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.
(white space) consists of one or more space (#x20) characters,
carriage returns, line feeds, or tabs.
White Space
[3]
::=
(#x20 | #x9 | #xD | #xA)+
Characters are classified for convenience as letters,
digits, or other characters. 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:
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
[4]
NameChar
::=
Letter
Digit
| '.' | '-' | '_' | ':' |
CombiningChar
Extender
[5]
Name
::=
Letter
| '_' | ':') (
NameChar
)*
[6]
Names
::=
Name
Name
)*
[7]
Nmtoken
::=
NameChar
)+
[8]
Nmtokens
::=
Nmtoken
Nmtoken
)*
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:
Although
the
EntityValue
production allows the definition
of an entity consisting of a single explicit
in the literal
(e.g.,
), 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
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
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.] Parameter entity references are not recognized
within comments.
Comments
[15]
Comment
::=
''
An example of a comment:
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
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. 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 "
" and end with the string "
]]>
":]
CDATA
Sections
[18]
CDSect
::=
CDStart
CData
CDEnd
[19]
CDStart
::=
'[20]
CData
::=
Char
* - (
Char
* ']]>'
Char
*))
[21]
CDEnd
::=
']]>'
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 "
and "
" are recognized as
character data
, not
markup
Hello, world!]]>
2.8 Prolog
and Document Type Declaration
Definition
XML documents 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
and so is this:
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
[22]
prolog
::=
XMLDecl
Misc
doctypedecl
Misc
*)?
[23]
XMLDecl
::=
'VersionInfo
EncodingDecl
SDDecl
? '?>'
[24]
VersionInfo
::=
'version'
Eq
("'"
VersionNum
"'" | '"'
VersionNum
'"')
/* */
[25]
Eq
::=
? '='
[26]
VersionNum
::=
([a-zA-Z0-9_.:] | '-')+
[27]
Misc
::=
Comment
PI
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
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 further
information, see
4 Physical Structures
Document
Type Definition
[28]
doctypedecl
::=
'Name
ExternalID
)?
('[' (
markupdecl
DeclSep
)*
']'
?)? '>'
[VC: Root Element Type]
[WFC: External Subset]
/* */
[28a]
DeclSep
::=
PEReference
[WFC: PE Between Declarations]
/* */
[29]
markupdecl
::=
elementdecl
AttlistDecl
EntityDecl
NotationDecl
PI
Comment
[VC: Proper Declaration/PE Nesting]
[WFC: PEs in Internal Subset]
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
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.)
Well-formedness constraint: External Subset
The
external subset, if any, must match the production for
extSubset
Well-formedness
constraint: PE Between Declarations
The replacement text of a parameter
entity reference in a
DeclSep
must match the production
extSubsetDecl
Like the internal
subset, the external subset and any external parameter entities referenced
in a
DeclSep
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 these external parameter entities may conditionally be ignored by using
the
conditional section
construct; this is not allowed in the internal subset.
External Subset
[30]
extSubset
::=
TextDecl
extSubsetDecl
[31]
extSubsetDecl
::=
markupdecl
conditionalSect
DeclSep
)*
/* */
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:
The
system identifier
hello.dtd
" gives the address (a URI reference) of a DTD for
the document.
The declarations can also be given locally, as in this
example:
]>
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
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
[32]
SDDecl
::=
'standalone'
Eq
(("'"
('yes' | 'no') "'") | ('"' ('yes' | 'no') '"'))
[VC: Standalone Document Declaration]
In a standalone document declaration, the value "yes"
indicates that there are no
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:
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 values are one or both of
"default" and "preserve". For example:
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
, the characters
passed to an application by the
XML
processor
must be as if the XML processor normalized all line breaks in
external parsed entities (including the document entity) on input, before
parsing, by translating both the two-character sequence #xD #xA and any #xD
that is not followed by #xA to a single #xA character.
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
, or its successor on the IETF Standards
Track.
Note:
[IETF
RFC 1766]
tags are constructed from two-letter language codes as defined
by
[ISO 639]
, from two-letter country codes as defined
by
[ISO 3166]
, or from language identifiers registered
with the Internet Assigned Numbers Authority
[IANA-LANGCODES]
It is expected that the successor to
[IETF RFC 1766]
will introduce three-letter language codes for languages not presently covered
by
[ISO 639]
(Productions 33 through
38 have been removed.)
For example:
The quick brown fox jumps over the lazy dog.
What colour is it?
What color is it?
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:
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
[39]
element
::=
EmptyElemTag
STag
content
ETag
[WFC: Element Type Match]
[VC: Element Valid]
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
between 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
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
[40]
STag
::=
'<'
Name
Attribute
)*
? '>'
[WFC: Unique Att Spec]
[41]
Attribute
::=
Name
Eq
AttValue
[VC: Attribute Value Type]
[WFC: No External Entity References]
[WFC: No < in Attribute Values]
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
.]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:
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
? '>'
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
[43]
content
::=
CharData
? ((
element
Reference
CDSect
PI
Comment
CharData
?)*
/* */
Definition
: 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
[44]
EmptyElemTag
::=
'<'
Name
Attribute
)*
? '/>'
[WFC: Unique Att Spec]
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 should be used, and should only be used, for elements
which are declared EMPTY.
Examples of empty elements:
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
[45]
elementdecl
::=
'Name
contentspec
'>'
[VC: Unique Element Type Declaration]
[46]
contentspec
::=
'EMPTY' | 'ANY' |
Mixed
children
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:
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
).][
Definition
: In this case, the constraint includes
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 (
cp
s), which consist of
names, choice lists of content particles, or sequence lists of content particles:
Element-content
Models
[47]
children
::=
choice
seq
) ('?'
| '*' | '+')?
[48]
cp
::=
Name
choice
seq
) ('?' | '*' | '+')?
[49]
choice
::=
'('
cp
'|'
cp
)+
')'
/* */
/* */
[VC: Proper Group/PE Nesting]
[50]
seq
::=
'('
cp
','
cp
)*
')'
/* */
[VC: Proper Group/PE Nesting]
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 parenthesized
groups. That is to say, if either of the opening or closing parentheses in
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 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:
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
[51]
Mixed
::=
'('
? '#PCDATA' (
? '|'
Name
)*
? ')*'
| '('
? '#PCDATA'
? ')'
[VC: Proper Group/PE Nesting]
[VC: No Duplicate Types]
where the
Name
s give the types
of elements that may appear as children. 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:
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
[52]
AttlistDecl
::=
'Name
AttDef
? '>'
[53]
AttDef
::=
Name
AttType
DefaultDecl
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 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. 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
[54]
AttType
::=
StringType
TokenizedType
EnumeratedType
[55]
StringType
::=
'CDATA'
[56]
TokenizedType
::=
'ID'
[VC: ID]
[VC: One ID per Element Type]
[VC: ID Attribute Default]
| 'IDREF'
[VC: IDREF]
| 'IDREFS'
[VC: IDREF]
| 'ENTITY'
[VC: Entity Name]
| 'ENTITIES'
[VC: Entity Name]
| 'NMTOKEN'
[VC: Name Token]
| 'NMTOKENS'
[VC: Name Token]
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
[57]
EnumeratedType
::=
NotationType
Enumeration
[58]
NotationType
::=
'NOTATION'
'('
Name
? '|'
Name
)*
? ')'
[VC: Notation Attributes]
[VC: One Notation Per Element Type]
[VC: No Notation on Empty Element]
[59]
Enumeration
::=
'('
Nmtoken
? '|'
Nmtoken
)*
? ')'
[VC: Enumeration]
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: One Notation Per Element Type
No element type may have
more than one
NOTATION
attribute specified.
Validity constraint: No Notation
on 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
[60]
DefaultDecl
::=
'#REQUIRED' | '#IMPLIED'
| (('#FIXED' S)?
AttValue
[VC: Required Attribute]
[VC: Attribute Default Legal]
[WFC: No < in Attribute Values]
[VC: Fixed Attribute Default]
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:
id ID #REQUIRED
name CDATA #IMPLIED>
type (bullets|ordered|glossary) "ordered">
method CDATA #FIXED "POST">
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 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 processor as if declared
CDATA
Following
are examples of attribute normalization. Given the following declarations:
the attribute specifications in the left column below would be
normalized to the character sequences of the middle column if the attribute
is declared
NMTOKENS
and to those of the right columns if
is declared
CDATA
Attribute specification
a is NMTOKENS
a is CDATA
a="
xyz"
x y z
#x20 #x20 x y z
a="&d;&d;A&a;&a;B&da;"
A #x20 B
#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
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
[61]
conditionalSect
::=
includeSect
ignoreSect
[62]
includeSect
::=
'extSubsetDecl
']]>'
/* */
[VC: Proper Conditional Section/PE Nesting]
[63]
ignoreSect
::=
'ignoreSectContents
']]>'
/* */
[VC: Proper Conditional Section/PE Nesting]
[64]
ignoreSectContents
::=
Ignore
('ignoreSectContents
']]>'
Ignore
)*
[65]
Ignore
::=
Char
* - (
Char
* ('| ']]>')
Char
*)
Validity
constraint: 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. 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:
]]>
]]>
Physical Structures
Definition
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
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
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, 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
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
[66]
CharRef
::=
'&#' [0-9]+ ';'
| '&#x' [0-9a-fA-F]+ ';'
[WFC: Legal Character]
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
[67]
Reference
::=
EntityRef
CharRef
[68]
EntityRef
::=
'&'
Name
';'
[WFC: Entity Declared]
[VC: Entity Declared]
[WFC: Parsed Entity]
[WFC: No Recursion]
[69]
PEReference
::=
'%'
Name
';'
[VC: Entity Declared]
[WFC: No Recursion]
[WFC: In DTD]
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'
", 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
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 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
This document was prepared on &docdate; and
is classified &security-level;.
Example of a parameter-entity reference:
SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >
%ISOLat2;
4.2 Entity
Declarations
Definition
Entities are declared thus:]
Entity Declaration
[70]
EntityDecl
::=
GEDecl
PEDecl
[71]
GEDecl
::=
'Name
EntityDef
'>'
[72]
PEDecl
::=
''%'
Name
PEDef
? '>'
[73]
EntityDef
::=
EntityValue
| (
ExternalID
NDataDecl
?)
[74]
PEDef
::=
EntityValue
ExternalID
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
Definition
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:
specification.">
4.2.2
External Entities
Definition
If the entity is not internal, it is an
external entity
, declared as
follows:]
External Entity Declaration
[75]
ExternalID
::=
'SYSTEM'
SystemLiteral
| 'PUBLIC'
PubidLiteral
SystemLiteral
[76]
NDataDecl
::=
'NDATA'
Name
[VC: Notation Declared]
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 URI reference (as
defined in
[IETF RFC 2396]
, updated by
[IETF
RFC 2732]
), 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
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 URI reference.
If the processor is unable to do so, it must use the URI reference specified
in the system literal. Before a match is attempted, all strings of white space
in the public identifier must be normalized to single space characters (#x20),
and leading and trailing white space must be removed.
Examples of external
entity declarations:
SYSTEM "http://www.textuality.com/boilerplate/OpenHatch.xml">
PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
"http://www.textuality.com/boilerplate/OpenHatch.xml">
SYSTEM "../grafix/OpenHatch.gif"
NDATA gif >
4.3
Parsed Entities
4.3.1
The Text Declaration
External parsed entities should each begin with
text declaration
Text Declaration
[77]
TextDecl
::=
'VersionInfo
EncodingDecl
? '?>'
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. 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
. All external parameter
entities are well-formed by definition.
Well-Formed External Parsed
Entity
[78]
extParsedEnt
::=
TextDecl
content
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 both the
UTF-8 and UTF-16 encodings. 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 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. 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
::=
'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-
" (where
is the part number) 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. It is recommended
that character encodings registered (as
charset
s) with the Internet
Assigned Numbers Authority
[IANA-CHARSETS]
, other than
those just listed, 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.
It is 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. 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 text declarations containing encoding declarations:
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
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
).
Entity Type
Character
Parameter
Internal General
External Parsed General
Unparsed
Reference in Content
Not recognized
Included
Included if
validating
Forbidden
Included
Reference in Attribute Value
Not recognized
Included in literal
Forbidden
Forbidden
Included
Occurs as Attribute Value
Not recognized
Forbidden
Forbidden
Notify
Not recognized
Reference in EntityValue
Included in literal
Bypassed
Bypassed
Forbidden
Included
Reference in DTD
Included as PE
Forbidden
Forbidden
Forbidden
Forbidden
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 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:
while this is not:
: 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:
© 1947 %pub;. &rights;" >
then the replacement text for the entity "
book
" is:
La Peste: Albert Camus,
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
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. If the entities
lt
or
amp
are declared, they must be declared as internal entities whose replacement
text is a character reference to the respective character (less-than sign
or ampersand) 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:
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
[82]
NotationDecl
::=
'Name
ExternalID
PublicID
? '>'
[VC: Unique Notation Name]
[83]
PublicID
::=
'PUBLIC'
PubidLiteral
Validity
constraint: 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.
Conformance
5.1 Validating
and Non-Validating Processors
Conforming
XML processors
fall into two classes: validating and
non-validating.
Validating and non-validating processors alike must
report violations of this specification's well-formedness constraints in the
content of the
document entity
and any other
parsed entities
that they read.
Definition
Validating
processors
must, at user option, report violations of the constraints
expressed by the declarations in the
DTD
, and failures to fulfill the validity constraints
given in this specification.] To accomplish this, validating XML processors
must read and process the entire DTD and all external parsed entities referenced
in the document.
Non-validating processors are required to check only
the
document entity
, including
the entire internal DTD subset, for well-formedness. [
Definition
: While they are not required to check
the document for validity, they are required to
process
all the declarations
they read in the internal DTD subset and in any parameter entity that they
read, up to the first reference to a parameter entity that they do
not
read; that is to say, they must use the information in those declarations
to
normalize
attribute values,
include
the replacement text of internal
entities, and supply
default attribute
values
.] Except when
standalone="yes"
, they must not
process
entity declarations
or
attribute-list declarations
encountered after a reference
to a parameter entity that is not read, since the entity may have contained
overriding declarations.
5.2
Using XML Processors
The behavior of a validating XML processor is
highly predictable; it must read every piece of a document and report all
well-formedness and validity violations. Less is required of a non-validating
processor; it need not read any part of the document other than the document
entity. This has two effects that may be important to users of XML processors:
Certain well-formedness errors, specifically those that require reading
external entities, may not be detected by a non-validating processor. Examples
include the constraints entitled
Entity Declared
Parsed Entity
, and
No
Recursion
, as well as some of the cases described as
forbidden
in
4.4 XML Processor Treatment of Entities and References
The information passed from the processor to the application may vary,
depending on whether the processor reads parameter and external entities.
For example, a non-validating processor may not
normalize
attribute values,
include
the replacement
text of internal entities, or supply
default
attribute values
, where doing so depends on having read declarations
in external or parameter entities.
For maximum reliability in interoperating between different XML processors,
applications which use non-validating processors should not rely on any behaviors
not required of such processors. Applications which require facilities such
as the use of default attributes or internal entities which are declared in
external entities should use validating XML processors.
6 Notation
The formal
grammar of XML is given in this specification using a simple Extended Backus-Naur
Form (EBNF) notation. Each rule in the grammar defines one symbol, in the
form
symbol ::= expression
Symbols are written with an initial capital letter if they are
the start symbol of a regular language, otherwise with an initial lower case
letter. Literal strings are quoted.
Within the expression on the right-hand
side of a rule, the following expressions are used to match strings of one
or more characters:
#xN
where
is a hexadecimal integer, the expression matches
the character in ISO/IEC 10646 whose canonical (UCS-4) code value, when interpreted
as an unsigned binary number, has the value indicated. The number of leading
zeros in the
#xN
form is insignificant; the number of leading
zeros in the corresponding code value is governed by the character encoding
in use and is not significant for XML.
[a-zA-Z]
[#xN-#xN]
matches any
Char
with a value in the range(s)
indicated (inclusive).
[abc]
[#xN#xN#xN]
matches any
Char
with a value among the characters
enumerated. Enumerations and ranges can be mixed in one set of brackets.
[^a-z]
[^#xN-#xN]
matches any
Char
with a value
outside
the range indicated.
[^abc]
[^#xN#xN#xN]
matches any
Char
with a value not among the
characters given. Enumerations and ranges of forbidden values can be mixed
in one set of brackets.
"string"
matches a literal string
matching
that given inside the double quotes.
'string'
matches a literal string
matching
that given inside the single quotes.
These symbols may be combined to match more complex patterns as follows,
where
and
represent simple expressions:
expression
expression
is treated as a unit and may be combined as
described in this list.
A?
matches
or nothing; optional
A B
matches
followed by
. This operator has
higher precedence than alternation; thus
A B | C D
is identical
to
(A B) | (C D)
A | B
matches
or
but not both.
A - B
matches any string that matches
but does not match
A+
matches one or more occurrences of
.Concatenation has
higher precedence than alternation; thus
A+ | B+
is identical
to
(A+) | (B+)
A*
matches zero or more occurrences of
. Concatenation
has higher precedence than alternation; thus
A* | B*
is identical
to
(A*) | (B*)
Other notations used in the productions are:
/* ... */
comment.
[ wfc: ... ]
well-formedness constraint; this identifies by name a constraint on
well-formed
documents associated
with a production.
[ vc: ... ]
validity constraint; this identifies by name a constraint on
valid
documents associated with a production.
References
A.1
Normative References
IANA-CHARSETS
(Internet Assigned Numbers Authority)
Official Names for Character
Sets
, ed. Keld Simonsen et al. See
ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets
IETF RFC 1766
IETF (Internet Engineering Task Force).
RFC 1766: Tags for the Identification
of Languages
, ed. H. Alvestrand. 1995. (See
.)
ISO/IEC 10646
ISO (International Organization for Standardization).
ISO/IEC 10646-1993
(E). Information technology -- Universal Multiple-Octet Coded Character Set
(UCS) -- Part 1: Architecture and Basic Multilingual Plane.
[Geneva]:
International Organization for Standardization, 1993 (plus amendments AM 1
through AM 7).
ISO/IEC 10646-2000
ISO (International Organization for Standardization).
ISO/IEC 10646-1:2000.
Information technology -- Universal Multiple-Octet Coded Character Set (UCS)
-- Part 1: Architecture and Basic Multilingual Plane.
[Geneva]: International
Organization for Standardization, 2000.
Unicode
The Unicode Consortium.
The Unicode Standard, Version 2.0.
Reading,
Mass.: Addison-Wesley Developers Press, 1996.
Unicode3
The Unicode Consortium.
The Unicode Standard, Version 3.0.
Reading,
Mass.: Addison-Wesley Developers Press, 2000. ISBN 0-201-61633-5.
A.2 Other References
Aho/Ullman
Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman.
Compilers: Principles,
Techniques, and Tools
. Reading: Addison-Wesley, 1986, rpt. corr. 1988.
Berners-Lee et al.
Berners-Lee, T., R. Fielding, and L. Masinter.
Uniform Resource
Identifiers (URI): Generic Syntax and Semantics
. 1997. (Work in progress;
see updates to RFC1738.)
Brüggemann-Klein
Brüggemann-Klein, Anne. Formal Models in Document Processing. Habilitationsschrift.
Faculty of Mathematics at the University of Freiburg, 1993. (See
ftp://ftp.informatik.uni-freiburg.de/documents/papers/brueggem/habil.ps
.)
Brüggemann-Klein and Wood
Brüggemann-Klein, Anne, and Derick Wood.
Deterministic Regular
Languages
. Universität Freiburg, Institut für Informatik,
Bericht 38, Oktober 1991. Extended abstract in A. Finkel, M. Jantzen, Hrsg.,
STACS 1992, S. 173-184. Springer-Verlag, Berlin 1992. Lecture Notes in Computer
Science 577. Full version titled
One-Unambiguous Regular Languages
in Information and Computation 140 (2): 229-253, February 1998.
Clark
James Clark. Comparison of SGML and XML. See
IANA-LANGCODES
(Internet Assigned Numbers Authority)
Registry of Language Tags
ed. Keld Simonsen et al. (See
.)
IETF RFC2141
IETF (Internet Engineering Task Force).
RFC 2141: URN Syntax
ed. R. Moats. 1997. (See
.)
IETF RFC 2279
IETF (Internet Engineering Task Force).
RFC 2279: UTF-8, a transformation
format of ISO 10646
, ed. F. Yergeau, 1998. (See
.)
IETF RFC 2376
IETF (Internet Engineering Task Force).
RFC 2376: XML Media Types
ed. E. Whitehead, M. Murata. 1998. (See
.)
IETF RFC 2396
IETF (Internet Engineering Task Force).
RFC 2396: Uniform Resource
Identifiers (URI): Generic Syntax
. T. Berners-Lee, R. Fielding, L.
Masinter. 1998. (See
.)
IETF RFC 2732
IETF (Internet Engineering Task Force).
RFC 2732: Format for Literal
IPv6 Addresses in URL's
. R. Hinden, B. Carpenter, L. Masinter. 1999.
(See
.)
IETF RFC 2781
IETF (Internet Engineering Task Force).
RFC 2781: UTF-16, an encoding
of ISO 10646
, ed. P. Hoffman, F. Yergeau. 2000. (See
.)
ISO 639
(International Organization for Standardization).
ISO 639:1988
(E). Code for the representation of names of languages.
[Geneva]: International
Organization for Standardization, 1988.
ISO 3166
(International Organization for Standardization).
ISO 3166-1:1997
(E). Codes for the representation of names of countries and their subdivisions
-- Part 1: Country codes
[Geneva]: International Organization for Standardization,
1997.
ISO 8879
ISO (International Organization for Standardization).
ISO 8879:1986(E).
Information processing -- Text and Office Systems -- Standard Generalized
Markup Language (SGML).
First edition -- 1986-10-15. [Geneva]: International
Organization for Standardization, 1986.
ISO/IEC 10744
ISO (International Organization for Standardization).
ISO/IEC 10744-1992
(E). Information technology -- Hypermedia/Time-based Structuring Language
(HyTime).
[Geneva]: International Organization for Standardization,
1992.
Extended Facilities Annexe.
[Geneva]: International Organization
for Standardization, 1996.
WEBSGML
ISO (International Organization for Standardization).
ISO 8879:1986
TC2. Information technology -- Document Description and Processing Languages.
[Geneva]: International Organization for Standardization, 1998. (See
.)
XML Names
Tim Bray, Dave Hollander, and Andrew Layman, editors.
Namespaces
in XML
. Textuality, Hewlett-Packard, and Microsoft. World Wide Web
Consortium, 1999. (See
.)
B Character
Classes
Following the characteristics defined in the Unicode standard,
characters are classed as base characters (among others, these contain the
alphabetic characters of the Latin alphabet), ideographic characters, and
combining characters (among others, this class contains most diacritics) Digits
and extenders are also distinguished.
Characters
[84]
Letter
::=
BaseChar
Ideographic
[85]
BaseChar
::=
[#x0041-#x005A] | [#x0061-#x007A] | [#x00C0-#x00D6] | [#x00D8-#x00F6]
| [#x00F8-#x00FF] | [#x0100-#x0131] | [#x0134-#x013E] | [#x0141-#x0148]
| [#x014A-#x017E] | [#x0180-#x01C3] | [#x01CD-#x01F0] | [#x01F4-#x01F5]
| [#x01FA-#x0217] | [#x0250-#x02A8] | [#x02BB-#x02C1] | #x0386
| [#x0388-#x038A] | #x038C | [#x038E-#x03A1] | [#x03A3-#x03CE]
| [#x03D0-#x03D6] | #x03DA | #x03DC | #x03DE | #x03E0
| [#x03E2-#x03F3] | [#x0401-#x040C] | [#x040E-#x044F] | [#x0451-#x045C]
| [#x045E-#x0481] | [#x0490-#x04C4] | [#x04C7-#x04C8] | [#x04CB-#x04CC]
| [#x04D0-#x04EB] | [#x04EE-#x04F5] | [#x04F8-#x04F9] | [#x0531-#x0556]
| #x0559 | [#x0561-#x0586] | [#x05D0-#x05EA] | [#x05F0-#x05F2]
| [#x0621-#x063A] | [#x0641-#x064A] | [#x0671-#x06B7] | [#x06BA-#x06BE]
| [#x06C0-#x06CE] | [#x06D0-#x06D3] | #x06D5 | [#x06E5-#x06E6]
| [#x0905-#x0939] | #x093D | [#x0958-#x0961] | [#x0985-#x098C]
| [#x098F-#x0990] | [#x0993-#x09A8] | [#x09AA-#x09B0] | #x09B2
| [#x09B6-#x09B9] | [#x09DC-#x09DD] | [#x09DF-#x09E1] | [#x09F0-#x09F1]
| [#x0A05-#x0A0A] | [#x0A0F-#x0A10] | [#x0A13-#x0A28] | [#x0A2A-#x0A30]
| [#x0A32-#x0A33] | [#x0A35-#x0A36] | [#x0A38-#x0A39] | [#x0A59-#x0A5C]
| #x0A5E | [#x0A72-#x0A74] | [#x0A85-#x0A8B] | #x0A8D
| [#x0A8F-#x0A91] | [#x0A93-#x0AA8] | [#x0AAA-#x0AB0] | [#x0AB2-#x0AB3]
| [#x0AB5-#x0AB9] | #x0ABD | #x0AE0 | [#x0B05-#x0B0C]
| [#x0B0F-#x0B10] | [#x0B13-#x0B28] | [#x0B2A-#x0B30] | [#x0B32-#x0B33]
| [#x0B36-#x0B39] | #x0B3D | [#x0B5C-#x0B5D] | [#x0B5F-#x0B61]
| [#x0B85-#x0B8A] | [#x0B8E-#x0B90] | [#x0B92-#x0B95] | [#x0B99-#x0B9A]
| #x0B9C | [#x0B9E-#x0B9F] | [#x0BA3-#x0BA4] | [#x0BA8-#x0BAA]
| [#x0BAE-#x0BB5] | [#x0BB7-#x0BB9] | [#x0C05-#x0C0C] | [#x0C0E-#x0C10]
| [#x0C12-#x0C28] | [#x0C2A-#x0C33] | [#x0C35-#x0C39] | [#x0C60-#x0C61]
| [#x0C85-#x0C8C] | [#x0C8E-#x0C90] | [#x0C92-#x0CA8] | [#x0CAA-#x0CB3]
| [#x0CB5-#x0CB9] | #x0CDE | [#x0CE0-#x0CE1] | [#x0D05-#x0D0C]
| [#x0D0E-#x0D10] | [#x0D12-#x0D28] | [#x0D2A-#x0D39] | [#x0D60-#x0D61]
| [#x0E01-#x0E2E] | #x0E30 | [#x0E32-#x0E33] | [#x0E40-#x0E45]
| [#x0E81-#x0E82] | #x0E84 | [#x0E87-#x0E88] | #x0E8A
| #x0E8D | [#x0E94-#x0E97] | [#x0E99-#x0E9F] | [#x0EA1-#x0EA3]
| #x0EA5 | #x0EA7 | [#x0EAA-#x0EAB] | [#x0EAD-#x0EAE]
| #x0EB0 | [#x0EB2-#x0EB3] | #x0EBD | [#x0EC0-#x0EC4]
| [#x0F40-#x0F47] | [#x0F49-#x0F69] | [#x10A0-#x10C5] | [#x10D0-#x10F6]
| #x1100 | [#x1102-#x1103] | [#x1105-#x1107] | #x1109
| [#x110B-#x110C] | [#x110E-#x1112] | #x113C | #x113E
| #x1140 | #x114C | #x114E | #x1150 | [#x1154-#x1155]
| #x1159 | [#x115F-#x1161] | #x1163 | #x1165 | #x1167
| #x1169 | [#x116D-#x116E] | [#x1172-#x1173] | #x1175
| #x119E | #x11A8 | #x11AB | [#x11AE-#x11AF] | [#x11B7-#x11B8]
| #x11BA | [#x11BC-#x11C2] | #x11EB | #x11F0 | #x11F9
| [#x1E00-#x1E9B] | [#x1EA0-#x1EF9] | [#x1F00-#x1F15] | [#x1F18-#x1F1D]
| [#x1F20-#x1F45] | [#x1F48-#x1F4D] | [#x1F50-#x1F57] | #x1F59
| #x1F5B | #x1F5D | [#x1F5F-#x1F7D] | [#x1F80-#x1FB4]
| [#x1FB6-#x1FBC] | #x1FBE | [#x1FC2-#x1FC4] | [#x1FC6-#x1FCC]
| [#x1FD0-#x1FD3] | [#x1FD6-#x1FDB] | [#x1FE0-#x1FEC] | [#x1FF2-#x1FF4]
| [#x1FF6-#x1FFC] | #x2126 | [#x212A-#x212B] | #x212E
| [#x2180-#x2182] | [#x3041-#x3094] | [#x30A1-#x30FA] | [#x3105-#x312C]
| [#xAC00-#xD7A3]
[86]
Ideographic
::=
[#x4E00-#x9FA5] | #x3007 | [#x3021-#x3029]
[87]
CombiningChar
::=
[#x0300-#x0345] | [#x0360-#x0361] | [#x0483-#x0486] | [#x0591-#x05A1]
| [#x05A3-#x05B9] | [#x05BB-#x05BD] | #x05BF | [#x05C1-#x05C2]
| #x05C4 | [#x064B-#x0652] | #x0670 | [#x06D6-#x06DC]
| [#x06DD-#x06DF] | [#x06E0-#x06E4] | [#x06E7-#x06E8] | [#x06EA-#x06ED]
| [#x0901-#x0903] | #x093C | [#x093E-#x094C] | #x094D
| [#x0951-#x0954] | [#x0962-#x0963] | [#x0981-#x0983] | #x09BC
| #x09BE | #x09BF | [#x09C0-#x09C4] | [#x09C7-#x09C8]
| [#x09CB-#x09CD] | #x09D7 | [#x09E2-#x09E3] | #x0A02
| #x0A3C | #x0A3E | #x0A3F | [#x0A40-#x0A42] | [#x0A47-#x0A48]
| [#x0A4B-#x0A4D] | [#x0A70-#x0A71] | [#x0A81-#x0A83] | #x0ABC
| [#x0ABE-#x0AC5] | [#x0AC7-#x0AC9] | [#x0ACB-#x0ACD] | [#x0B01-#x0B03]
| #x0B3C | [#x0B3E-#x0B43] | [#x0B47-#x0B48] | [#x0B4B-#x0B4D]
| [#x0B56-#x0B57] | [#x0B82-#x0B83] | [#x0BBE-#x0BC2] | [#x0BC6-#x0BC8]
| [#x0BCA-#x0BCD] | #x0BD7 | [#x0C01-#x0C03] | [#x0C3E-#x0C44]
| [#x0C46-#x0C48] | [#x0C4A-#x0C4D] | [#x0C55-#x0C56] | [#x0C82-#x0C83]
| [#x0CBE-#x0CC4] | [#x0CC6-#x0CC8] | [#x0CCA-#x0CCD] | [#x0CD5-#x0CD6]
| [#x0D02-#x0D03] | [#x0D3E-#x0D43] | [#x0D46-#x0D48] | [#x0D4A-#x0D4D]
| #x0D57 | #x0E31 | [#x0E34-#x0E3A] | [#x0E47-#x0E4E]
| #x0EB1 | [#x0EB4-#x0EB9] | [#x0EBB-#x0EBC] | [#x0EC8-#x0ECD]
| [#x0F18-#x0F19] | #x0F35 | #x0F37 | #x0F39 | #x0F3E
| #x0F3F | [#x0F71-#x0F84] | [#x0F86-#x0F8B] | [#x0F90-#x0F95]
| #x0F97 | [#x0F99-#x0FAD] | [#x0FB1-#x0FB7] | #x0FB9
| [#x20D0-#x20DC] | #x20E1 | [#x302A-#x302F] | #x3099
| #x309A
[88]
Digit
::=
[#x0030-#x0039] | [#x0660-#x0669] | [#x06F0-#x06F9] | [#x0966-#x096F]
| [#x09E6-#x09EF] | [#x0A66-#x0A6F] | [#x0AE6-#x0AEF] | [#x0B66-#x0B6F]
| [#x0BE7-#x0BEF] | [#x0C66-#x0C6F] | [#x0CE6-#x0CEF] | [#x0D66-#x0D6F]
| [#x0E50-#x0E59] | [#x0ED0-#x0ED9] | [#x0F20-#x0F29]
[89]
Extender
::=
#x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46
| #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309D-#x309E]
| [#x30FC-#x30FE]
The character classes defined here can be derived from
the Unicode 2.0 character database as follows:
Name start characters must have one of the categories Ll, Lu, Lo, Lt,
Nl.
Name characters other than Name-start characters must have one of the
categories Mc, Me, Mn, Lm, or Nd.
Characters in the compatibility area (i.e. with character code greater
than #xF900 and less than #xFFFE) are not allowed in XML names.
Characters which have a font or compatibility decomposition (i.e. those
with a "compatibility formatting tag" in field 5 of the database -- marked
by field 5 beginning with a "<") are not allowed.
The following characters are treated as name-start characters rather
than name characters, because the property file classifies them as Alphabetic:
[#x02BB-#x02C1], #x0559, #x06E5, #x06E6.
Characters #x20DD-#x20E0 are excluded (in accordance with Unicode 2.0,
section 5.14).
Character #x00B7 is classified as an extender, because the property
list so identifies it.
Character #x0387 is added as a name character, because #x00B7 is its
canonical equivalent.
Characters ':' and '_' are allowed as name-start characters.
Characters '-' and '.' are allowed as name characters.
C XML and
SGML (Non-Normative)
XML is designed to be a subset of SGML, in that
every XML document should also be a conforming SGML document. For a detailed
comparison of the additional restrictions that XML places on documents beyond
those of SGML, see
[Clark]
D Expansion of Entity and Character References (Non-Normative)
This
appendix contains some examples illustrating the sequence of entity- and character-reference
recognition and expansion, as specified in
4.4 XML Processor
Treatment of Entities and References
If the DTD contains the
declaration
An ampersand (&) may be escaped
numerically (&#38;) or with a general entity
(&).
then the XML processor will recognize the character references
when it parses the entity declaration, and resolve them before storing the
following string as the value of the entity "
example
":
An ampersand (&) may be escaped
numerically (&) or with a general entity
(&).
A reference in the document to "
&example;
" will
cause the text to be reparsed, at which time the start- and end-tags of the
element will be recognized and the three references will be recognized and
expanded, resulting in a
element with the following content
(all data, no delimiters or markup):
An ampersand (&) may be escaped
numerically (&) or with a general entity
(&).
A more complex example will illustrate the rules and their effects
fully. In the following example, the line numbers are solely for reference.
1
2 3
4
5 ' >
6 %xx;
7 ]>
8
This produces the following:
in line 4, the reference to character 37 is expanded immediately, and
the parameter entity "
xx
" is stored in the symbol table with
the value "
%zz;
". Since the replacement text is not rescanned,
the reference to parameter entity "
zz
" is not recognized. (And
it would be an error if it were, since "
zz
" is not yet declared.)
in line 5, the character reference "
<
" is expanded
immediately and the parameter entity "
zz
" is stored with the
replacement text "
", which
is a well-formed entity declaration.
in line 6, the reference to "
xx
" is recognized, and the
replacement text of "
xx
" (namely "
%zz;
") is parsed.
The reference to "
zz
" is recognized in its turn, and its replacement
text ("
") is parsed. The general
entity "
tricky
" has now been declared, with the replacement text
error-prone
".
in line 8, the reference to the general entity "
tricky
is recognized, and it is expanded, so the full content of the
test
element is the self-describing (and ungrammatical) string
This sample
shows a error-prone method.
E Deterministic
Content Models (Non-Normative)
As noted in
3.2.1
Element Content
, it is required that content models in element type
declarations be deterministic. This requirement is
for compatibility
with SGML (which calls deterministic
content models "unambiguous"); XML processors built using SGML systems may
flag non-deterministic content models as errors.
For example, the content
model
((b, c) | (b, d))
is non-deterministic, because given an
initial
the XML processor cannot know which
in the model is being matched without looking ahead to see which element follows
the
. In this case, the two references to
can
be collapsed into a single reference, making the model read
(b, (c |
d))
. An initial
now clearly matches only a single name
in the content model. The processor doesn't need to look ahead to see what
follows; either
or
would be accepted.
More
formally: a finite state automaton may be constructed from the content model
using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi,
and Ullman
[Aho/Ullman]
. In many such algorithms, a follow
set is constructed for each position in the regular expression (i.e., each
leaf node in the syntax tree for the regular expression); if any position
has a follow set in which more than one following position is labeled with
the same element type name, then the content model is in error and may be
reported as an error.
Algorithms exist which allow many but not all
non-deterministic content models to be reduced automatically to equivalent
deterministic models; see Brüggemann-Klein 1991
[Brüggemann-Klein]
F Autodetection of Character
Encodings (Non-Normative)
The XML encoding declaration functions as
an internal label on each entity, indicating which character encoding is in
use. Before an XML processor can read the internal label, however, it apparently
has to know what character encoding is in use--which is what the internal
label is trying to indicate. In the general case, this is a hopeless situation.
It is not entirely hopeless in XML, however, because XML limits the general
case in two ways: each implementation is assumed to support only a finite
set of character encodings, and the XML encoding declaration is restricted
in position and content in order to make it feasible to autodetect the character
encoding in use in each entity in normal cases. Also, in many cases other
sources of information are available in addition to the XML data stream itself.
Two cases may be distinguished, depending on whether the XML entity is presented
to the processor without, or with, any accompanying (external) information.
We consider the first case first.
F.1
Detection Without External Encoding Information
Because each XML entity
not accompanied by external encoding information and not in UTF-8 or UTF-16
encoding
must
begin with an XML encoding declaration, in which the
first characters must be '
', any conforming processor
can detect, after two to four octets of input, which of the following cases
apply. In reading this list, it may help to know that in UCS-4, '<' is
#x0000003C
" and '?' is "
#x0000003F
", and the Byte
Order Mark required of UTF-16 data streams is "
#xFEFF
". The notation
##
is used to denote any byte value except that two consecutive
##
cannot be both 00.
With a Byte Order Mark:
00 00 FE FF
UCS-4, big-endian machine (1234 order)
FF FE 00 00
UCS-4, little-endian machine (4321 order)
00 00 FF FE
UCS-4, unusual octet order (2143)
FE FF 00 00
UCS-4, unusual octet order (3412)
FE FF ## ##
UTF-16, big-endian
FF FE ## ##
UTF-16, little-endian
EF BB BF
UTF-8
Without a Byte Order Mark:
00 00 00 3C
UCS-4 or other encoding with a 32-bit code unit
and ASCII characters encoded as ASCII values, in respectively big-endian (1234),
little-endian (4321) and two unusual byte orders (2143 and 3412). The encoding
declaration must be read to determine which of UCS-4 or other supported 32-bit
encodings applies.
3C 00 00 00
00 00 3C 00
00 3C 00 00
00 3C 00 3F
UTF-16BE or big-endian ISO-10646-UCS-2 or other
encoding with a 16-bit code unit in big-endian order and ASCII characters
encoded as ASCII values (the encoding declaration must be read to determine
which)
3C 00 3F 00
UTF-16LE or little-endian ISO-10646-UCS-2 or other
encoding with a 16-bit code unit in little-endian order and ASCII characters
encoded as ASCII values (the encoding declaration must be read to determine
which)
3C 3F 78 6D
UTF-8, ISO 646, ASCII, some part of ISO 8859,
Shift-JIS, EUC, or any other 7-bit, 8-bit, or mixed-width encoding which ensures
that the characters of ASCII have their normal positions, width, and values;
the actual encoding declaration must be read to detect which of these applies,
but since all of these encodings use the same bit patterns for the relevant
ASCII characters, the encoding declaration itself may be read reliably
4C 6F A7 94
EBCDIC (in some flavor; the full encoding declaration
must be read to tell which code page is in use)
Other
UTF-8 without an encoding declaration, or else
the data stream is mislabeled (lacking a required encoding declaration), corrupt,
fragmentary, or enclosed in a wrapper of some kind
Note:
In
cases above which do not require reading the encoding declaration to determine
the encoding, section 4.3.3 still requires that the encoding declaration,
if present, be read and that the encoding name be checked to match the actual
encoding of the entity. Also, it is possible that new character encodings
will be invented that will make it necessary to use the encoding declaration
to determine the encoding, in cases where this is not required at present.
This
level of autodetection is enough to read the XML encoding declaration and
parse the character-encoding identifier, which is still necessary to distinguish
the individual members of each family of encodings (e.g. to tell UTF-8 from
8859, and the parts of 8859 from each other, or to distinguish the specific
EBCDIC code page in use, and so on).
Because the contents of the encoding
declaration are restricted to characters from the ASCII repertoire (however
encoded), a processor can reliably read the entire encoding declaration as
soon as it has detected which family of encodings is in use. Since in practice,
all widely used character encodings fall into one of the categories above,
the XML encoding declaration allows reasonably reliable in-band labeling of
character encodings, even when external sources of information at the operating-system
or transport-protocol level are unreliable. Character encodings such as UTF-7
that make overloaded usage of ASCII-valued bytes may fail to be reliably detected.
Once
the processor has detected the character encoding in use, it can act appropriately,
whether by invoking a separate input routine for each case, or by calling
the proper conversion function on each character of input.
Like any
self-labeling system, the XML encoding declaration will not work if any software
changes the entity's character set or encoding without updating the encoding
declaration. Implementors of character-encoding routines should be careful
to ensure the accuracy of the internal and external information used to label
the entity.
F.2
Priorities in the Presence of External Encoding Information
The second
possible case occurs when the XML entity is accompanied by encoding information,
as in some file systems and some network protocols. When multiple sources
of information are available, their relative priority and the preferred method
of handling conflict should be specified as part of the higher-level protocol
used to deliver XML. In particular, please refer to
[IETF
RFC 2376]
or its successor, which defines the
text/xml
and
application/xml
MIME types and provides some useful guidance. In the interests of interoperability,
however, the following rule is recommended.
If an XML entity is in a file, the Byte-Order Mark and encoding declaration
are used (if present) to determine the character encoding.
G W3C
XML Working Group (Non-Normative)
This specification was prepared
and approved for publication by the W3C XML Working Group (WG). WG approval
of this specification does not necessarily imply that all WG members voted
for its approval. The current and former members of the XML WG are:
Jon Bosak, Sun (
Chair
James Clark (
Technical Lead
Tim Bray, Textuality and Netscape (
XML Co-editor
Jean Paoli, Microsoft (
XML Co-editor
C. M. Sperberg-McQueen, U. of Ill. (
XML Co-editor
Dan Connolly, W3C (
W3C Liaison
Paula Angerstein, Texcel
Steve DeRose, INSO
Dave Hollander, HP
Eliot Kimber, ISOGEN
Eve Maler, ArborText
Tom Magliery, NCSA
Murray Maloney, SoftQuad, Grif SA, Muzmo and Veo Systems
MURATA Makoto (FAMILY Given), Fuji Xerox Information Systems
Joel Nava, Adobe
Conleth O'Connell, Vignette
Peter Sharpe, SoftQuad
John Tigue, DataChannel
H W3C XML Core
Group (Non-Normative)
The second edition of this specification was
prepared by the W3C XML Core Working Group (WG). The members of the WG at
the time of publication of this edition were:
Paula Angerstein, Vignette
Daniel Austin, Ask Jeeves
Tim Boland
Allen Brown, Microsoft
Dan Connolly, W3C (
Staff Contact
John Cowan, Reuters Limited
John Evdemon, XMLSolutions Corporation
Paul Grosso, Arbortext (
Co-Chair
Arnaud Le Hors, IBM (
Co-Chair
Eve Maler, Sun Microsystems (
Second Edition Editor
Jonathan Marsh, Microsoft
MURATA Makoto (FAMILY Given), IBM
Mark Needleman, Data Research Associates
David Orchard, Jamcracker
Lew Shannon, NCR
Richard Tobin, University of Edinburgh
Daniel Veillard, W3C
Dan Vint, Lexica
Norman Walsh, Sun Microsystems
François Yergeau, Alis Technologies (
Errata List Editor
Kongyi Zhou, Oracle
I Production
Notes (Non-Normative)
This Second Edition was encoded in the
XMLspec
DTD
(which has
documentation
available). The HTML versions were produced with a combination of the
xmlspec.xsl
diffspec.xsl
, and
REC-xml-2e.xsl
XSLT
stylesheets. The PDF version was produced with the
html2ps
facility and a distiller program.