Extensible Markup Language (XML) 1.0 (Third Edition)
Extensible Markup Language (XML) 1.0 (Third Edition)
W3C Recommendation 04 February 2004
This version:
Latest version:
Previous version:
Editors:
Tim Bray, Textuality and Netscape
Jean Paoli, Microsoft
C. M. Sperberg-McQueen, W3C
Eve Maler, Sun Microsystems, Inc.
- Second Edition
François Yergeau
- Third Edition
Please refer to the
errata
for this document, which may include some normative corrections.
This document is also available in these non-normative formats:
XML
and
XHTML with color-coded revision indicators
See also
translations
W3C
MIT
ERCIM
Keio
), All Rights Reserved. W3C
liability
trademark
document use
and
software licensing
rules apply.
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 section describes the status of this document at the time of its publication.
Other documents may supersede this document. A list of current W3C publications and the
latest revision of this technical report can be found in the
W3C technical reports index
at
This document is a
Recommendation
of the W3C. It 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
XML Core Working Group
as part of the
XML Activity
The English version of this specification is the only normative version. However,
for translations of this document, see
This third edition is
not
a new version of XML. As a convenience to readers,
it incorporates the changes dictated by the accumulated errata (available at
) to the
Second Edition of XML 1.0, dated 6
October 2000
. In addition, markup has been introduced on a significant portion of
the prescriptions of the specification, clarifying when prescriptive keywords such as
MUST
SHOULD
and
MAY
are used
in the formal sense defined in
[IETF RFC 2119]
. For the convenience of readers,
an
XHTML version with color-coded revision indicators
is
also provided; this version highlights each change due to an erratum published in the
errata list
, together with a link to the particular
erratum in that list. Most of the errata in the list provide a rationale for the change.
An implementation report is available at
Documentation of intellectual property possibly relevant to this recommendation
may be found at the Working Group's public
IPR disclosure page
Please report errors in this document to
xml-editor@w3.org
archives
are available.
The errata list for this third edition is available at
Test Suite
is maintained to help assessing conformance to this specification.
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
4.4.9
Error
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 Working 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 Working Group'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
[Unicode]
and ISO/IEC 10646
[ISO/IEC 10646]
for characters, Internet RFC
3066
[IETF RFC 3066]
for language identification tags, ISO 639
[ISO 639]
for language name codes, and ISO 3166
[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 key words
MUST
MUST NOT
REQUIRED
SHALL
SHALL NOT
SHOULD
SHOULD NOT
RECOMMENDED
MAY
, and
OPTIONAL
, when
EMPHASIZED
, are to be interpreted as described in
[IETF RFC 2119]
. In addition,
the terms defined in the following list are used in building
those definitions and in describing the actions of an XML processor:
error
Definition
: A violation of the rules of this specification;
results are undefined.
Unless otherwise specified, failure to observe a prescription of this specification indicated by one of the keywords
MUST
REQUIRED
MUST NOT
SHALL
and
SHALL NOT
is an error.
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.]
2 Documents
Definition
: A data object is an
XML
document
if it is
well-formed
as defined in this specification. A well-formed XML document
MAY
in addition
be
valid
if it meets certain further constraints.]
Each XML document has both a logical and a physical structure. Physically,
the document is composed of units called
entities
An entity
MAY
refer
to other entities to
cause their inclusion in the document. A document begins in a "root"
or
document entity
. Logically, the document
is composed of declarations, elements, comments, character references, and
processing instructions, all of which are indicated in the document by explicit
markup. The logical and physical structures
MUST
nest properly, as described
in
4.3.2 Well-Formed Parsed Entities
2.1 Well-Formed XML Documents
Definition
: A textual object is a
well-formed
XML document if:]
Taken as a whole, it matches the production labeled
document
It meets all the well-formedness constraints given in this specification.
Each of the
parsed entities
which is referenced directly or indirectly within the document is
well-formed
Document
[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
child
of
.]
2.2 Characters
Definition
: A parsed entity contains
text
a sequence of
characters
, which may
represent markup or character data.] [
Definition
: A
character
is an atomic unit of text as specified by
ISO/IEC 10646:2000
[ISO/IEC 10646]
. 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
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
Unicode 3.1
[Unicode3]
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
Note:
Document authors are encouraged to avoid
"compatibility characters", as defined
in section 6.8 of
[Unicode]
(see also D21 in section 3.6 of
[Unicode3]
). The characters defined in the following ranges are also
discouraged. They are either control characters or permanently undefined Unicode
characters:
[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDDF],
[#1FFFE-#x1FFFF], [#2FFFE-#x2FFFF], [#3FFFE-#x3FFFF],
[#4FFFE-#x4FFFF], [#5FFFE-#x5FFFF], [#6FFFE-#x6FFFF],
[#7FFFE-#x7FFFF], [#8FFFE-#x8FFFF], [#9FFFE-#x9FFFF],
[#AFFFE-#xAFFFF], [#BFFFE-#xBFFFF], [#CFFFE-#xCFFFF],
[#DFFFE-#xDFFFF], [#EFFFE-#xEFFFF], [#FFFFE-#xFFFFF],
[#10FFFE-#x10FFFF].
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)+
Note:
The presence of #xD in the above production is
maintained purely for backward compatibility with the
First Edition
As explained in
2.11 End-of-Line Handling
all #xD characters literally present in an XML document
are either removed or replaced by #xA characters before
any other processing is done. The only way to get a #xD character to match this production is to
use a character reference in an entity value literal.
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
with
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
(#x20
Name
)*
[7]
Nmtoken
::=
NameChar
)+
[8]
Nmtokens
::=
Nmtoken
(#x20
Nmtoken
)*
Note:
The
Names
and
Nmtokens
productions are used to define the validity
of tokenized attribute values after normalization (see
3.3.1 Attribute Types
).
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 a
general
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 (<)
MUST NOT
appear
in their literal form
, except
when used as markup delimiters, or
within a
comment
, a
processing
instruction
, or a
CDATA section
If they are needed elsewhere, they
MUST
be
escaped
using either
numeric character references
or the strings "
&
" and "
<
respectively. The right angle bracket (>)
MAY
be represented using the string "
>
",
and
MUST
for compatibility
, be escaped
using
either
>
" 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
and does not include the CDATA-section-close
delimiter, "
]]>
. 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
MUST NOT
be
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
MUST NOT
be
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 function of the markup in an XML document is to describe its storage and
logical structure and to associate
attribute
name-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
::=
'1.0'
[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
: A
markup declaration
is an
element type declaration
, an
attribute-list declaration
, an
entity
declaration
, or a
notation declaration
.]
These declarations
MAY
be contained in whole or in part within
parameter
entities
, as described in the well-formedness and validity constraints
below. For further
information, see
4 Physical Structures
Document Type Definition
[28]
doctypedecl
::=
'Name
ExternalID
)?
('['
intSubset
']'
?)? '>'
[VC: Root Element Type]
[WFC: External Subset]
[28a]
DeclSep
::=
PEReference
[WFC: PE Between Declarations]
[28b]
intSubset
::=
markupdecl
DeclSep
)*
[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
MUST NOT
occur within markup declarations; they
MAY
occur where markup declarations can occur
(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
but is
allowed in external parameter entities referenced 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
MUST
be
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
tokenized types, where the
attribute appears in the document with a value such that
normalization
will produce a different value from that which would be produced
in the absence of the declaration, 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.
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
overridden
with
another instance of the
xml:space
attribute.
This specification does not give meaning to any value of
xml:space
other than "default" and "preserve". It is an error for other values to be specified; the XML processor
MAY
report the error or
MAY
recover by ignoring the attribute specification or by reporting the (erroneous) value to the application. Applications may ignore or reject erroneous values.
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
XML
processor
MUST
behave as if it
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 3066]
Tags
for the Identification of Languages
, or its successor
; in addition, the empty string
MAY
be specified
(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.
In particular, the empty value of
xml:lang
is used on an element B to override a specification of
xml:lang
on an enclosing element A, without specifying another language. Within B, it is considered that there is no language information available, just as if
xml:lang
had not been specified on B or any of its ancestors.
Note:
Language information may also be provided by external transport protocols (e.g. HTTP or
MIME). When available, this information may be used by XML applications, but the more local
information provided by
xml:lang
should be considered to override it.
A simple declaration for
xml:lang
might take the form
xml:lang
CDATA
#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:
CDATA
'fr'>
CDATA
'en'>
CDATA
'en'>
3 Logical Structures
Definition
: Each
XML
document
contains one or more
elements
, the boundaries
of which are either delimited by
start-tags
and
end-tags
, or, for
empty
elements, by an
empty-element tag
. Each
element has a type, identified by name, sometimes called its "generic
identifier" (GI), and
MAY
have a set of attribute specifications.]
Each attribute specification has a
name
and a
value
Element
[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
(not even entity
references, comments, PIs or white space)
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
, comments and
PIs (i.e. markup matching production [27]
Misc
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
or a reference
to an entity whose replacement text is character references expanding to white
space
do
not
match the nonterminal
, and
hence cannot appear in these positions
; however, a
reference to an internal entity with a literal value consisting of character
references expanding to white space does match
, since its
replacement text is the white space resulting from expansion of the character
references
The declaration matches
Mixed
and the content
(after replacing
any entity references with their replacement text)
consists of
character data
comments
PIs
and
child elements
whose types match names in the
content model.
The declaration matches
ANY
, and the
content
(after replacing
any entity references with their replacement text)
consists of character data and
child elements
whose types
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
An attribute name
MUST NOT
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
MUST NOT
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
An element
type
MUST NOT
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 a
content
model
, a simple grammar governing the allowed types of the
child elements and the order in which they are allowed to appear.]
The grammar is built on content particles (
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
the content model
allows an element to 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
MUST NOT
appear outside of
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
3.3.3 Attribute-Value Normalization
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
An element
type
MUST NOT
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
MUST
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]
[VC: No Duplicate
Tokens]
[59]
Enumeration
::=
'('
Nmtoken
? '|'
Nmtoken
)*
? ')'
[VC: Enumeration]
[VC: No Duplicate
Tokens]
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
An element type
MUST NOT
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: No Duplicate
Tokens
The notation names in a single
NotationType
attribute declaration, as well as the
NmToken
s in a single
Enumeration
attribute declaration,
MUST
all be distinct.
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
is
to
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 Value Syntactically Correct]
[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.
When an XML processor encounters
an
element
without a specification for an attribute for which it has read a default
value declaration, it
MUST
report the attribute with the declared default
value to the application
.]
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 Value Syntactically Correct
The declared default value
MUST
meet the
syntactic
constraints of the declared attribute type.
Note that only the
syntactic constraints of the type are required here; other constraints (e.g.
that the value be the name of a declared unparsed entity, for an attribute of
type ENTITY) may come into play if the declared default value is actually used
(an element without a specification for this attribute occurs).
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
It
is an error if an
attribute
value
contains a
reference
to an
entity for which no declaration has been read.
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
#x20 B #x20 #x20
a=
"
A
B
"
#xD #xD A #xA #xA B #xD #xA
#xD #xD A #xA #xA B #xD #xA
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
or
of external parameter entities
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
MUST
be considered
part of the DTD. If the keyword of
the conditional section is
IGNORE
, then the contents of the conditional
section
MUST
be considered as
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
MUST
be
ignored. The contents
of an ignored conditional section
MUST
be
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
MUST NOT
be
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:
]]>
]]>
4 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
The contents of a
parsed
entity
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 non-validating processors are
not
obligated to
to read and process entity declarations occurring in parameter entities or in
the external subset
; 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
MUST NOT
appear outside
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
meant to be
converted to
a URI reference
(as defined in
[IETF RFC 2396]
, updated by
[IETF RFC 2732]
),
as part of the
process of dereferencing it
to obtain input for the XML processor to construct the
entity's replacement text.] It is an error for a fragment identifier
(beginning with a
character) to be part of a system identifier.
Unless otherwise provided by information outside the scope of this specification
(e.g. a special XML element type defined by a particular DTD, or a processing
instruction defined by a particular application specification), relative URIs
are relative to the location of the resource within which the entity declaration
occurs.
This is defined to
be the external entity containing the '<' which starts the declaration, at the
point when it is parsed as a declaration.
A URI might thus be relative to the
document
entity
, to the entity containing the
external
DTD subset
, or to some other
external parameter
entity
Attempts to
retrieve the resource identified by a URI
MAY
be redirected at the parser
level (for example, in an entity resolver) or below (at the protocol level,
for example, via an HTTP
Location:
header). In the absence of additional
information outside the scope of this specification within the resource,
the base URI of a resource is always the URI of the actual resource returned.
In other words, it is the URI of the resource retrieved after all redirection
has occurred.
System
identifiers (and other XML strings meant to be used as URI references)
MAY
contain
characters that, according to
[IETF RFC 2396]
and
[IETF RFC 2732]
must be escaped before a URI can be used to retrieve the referenced resource. The
characters to be escaped are the control characters #x0 to #x1F and #x7F (most of
which cannot appear in XML), space #x20, the delimiters '<' #x3C, '>' #x3E and
'"' #x22, the
unwise
characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and
'`' #x60, as well as all characters above #x7F. Since escaping is not always a fully
reversible process, it
MUST
be performed only when absolutely necessary and as late
as possible in a processing chain. In particular, neither the process of converting
a relative URI to an absolute one nor the process of passing a URI reference to a
process or software component responsible for dereferencing it
SHOULD
trigger escaping.
When escaping does occur, it
MUST
be performed as follows:
Each
character
to be escaped
is
represented in
UTF-8
[Unicode3]
as one or more bytes.
The resulting bytes
are escaped with
the URI escaping mechanism (that is, converted to
HH
where HH is the hexadecimal notation of the byte value).
The original character is replaced by the resulting character sequence.
Definition
: In addition to a system
identifier, an external identifier
MAY
include a
public identifier
.]
An XML processor attempting to retrieve the entity's content
MAY
use
any combination of
the public and system identifiers as well as additional information outside the
scope of this specification
to try to generate an alternative URI reference.
If the processor is unable to do so, it
MUST
use the URI
reference specified in the system literal. Before a match is attempted,
all strings of white space in the public identifier
MUST
be normalized to
single space characters (#x20), and leading and trailing white space
MUST
be removed.
Examples of external entity declarations:
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 a
text declaration
Text Declaration
[77]
TextDecl
::=
'VersionInfo
EncodingDecl
? '?>'
The text declaration
MUST
be provided literally, not by reference
to a parsed entity.
The
text declaration
MUST NOT
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
general
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
and entities
encoded in UTF-8
MAY
begin with the Byte Order Mark described by
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
fatal 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
byte
sequences that are not legal in that encoding.
Specifically, it is a
fatal error if an entity encoded in UTF-8 contains any irregular code unit sequences,
as defined in Unicode 3.1
[Unicode3]
Unless an encoding
is determined by a higher-level protocol,
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
Error
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
, except in the
EntityValue
in an entity declaration
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
MUST
be
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
MUST
always be
treated as a normal data
character and
MUST NOT
terminate the literal. For example, this is well-formed:
while this is not:
For an
internal entity,
the
literal
entity value
is the quoted string actually present in the entity declaration,
corresponding to the non-terminal
EntityValue
.] [
Definition
: For an external entity, the
literal
entity value
is the exact text contained in the entity.] [
Definition
For an
internal entity,
the
replacement text
is the content of the entity, after replacement of character references and
parameter-entity references.] [
Definition
: For
an external entity, the
replacement text
is the content of the entity,
after stripping the text declaration (leaving any surrounding whitespace) if there
is one but without any replacement of character references or 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
(or
included in literal
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
D Expansion of Entity and Character References
4.6 Predefined Entities
Definition
: Entity and character references
MAY
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
OPTIONAL
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
A given
Name
MUST NOT
be declared in more than one notation declaration.
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.
5 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
; when
standalone="yes"
, processors
MUST
process these declarations
Note
that when processing invalid documents with a non-validating
processor the application may not be presented with consistent
information. For example, several requirements for uniqueness
within the document may not be met, including more than one element
with the same id, duplicate declarations of elements or notations
with the same name, etc. In these cases the behavior of the parser
with respect to reporting such information to the application is
undefined.
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 fail to
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 fail to
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
DTD
facilities
not related to validation (
such
as the
declaration
of default attributes
and
internal entities
that are or may be specified
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 lowercase 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
whose
number
(code point) in
ISO/IEC 10646
is
. The number of leading zeros in the
#xN
form is insignificant.
[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
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.
A References
A.1 Normative References
IANA-CHARSETS
(Internet
Assigned Numbers Authority)
Official Names for Character Sets
ed. Keld Simonsen et al. (See http://www.iana.org/assignments/character-sets.)
IETF RFC 2119
IETF
(Internet Engineering Task Force).
RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
Scott Bradner, 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)
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 http://www.ietf.org/rfc/rfc2396.txt.)
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 http://www.ietf.org/rfc/rfc2732.txt.)
IETF RFC 3066
IETF
(Internet Engineering Task Force).
RFC 3066: Tags for the Identification
of Languages
, ed. H. Alvestrand. 2001. (See http://www.ietf.org/rfc/rfc3066.txt.)
ISO/IEC 10646
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
and
ISO/IEC 10646-2:2001.
Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 2:
Supplementary Planes
, as, from time to time, amended, replaced by a new edition or
expanded by the addition of new parts. [Geneva]: International Organization for Standardization.
(See
for the latest version.)
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.
, defined by:
The Unicode Standard, Version 3.0
(Reading, MA,
Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the
Unicode
Standard Annex #27: Unicode 3.1
and the
Unicode Standard Annex #28: Unicode
3.2
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.
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 http://www.w3.org/TR/NOTE-sgml-xml-971215.)
IANA-LANGCODES
(Internet
Assigned Numbers Authority)
Registry of Language Tags
ed. Keld Simonsen et al. (See http://www.iana.org/assignments/language-tags.)
IETF RFC 2141
IETF
(Internet Engineering Task Force).
RFC 2141: URN Syntax
, ed.
R. Moats. 1997. (See http://www.ietf.org/rfc/rfc2141.txt.)
IETF RFC 3023
IETF
(Internet Engineering Task Force).
RFC 3023: XML Media Types
eds. M. Murata, S. St.Laurent, D. Kohn. 2001. (See http://www.ietf.org/rfc/rfc3023.txt.)
IETF RFC 2781
IETF
(Internet Engineering Task Force).
RFC 2781: UTF-16, an encoding
of ISO 10646
, ed. P. Hoffman, F. Yergeau. 2000. (See http://www.ietf.org/rfc/rfc2781.txt.)
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 http://www.sgmlsource.com/8879/n0029.htm.)
XML Names
Tim Bray,
Dave Hollander, and Andrew Layman, editors.
Namespaces in XML
Textuality, Hewlett-Packard, and Microsoft. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/REC-xml-names/.)
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
##
s 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 3023]
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
participants
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
Working
Group (Non-Normative)
The
third
edition of this specification was prepared by the W3C XML Core
Working Group (WG). The
participants in
the WG at the time of publication of this
edition were:
Leonid Arbouzov, Sun Microsystems
Mary Brady
John Cowan
John Evdemon, Microsoft
Andrew Fang, Arbortext
Paul Grosso, Arbortext (
Co-Chair
Arnaud Le Hors, IBM
Dmitry Lenkov, Oracle
Anjana Manian, Oracle
Glenn Marcy, IBM
Jonathan Marsh, Microsoft
Sandra Martinez, NIST
Liam Quin, W3C (
Staff Contact
Lew Shannon
Richard Tobin, University of Edinburgh
Daniel Veillard
Norman Walsh, Sun Microsystems (
Co-Chair
François Yergeau (
Third Edition Editor
I Production Notes (Non-Normative)
This Third Edition was encoded in a
slightly modified version of the
XMLspec DTD, v2.5
The XHTML versions were produced with a combination of the
xmlspec.xsl
diffspec.xsl
and
REC-xml-3e.xsl
XSLT stylesheets.