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Abstract
The Extensible Markup Language (XML) is a subset of SGML that is completely
described in this document. Its goal is to enable generic SGML to be served,
received, and processed on the Web in the way that is now possible with HTML.
XML has been designed for ease of implementation and for interoperability with
both SGML and HTML.
Status of this document
This document has been reviewed by W3C Members and other interested parties
and has been endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited as a normative reference
from another document. W3C's role in making the Recommendation is to draw
attention to the specification and to promote its widespread deployment. This
enhances the functionality and interoperability of the Web.
This document specifies a syntax created by subsetting an existing, widely
used international text processing standard (Standard Generalized Markup
Language, ISO 8879:1986(E) as amended and corrected) for use on the World
Wide Web. It is a product of the W3C XML Activity, details of which can
be found at http://www.w3.org/XML.
A list of current W3C Recommendations and other technical documents can
be found at http://www.w3.org/TR.
This specification uses the term URI, which is defined by [Berners-Lee et
al.], a work in progress expected to update [IETF RFC1738] and
[IETF RFC1808].
The list of known errors in this specification is available at http://www.w3.org/XML/xml-19980210-errata.
Please report errors in this document to xml-editor@w3.org.
Extensible Markup Language (XML) 1.0
Table of Contents1. Introduction 1.1
Origin and
Goals 1.2 Terminology 2.
Documents 2.1
Well-Formed
XML Documents 2.2 Characters 2.3
Common
Syntactic Constructs 2.4 Character Data and
Markup 2.5 Comments 2.6
Processing
Instructions 2.7 CDATA
Sections 2.8 Prolog and
Document Type Declaration 2.9 Standalone Document
Declaration 2.10 White Space
Handling 2.11 End-of-Line
Handling 2.12 Language
Identification 3. Logical
Structures 3.1 Start-Tags,
End-Tags, and Empty-Element Tags 3.2 Element Type
Declarations 3.2.1 Element
Content 3.2.2 Mixed
Content 3.3 Attribute-List
Declarations 3.3.1 Attribute
Types 3.3.2 Attribute
Defaults 3.3.3 Attribute-Value
Normalization 3.4 Conditional
Sections 4. Physical
Structures 4.1 Character and
Entity References 4.2 Entity
Declarations 4.2.1 Internal
Entities 4.2.2 External
Entities 4.3 Parsed
Entities 4.3.1 The Text
Declaration 4.3.2 Well-Formed Parsed
Entities 4.3.3 Character
Encoding in Entities 4.4 XML Processor
Treatment of Entities and
References 4.4.1 Not
Recognized 4.4.2 Included 4.4.3
Included
If Validating 4.4.4 Forbidden 4.4.5
Included in
Literal 4.4.6 Notify 4.4.7
Bypassed 4.4.8
Included as
PE 4.5 Construction
of Internal Entity Replacement Text 4.6 Predefined
Entities 4.7 Notation
Declarations 4.8 Document
Entity 5. Conformance 5.1
Validating and
Non-Validating Processors 5.2 Using XML
Processors 6. Notation
AppendicesA. References A.1
Normative
References A.2 Other
References B. Character
Classes C. XML and SGML
(Non-Normative) D. Expansion of
Entity and Character References (Non-Normative) E. Deterministic
Content Models (Non-Normative) F. Autodetection of
Character Encodings (Non-Normative) G. W3C XML Working
Group (Non-Normative)
Extensible Markup Language, abbreviated XML, describes a class of data
objects called XML documents
and partially describes the behavior of computer programs which process them.
XML is an application profile or restricted form of SGML, the Standard
Generalized Markup Language [ISO 8879]. By
construction, XML documents are conforming SGML documents.
XML documents are made up of storage units called entities, which
contain either parsed or unparsed data. Parsed data is made up of characters,
some of which form character
data, and some of which form markup. Markup
encodes a description of the document's storage layout and logical structure.
XML provides a mechanism to impose constraints on the storage layout and logical
structure.
A software module called an XML processor is
used to read XML documents and provide access to their content and structure. It is assumed that an XML processor is doing its work on behalf
of another module, called the application. This specification describes
the required behavior of an XML processor in terms of how it must read XML data
and the information it must provide to the application.
XML was developed by an XML Working Group (originally known as the SGML
Editorial Review Board) formed under the auspices of the World Wide Web
Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with
the active participation of an XML Special Interest Group (previously known as
the SGML Working Group) also organized by the W3C. The membership of the XML
Working Group is given in an appendix. Dan Connolly served as the WG's contact
with the W3C.
The design goals for XML are:
- XML shall be straightforwardly usable over the Internet.
- XML shall support a wide variety of applications.
- XML shall be compatible with SGML.
- It shall be easy to write programs which process XML documents.
- The number of optional features in XML is to be kept to the absolute
minimum, ideally zero.
- XML documents should be human-legible and reasonably clear.
- The XML design should be prepared quickly.
- The design of XML shall be formal and concise.
- XML documents shall be easy to create.
- Terseness in XML markup is of minimal importance.
This specification, together with associated standards (Unicode and ISO/IEC
10646 for characters, Internet RFC 1766 for language identification tags, ISO
639 for language name codes, and ISO 3166 for country name codes), provides all
the information necessary to understand XML Version 1.0 and construct computer
programs to process it.
This version of the XML specification may be distributed freely, as long as
all text and legal notices remain intact.
The terminology used to describe XML documents is defined in the body of this
specification. The terms defined in the following list are used in building
those definitions and in describing the actions of an XML processor:
- may
- Conforming documents and XML processors are permitted
to but need not behave as described.
- must
- Conforming documents and XML processors are required to behave as
described; otherwise they are in error.
- error
- A violation of the rules of this specification;
results are undefined. Conforming software may detect and report an error and
may recover from it.
- fatal error
- An error which a conforming XML
processor must detect and report to the application. After encountering a
fatal error, the processor may continue processing the data to search for
further errors and may report such errors to the application. In order to
support correction of errors, the processor may make unprocessed data from the
document (with intermingled character data and markup) available to the
application. Once a fatal error is detected, however, the processor must not
continue normal processing (i.e., it must not continue to pass character data
and information about the document's logical structure to the application in
the normal way).
- at user option
- Conforming software may or must (depending on the modal verb in the
sentence) behave as described; if it does, it must provide users a means to
enable or disable the behavior described.
- validity constraint
- A rule which applies to all valid XML
documents. Violations of validity constraints are errors; they must, at user
option, be reported by validating XML
processors.
- well-formedness constraint
- A rule which applies to all well-formed
XML documents. Violations of well-formedness constraints are fatal errors.
- match
- (Of strings or names:) Two strings or names being
compared must be identical. Characters with multiple possible representations
in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic
forms) match only if they have the same representation in both strings. At
user option, processors may normalize such characters to some canonical form.
No case folding is performed. (Of strings and rules in the grammar:) A string
matches a grammatical production if it belongs to the language generated by
that production. (Of content and content models:) An element matches its
declaration when it conforms in the fashion described in the constraint "Element
Valid".
- for compatibility
- A feature of XML included solely to ensure that XML
remains compatible with SGML.
- for interoperability
- A non-binding recommendation included to increase
the chances that XML documents can be processed by the existing installed base
of SGML processors which predate the WebSGML Adaptations Annex to ISO 8879.
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".
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.
Matching the document
production implies that:
- It contains one or more elements.
- There is exactly one element, called the root,
or document element, no part of which appears in the content of
any other element. For all other elements, if the start-tag is in the content
of another element, the end-tag is in the content of the same element. More
simply stated, the elements, delimited by start- and end-tags, nest properly
within each other.
As a consequence of this, for each non-root
element C in the document, there is one other element
P in the document such that C is in the content of
P , but is not in the content of any other element that is in the
content of P . P is referred to as the parent of
C , and C as a child of P .
A parsed entity contains text, a sequence of characters,
which may represent markup or character data. A
character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646].
Legal characters are tab, carriage return, line feed, and the legal graphic
characters of Unicode and ISO/IEC 10646. The use of "compatibility characters",
as defined in section 6.8 of [Unicode], is
discouraged.
Character Range |
[2] |
Char |
::= |
#x9 | #xA | #xD
| [#x20-#xD7FF] | [#xE000-#xFFFD]
| [#x10000-#x10FFFF] |
/* |
any Unicode character, excluding
the surrogate blocks, FFFE, and FFFF.
*/ | |
The mechanism for encoding character code points into bit patterns may vary
from entity to entity. All XML processors must accept the UTF-8 and UTF-16
encodings of 10646; the mechanisms for signaling which of the two is in use, or
for bringing other encodings into play, are discussed later, in "4.3.3 Character
Encoding in Entities".
This section defines some symbols used widely in the grammar.
S
(white space) consists of one or more space (#x20) characters, carriage returns,
line feeds, or tabs.
White Space |
[3] |
S |
::= |
(#x20 | #x9 | #xD
| #xA)+ | |
Characters are classified for convenience as letters, digits, or other
characters. Letters consist of an alphabetic or syllabic base character possibly
followed by one or more combining characters, or of an ideographic character.
Full definitions of the specific characters in each class are given in "B. Character
Classes".
A Name is a token beginning with a letter or one
of a few punctuation characters, and continuing with letters, digits, hyphens,
underscores, colons, or full stops, together known as name characters. Names
beginning with the string "xml ", or any string which would match
(('X'|'x') ('M'|'m') ('L'|'l')) , are reserved for standardization
in this or future versions of this specification.
Note: The colon character within XML names is reserved for
experimentation with name spaces. Its meaning is expected to be standardized at
some future point, at which point those documents using the colon for
experimental purposes may need to be updated. (There is no guarantee that any
name-space mechanism adopted for XML will in fact use the colon as a name-space
delimiter.) In practice, this means that authors should not use the colon in XML
names except as part of name-space experiments, but that XML processors should
accept the colon as a name character.
An Nmtoken
(name token) is any mixture of name characters.
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]
| [-'()+,./:=?;!*#@$_%] | |
Text
consists of intermingled character data
and markup. Markup takes the form of start-tags, end-tags, empty-element
tags, entity
references, character
references, comments, CDATA section
delimiters, document type
declarations, and processing
instructions.
All text that is not markup constitutes the
character data of the document.
The ampersand character (&) and the left angle bracket (<) may appear
in their literal form only when used as markup delimiters, or within a
comment, a
processing
instruction, or a CDATA
section. They are also legal within the literal entity
value of an internal entity declaration; see "4.3.2 Well-Formed
Parsed Entities". If they are needed elsewhere, they must be escaped using
either numeric
character references or the strings "& " and
"< " respectively. The right angle bracket (>) may be
represented using the string "> ", and must, for
compatibility, be escaped using "> " or a character
reference when it appears in the string "]]> " in content, when
that string is not marking the end of a CDATA
section.
In the content of elements, character data is any string of characters which
does not contain the start-delimiter of any markup. In a CDATA section,
character data is any string of characters not including the CDATA-section-close
delimiter, "]]> ".
To allow attribute values to contain both single and double quotes, the
apostrophe or single-quote character (') may be represented as
"' ", and the double-quote character (") as
"" ".
Character Data |
[14] |
CharData |
::= |
[^<&]* - ([^<&]*
']]>'
[^<&]*) | |
Comments may appear anywhere in a document
outside other markup; in
addition, they may appear within the document type declaration at places allowed
by the grammar. They are not part of the document's character
data; an XML processor may, but need not, make it possible for an
application to retrieve the text of comments. For
compatibility, the string "-- " (double-hyphen) must not occur
within comments.
Comments |
[15] |
Comment |
::= |
'<!--' ((Char -
'-') | ('-' (Char -
'-')))*
'-->' | |
An example of a comment:
<!-- declarations for <head> & <body> --> |
Processing instructions (PIs) allow documents to
contain instructions for applications.
Processing Instructions |
[16] |
PI |
::= |
'<?' PITarget
(S (Char*
- (Char*
'?>' Char*)))?
'?>' |
[17] |
PITarget |
::= |
Name -
(('X' | 'x') ('M' | 'm') ('L'
| 'l')) | |
PIs are not part of the document's character
data, but must be passed through to the application. The PI begins with a
target (PITarget )
used to identify the application to which the instruction is directed. The
target names "XML ", "xml ", and so on are reserved for
standardization in this or future versions of this specification. The XML Notation
mechanism may be used for formal declaration of PI targets.
CDATA sections may occur anywhere character
data may occur; they are used to escape blocks of text containing characters
which would otherwise be recognized as markup. CDATA sections begin with the
string "<![CDATA[ " and end with the string
"]]> ":
Within a CDATA section, only the CDEnd
string is recognized as markup, so that left angle brackets and ampersands may
occur in their literal form; they need not (and cannot) be escaped using
"< " and "& ". CDATA sections cannot
nest.
An example of a CDATA section, in which "<greeting> " and
"</greeting> " are recognized as character
data, not markup:
<![CDATA[<greeting>Hello, world!</greeting>]]> |
XML documents may, and should, begin with an XML
declaration which specifies the version of XML being used. For example, the
following is a complete XML document, well-formed
but not valid:
<?xml version="1.0"?> <greeting>Hello, world!</greeting>
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and so is this:
<greeting>Hello, world!</greeting>
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The version number "1.0 " should be used to indicate conformance
to this version of this specification; it is an error for a document to use the
value "1.0 " if it does not conform to this version of this
specification. It is the intent of the XML working group to give later versions
of this specification numbers other than "1.0 ", but this intent
does not indicate a commitment to produce any future versions of XML, nor if any
are produced, to use any particular numbering scheme. Since future versions are
not ruled out, this construct is provided as a means to allow the possibility of
automatic version recognition, should it become necessary. Processors may signal
an error if they receive documents labeled with versions they do not support.
The function of the markup in an XML document is to describe its storage and
logical structure and to associate attribute-value pairs with its logical
structures. XML provides a mechanism, the document type
declaration, to define constraints on the logical structure and to support
the use of predefined storage units. An XML document is
valid if it has an associated document type declaration and if the
document complies with the constraints expressed in it.
The document type declaration must appear before the first element in the
document.
The XML document type declaration contains or
points to markup
declarations that provide a grammar for a class of documents. This grammar
is known as a document type definition, or DTD. The document type
declaration can point to an external subset (a special kind of external entity)
containing markup declarations, or can contain the markup declarations directly
in an internal subset, or can do both. The DTD for a document consists of both
subsets taken together.
A markup declaration is an element type
declaration, an attribute-list
declaration, an entity
declaration, or a notation
declaration. These declarations may be contained in whole or in part within
parameter
entities, as described in the well-formedness and validity constraints
below. For fuller information, see "4. Physical
Structures".
The markup declarations may be made up in whole or in part of the replacement
text of parameter
entities. The productions later in this specification for individual
nonterminals (elementdecl ,
AttlistDecl ,
and so on) describe the declarations after all the parameter entities
have been included.
Validity Constraint: Root Element Type The Name in the
document type declaration must match the element type of the root element.
Validity Constraint: Proper Declaration/PE Nesting Parameter-entity
replacement
text must be properly nested with markup declarations. That is to say, if
either the first character or the last character of a markup declaration
(markupdecl
above) is contained in the replacement text for a parameter-entity
reference, both must be contained in the same replacement text.
Well-Formedness Constraint: PEs in Internal Subset In the internal
DTD subset, parameter-entity
references can occur only where markup declarations can occur, not within
markup declarations. (This does not apply to references that occur in external
parameter entities or to the external subset.)
Like the internal subset, the external subset and any external parameter
entities referred to in the DTD must consist of a series of complete markup
declarations of the types allowed by the non-terminal symbol markupdecl ,
interspersed with white space or parameter-entity
references. However, portions of the contents of the external subset or of
external parameter entities may conditionally be ignored by using the conditional
section construct; this is not allowed in the internal subset.
The external subset and external parameter entities also differ from the
internal subset in that in them, parameter-entity
references are permitted within markup declarations, not only
between markup declarations.
An example of an XML document with a document type declaration:
<?xml version="1.0"?> <!DOCTYPE greeting SYSTEM "hello.dtd"> <greeting>Hello, world!</greeting>
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The system
identifier "hello.dtd " gives the URI of a DTD for the
document.
The declarations can also be given locally, as in this example:
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE greeting [ <!ELEMENT greeting (#PCDATA)> ]> <greeting>Hello, world!</greeting>
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If both the external and internal subsets are used, the internal subset is
considered to occur before the external subset. This has the effect that entity
and attribute-list declarations in the internal subset take precedence over
those in the external subset.
Markup declarations can affect the content of the document, as passed from an
XML
processor to an application; examples are attribute defaults and entity
declarations. The standalone document declaration, which may appear as a
component of the XML declaration, signals whether or not there are such
declarations which appear external to the document entity.
Standalone Document Declaration |
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In a standalone document declaration, the value "yes " indicates
that there are no markup declarations external to the document entity
(either in the DTD external subset, or in an external parameter entity
referenced from the internal subset) which affect the information passed from
the XML processor to the application. The value "no " indicates that
there are or may be such external markup declarations. Note that the standalone
document declaration only denotes the presence of external
declarations; the presence, in a document, of references to external
entities, when those entities are internally declared, does not change
its standalone status.
If there are no external markup declarations, the standalone document
declaration has no meaning. If there are external markup declarations but there
is no standalone document declaration, the value "no " is
assumed.
Any XML document for which standalone="no" holds can be
converted algorithmically to a standalone document, which may be desirable for
some network delivery applications.
Validity Constraint: Standalone Document Declaration The standalone
document declaration must have the value "no " if any external
markup declarations contain declarations of:
- attributes with default
values, if elements to which these attributes apply appear in the document
without specifications of values for these attributes, or
- entities (other than
amp , lt , gt ,
apos , quot ), if references to
those entities appear in the document, or
- attributes with values subject to normalization,
where the attribute appears in the document with a value which will change as
a result of normalization, or
- element types with element
content, if white space occurs directly within any instance of those
types.
An example XML declaration with a standalone document declaration:
<?xml version="1.0" standalone='yes'?> |
In editing XML documents, it is often convenient to use "white space"
(spaces, tabs, and blank lines, denoted by the nonterminal S in this
specification) to set apart the markup for greater readability. Such white space
is typically not intended for inclusion in the delivered version of the
document. On the other hand, "significant" white space that should be preserved
in the delivered version is common, for example in poetry and source code.
An XML
processor must always pass all characters in a document that are not markup
through to the application. A validating XML
processor must also inform the application which of these characters
constitute white space appearing in element
content.
A special attribute named
xml:space may be attached to an element to signal an intention that
in that element, white space should be preserved by applications. In valid
documents, this attribute, like any other, must be declared if it
is used. When declared, it must be given as an enumerated
type whose only possible values are "default " and
"preserve ". For example:
<!ATTLIST poem xml:space (default|preserve) 'preserve'> |
The value "default " signals that applications' default
white-space processing modes are acceptable for this element; the value
"preserve " indicates the intent that applications preserve all the
white space. This declared intent is considered to apply to all elements within
the content of the element where it is specified, unless overriden with another
instance of the xml:space attribute.
The root
element of any document is considered to have signaled no intentions as
regards application space handling, unless it provides a value for this
attribute or the attribute is declared with a default value.
XML parsed
entities are often stored in computer files which, for editing convenience,
are organized into lines. These lines are typically separated by some
combination of the characters carriage-return (#xD) and line-feed (#xA).
To simplify the tasks of applications,
wherever an external parsed entity or the literal entity value of an internal
parsed entity contains either the literal two-character sequence "#xD#xA" or a
standalone literal #xD, an XML processor
must pass to the application the single character #xA. (This behavior can
conveniently be produced by normalizing all line breaks to #xA on input, before
parsing.)
In document processing, it is often useful to identify the natural or formal
language in which the content is written. A special attribute named
xml:lang may be inserted in documents to specify the language used
in the contents and attribute values of any element in an XML document. In valid
documents, this attribute, like any other, must be declared if it
is used. The values of the attribute are language identifiers as defined by [IETF RFC 1766],
"Tags for the Identification of Languages":
Language Identification |
[33] |
LanguageID |
::= |
Langcode
('-' Subcode)* |
[34] |
Langcode |
::= |
ISO639Code
| IanaCode
| UserCode |
[35] |
ISO639Code |
::= |
([a-z] | [A-Z]) ([a-z]
| [A-Z]) |
[36] |
IanaCode |
::= |
('i' | 'I') '-' ([a-z]
| [A-Z])+ |
[37] |
UserCode |
::= |
('x' | 'X') '-' ([a-z]
| [A-Z])+ |
[38] |
Subcode |
::= |
([a-z]
| [A-Z])+ | |
The Langcode
may be any of the following:
- a two-letter language code as defined by [ISO 639], "Codes
for the representation of names of languages"
- a language identifier registered with the Internet Assigned Numbers
Authority [IANA]; these begin
with the prefix "
i- " (or "I- ")
- a language identifier assigned by the user, or agreed on between parties
in private use; these must begin with the prefix "
x- " or
"X- " in order to ensure that they do not conflict with names
later standardized or registered with IANA
There may be any number of Subcode
segments; if the first subcode segment exists and the Subcode consists of two
letters, then it must be a country code from [ISO 3166], "Codes
for the representation of names of countries." If the first subcode consists of
more than two letters, it must be a subcode for the language in question
registered with IANA, unless the Langcode
begins with the prefix "x- " or "X- ".
It is customary to give the language code in lower case, and the country code
(if any) in upper case. Note that these values, unlike other names in XML
documents, are case insensitive.
For example:
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p> <p xml:lang="en-GB">What colour is it?</p> <p xml:lang="en-US">What color is it?</p> <sp who="Faust" desc='leise' xml:lang="de"> <l>Habe nun, ach! Philosophie,</l> <l>Juristerei, und Medizin</l> <l>und leider auch Theologie</l> <l>durchaus studiert mit heißem Bemüh'n.</l> </sp> |
The intent declared with xml:lang is considered to apply to all
attributes and content of the element where it is specified, unless overridden
with an instance of xml:lang on another element within that
content.
A simple declaration for xml:lang might take the form
xml:lang NMTOKEN #IMPLIED |
but specific default values may also be given, if appropriate. In a
collection of French poems for English students, with glosses and notes in
English, the xml:lang attribute might be declared this way:
<!ATTLIST poem xml:lang NMTOKEN 'fr'> <!ATTLIST gloss xml:lang NMTOKEN 'en'> <!ATTLIST note xml:lang NMTOKEN 'en'> |
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.
This specification does not constrain the semantics, use, or (beyond syntax)
names of the element types and attributes, except that names beginning with a
match to (('X'|'x')('M'|'m')('L'|'l')) are reserved for
standardization in this or future versions of this specification.
Well-Formedness Constraint: Element Type Match The Name in an
element's end-tag must match the element type in the start-tag.
Validity Constraint: Element Valid An element is valid if there is
a declaration matching elementdecl
where the Name
matches the element type, and one of the following holds:
- The declaration matches
EMPTY and the element has no content.
- The declaration matches
children
and the sequence of child
elements belongs to the language generated by the regular expression in
the content model, with optional white space (characters matching the
nonterminal S ) between
each pair of child elements.
- The declaration matches
Mixed
and the content consists of character
data and child
elements whose types match names in the content model.
- The declaration matches
ANY , and the types of any child
elements have been declared.
The beginning of every non-empty XML element is marked by
a start-tag.
The Name in the
start- and end-tags gives the element's type. The
Name -AttValue
pairs are referred to as the attribute specifications of the element, with the Name in
each pair referred to as the attribute name and the content of the AttValue
(the text between the ' or " delimiters) as the
attribute value.
Well-Formedness Constraint: Unique Att Spec No attribute name may
appear more than once in the same start-tag or empty-element tag.
Validity Constraint: Attribute Value Type The attribute must have
been declared; the value must be of the type declared for it. (For attribute
types, see "3.3 Attribute-List
Declarations".)
Well-Formedness Constraint: No External Entity References Attribute
values cannot contain direct or indirect entity references to external entities.
Well-Formedness Constraint: No < in Attribute
Values The replacement
text of any entity referred to directly or indirectly in an attribute value
(other than "< ") must not contain a < .
An example of a start-tag:
<termdef id="dt-dog" term="dog"> |
The end of every element that begins with a start-tag
must be marked by an end-tag containing a name that echoes the element's
type as given in the start-tag:
End-tag |
[42] |
ETag |
::= |
'</' Name
S?
'>' | |
An example of an end-tag:
The text between the
start-tag and end-tag is called the element's content:
If an element is empty, it must be represented
either by a start-tag immediately followed by an end-tag or by an empty-element
tag. An empty-element tag takes a special form:
Empty-element tags may be used for any element which has no content, whether
or not it is declared using the keyword EMPTY . For
interoperability, the empty-element tag must be used, and can only be used,
for elements which are declared
EMPTY .
Examples of empty elements:
<IMG align="left" src="http://www.w3.org/Icons/WWW/w3c_home" /> <br></br> <br/> |
The element
structure of an XML document
may, for validation
purposes, be constrained using element type and attribute-list declarations. An
element type declaration constrains the element's content.
Element type declarations often constrain which element types can appear as
children
of the element. At user option, an XML processor may issue a warning when a
declaration mentions an element type for which no declaration is provided, but
this is not an error.
An element type declaration takes the form:
where the Name gives
the element type being declared.
Validity Constraint: Unique Element Type Declaration No element
type may be declared more than once.
Examples of element type declarations:
<!ELEMENT br EMPTY> <!ELEMENT p (#PCDATA|emph)* > <!ELEMENT %name.para; %content.para; > <!ELEMENT container ANY> |
An element type has
element content when elements of that type must contain only child
elements (no character data), optionally separated by white space (characters
matching the nonterminal S ). In this
case, the constraint includes a content model, a simple grammar governing the
allowed types of the child elements and the order in which they are allowed to
appear. The grammar is built on content particles (cp s), which
consist of names, choice lists of content particles, or sequence lists of
content particles:
where each Name is the
type of an element which may appear as a child. Any
content particle in a choice list may appear in the element
content at the location where the choice list appears in the grammar;
content particles occurring in a sequence list must each appear in the element
content in the order given in the list. The optional character following a
name or list governs whether the element or the content particles in the list
may occur one or more (+ ), zero or more (* ), or zero
or one times (? ). The absence of such an operator means that the
element or content particle must appear exactly once. This syntax and meaning
are identical to those used in the productions in this specification.
The content of an element matches a content model if and only if it is
possible to trace out a path through the content model, obeying the sequence,
choice, and repetition operators and matching each element in the content
against an element type in the content model. For
compatibility, it is an error if an element in the document can match more
than one occurrence of an element type in the content model. For more
information, see "E. Deterministic
Content Models".
Validity Constraint: Proper Group/PE Nesting Parameter-entity replacement
text must be properly nested with parenthetized groups. That is to say, if
either of the opening or closing parentheses in a choice ,
seq , or
Mixed
construct is contained in the replacement text for a parameter entity,
both must be contained in the same replacement text. For
interoperability, if a parameter-entity reference appears in a choice ,
seq , or
Mixed
construct, its replacement text should not be empty, and neither the first nor
last non-blank character of the replacement text should be a connector
(| or , ).
Examples of element-content models:
<!ELEMENT spec (front, body, back?)> <!ELEMENT div1 (head, (p | list | note)*, div2*)> <!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*> |
An element type has mixed
content when elements of that type may contain character data, optionally
interspersed with child
elements. In this case, the types of the child elements may be constrained, but
not their order or their number of occurrences:
Mixed-content Declaration |
|
where the Name s give
the types of elements that may appear as children.
Validity Constraint: No Duplicate Types The same name must not
appear more than once in a single mixed-content declaration.
Examples of mixed content declarations:
<!ELEMENT p (#PCDATA|a|ul|b|i|em)*> <!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* > <!ELEMENT b (#PCDATA)> |
Attributes
are used to associate name-value pairs with elements.
Attribute specifications may appear only within start-tags and empty-element
tags; thus, the productions used to recognize them appear in "3.1 Start-Tags,
End-Tags, and Empty-Element Tags". Attribute-list declarations may be used:
- To define the set of attributes pertaining to a given element type.
- To establish type constraints for these attributes.
- To provide default
values for attributes.
Attribute-list declarations specify the name,
data type, and default value (if any) of each attribute associated with a given
element type:
Attribute-list Declaration |
|
The Name in the
AttlistDecl
rule is the type of an element. At user option, an XML processor may issue a
warning if attributes are declared for an element type not itself declared, but
this is not an error. The Name in the
AttDef
rule is the name of the attribute.
When more than one AttlistDecl
is provided for a given element type, the contents of all those provided are
merged. When more than one definition is provided for the same attribute of a
given element type, the first declaration is binding and later declarations are
ignored. For
interoperability, writers of DTDs may choose to provide at most one
attribute-list declaration for a given element type, at most one attribute
definition for a given attribute name, and at least one attribute definition in
each attribute-list declaration. For interoperability, an XML processor may at
user option issue a warning when more than one attribute-list declaration is
provided for a given element type, or more than one attribute definition is
provided for a given attribute, but this is not an error.
XML attribute types are of three kinds: a string type, a set of tokenized
types, and enumerated types. The string type may take any literal string as a
value; the tokenized types have varying lexical and semantic constraints, as
noted:
Validity Constraint: ID Values of type ID must match
the Name
production. A name must not appear more than once in an XML document as a value
of this type; i.e., ID values must uniquely identify the elements which bear
them.
Validity Constraint: One ID per Element Type No element type may
have more than one ID attribute specified.
Validity Constraint: ID Attribute Default An ID attribute must have
a declared default of #IMPLIED or #REQUIRED .
Validity Constraint: IDREF Values of type IDREF must
match the Name
production, and values of type IDREFS must match Names ;
each Name must
match the value of an ID attribute on some element in the XML document; i.e.
IDREF values must match the value of some ID attribute.
Validity Constraint: Entity Name Values of type ENTITY
must match the Name
production, values of type ENTITIES must match Names ;
each Name must
match the name of an unparsed
entity declared in the DTD.
Validity Constraint: Name Token Values of type NMTOKEN
must match the Nmtoken
production; values of type NMTOKENS must match Nmtokens.
Enumerated attributes can take one of a list
of values provided in the declaration. There are two kinds of enumerated types:
Enumerated Attribute Types |
|
A NOTATION attribute identifies a notation,
declared in the DTD with associated system and/or public identifiers, to be used
in interpreting the element to which the attribute is attached.
Validity Constraint: Notation Attributes Values of this type must
match one of the notation names
included in the declaration; all notation names in the declaration must be
declared.
Validity Constraint: Enumeration Values of this type must match one
of the Nmtoken
tokens in the declaration.
For
interoperability, the same Nmtoken
should not occur more than once in the enumerated attribute types of a single
element type.
An attribute
declaration provides information on whether the attribute's presence is
required, and if not, how an XML processor should react if a declared attribute
is absent in a document.
In an attribute declaration, #REQUIRED means that the attribute
must always be provided, #IMPLIED that no default value is
provided. If the declaration is neither
#REQUIRED nor #IMPLIED , then the AttValue
value contains the declared default value; the #FIXED
keyword states that the attribute must always have the default value. If a
default value is declared, when an XML processor encounters an omitted
attribute, it is to behave as though the attribute were present with the
declared default value.
Validity Constraint: Required Attribute If the default declaration
is the keyword #REQUIRED , then the attribute must be specified for
all elements of the type in the attribute-list declaration.
Validity Constraint: Attribute Default Legal The declared default
value must meet the lexical constraints of the declared attribute type.
Validity Constraint: Fixed Attribute Default If an attribute has a
default value declared with the #FIXED keyword, instances of that
attribute must match the default value.
Examples of attribute-list declarations:
<!ATTLIST termdef id ID #REQUIRED name CDATA #IMPLIED> <!ATTLIST list type (bullets|ordered|glossary) "ordered"> <!ATTLIST form method CDATA #FIXED "POST"> |
Before the value of an attribute is passed to the application or checked for
validity, the XML processor must normalize it as follows:
- a character reference is processed by appending the referenced character
to the attribute value
- an entity reference is processed by recursively processing the replacement
text of the entity
- a whitespace character (#x20, #xD, #xA, #x9) is processed by appending
#x20 to the normalized value, except that only a single #x20 is appended for a
"#xD#xA" sequence that is part of an external parsed entity or the literal
entity value of an internal parsed entity
- other characters are processed by appending them to the normalized value
If the declared value is not CDATA, then the XML processor must further
process the normalized attribute value by discarding any leading and trailing
space (#x20) characters, and by replacing sequences of space (#x20) characters
by a single space (#x20) character.
All attributes for which no declaration has been read should be treated by a
non-validating parser as if declared CDATA .
Conditional sections are portions of the
document type
declaration external subset which are included in, or excluded from, the
logical structure of the DTD based on the keyword which governs them.
Like the internal and external DTD subsets, a conditional section may contain
one or more complete declarations, comments, processing instructions, or nested
conditional sections, intermingled with white space.
If the keyword of the conditional section is INCLUDE , then the
contents of the conditional section are part of the DTD. If the keyword of the
conditional section is IGNORE , then the contents of the conditional
section are not logically part of the DTD. Note that for reliable parsing, the
contents of even ignored conditional sections must be read in order to detect
nested conditional sections and ensure that the end of the outermost (ignored)
conditional section is properly detected. If a conditional section with a
keyword of INCLUDE occurs within a larger conditional section with
a keyword of IGNORE , both the outer and the inner conditional
sections are ignored.
If the keyword of the conditional section is a parameter-entity reference,
the parameter entity must be replaced by its content before the processor
decides whether to include or ignore the conditional section.
An example:
<!ENTITY % draft 'INCLUDE' > <!ENTITY % final 'IGNORE' > <![%draft;[ <!ELEMENT book (comments*, title, body, supplements?)> ]]> <![%final;[ <!ELEMENT book (title, body, supplements?)> ]]>
|
An XML document may consist of one or many storage
units. These are called entities; they all have content and are
all (except for the document entity, see below, and the external DTD
subset) identified by name. Each XML document has one entity called
the document
entity, which serves as the starting point for the XML processor
and may contain the whole document.
Entities may be either parsed or unparsed. A
parsed entity's contents are referred to as its replacement
text; this text is considered
an integral part of the document.
An unparsed entity is a resource whose
contents may or may not be text, and if text,
may not be XML. Each unparsed entity has an associated notation,
identified by name. Beyond a requirement that an XML processor make the
identifiers for the entity and notation available to the application, XML places
no constraints on the contents of unparsed entities.
Parsed entities are invoked by name using entity references; unparsed
entities by name, given in the value of ENTITY or
ENTITIES attributes.
General entities are entities for use within
the document content. In this specification, general entities are sometimes
referred to with the unqualified term entity when this leads to no
ambiguity. Parameter entities are parsed entities for use
within the DTD. These two types of entities use different forms of reference and
are recognized in different contexts. Furthermore, they occupy different
namespaces; a parameter entity and a general entity with the same name are two
distinct entities.
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.
An entity reference refers to the content of a
named entity. References to parsed general entities use
ampersand (& ) and semicolon (; ) as delimiters. Parameter-entity references use percent-sign
(% ) and semicolon (; ) as delimiters.
Well-Formedness Constraint: Entity Declared In a document without
any DTD, a document with only an internal DTD subset which contains no parameter
entity references, or a document with "standalone='yes' ", the
Name given
in the entity reference must match that in an
entity
declaration, except that well-formed documents need not declare any of the
following entities: amp , lt , gt ,
apos , quot . The declaration of a parameter entity must
precede any reference to it. Similarly, the declaration of a general entity must
precede any reference to it which appears in a default value in an
attribute-list declaration. Note that if entities are declared in the external
subset or in external parameter entities, a non-validating processor is not obligated
to read and process their declarations; for such documents, the rule that an
entity must be declared is a well-formedness constraint only if standalone='yes'.
Validity Constraint: Entity Declared In a document with an external
subset or external parameter entities with "standalone='no' ", the
Name given
in the entity reference must match that in an
entity
declaration. For interoperability, valid documents should declare the
entities amp , lt , gt , apos ,
quot , in the form specified in "4.6 Predefined
Entities". The declaration of a parameter entity must precede any reference
to it. Similarly, the declaration of a general entity must precede any reference
to it which appears in a default value in an attribute-list declaration.
Well-Formedness Constraint: Parsed Entity An entity reference must
not contain the name of an unparsed
entity. Unparsed entities may be referred to only in attribute
values declared to be of type ENTITY or ENTITIES .
Well-Formedness Constraint: No Recursion A parsed entity must not
contain a recursive reference to itself, either directly or indirectly.
Well-Formedness Constraint: In DTD Parameter-entity references may
only appear in the DTD.
Examples of character and entity references:
Type <key>less-than</key> (<) to save options. This document was prepared on &docdate; and is classified &security-level;. |
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... --> <!ENTITY % ISOLat2 SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" > <!-- ... now reference it. --> %ISOLat2; |
Entities are declared thus:
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.
If the entity definition is an EntityValue ,
the defined entity is called an internal entity. There is no separate
physical storage object, and the content of the entity is given in the
declaration. Note that some processing of entity and character references in the
literal entity
value may be required to produce the correct replacement
text: see "4.5 Construction
of Internal Entity Replacement Text".
An internal entity is a parsed
entity.
Example of an internal entity declaration:
<!ENTITY Pub-Status "This is a pre-release of the specification."> |
If the entity is not internal, it is an external
entity, declared as follows:
External Entity Declaration |
|
If the NDataDecl
is present, this is a general unparsed
entity; otherwise it is a parsed entity.
Validity Constraint: Notation Declared The Name must
match the declared name of a notation.
The SystemLiteral
is called the entity's system identifier. It is a URI, which may be used
to retrieve the entity. Note that the hash mark (# ) and fragment
identifier frequently used with URIs are not, formally, part of the URI itself;
an XML processor may signal an error if a fragment identifier is given as part
of a system identifier. Unless otherwise provided by information outside the
scope of this specification (e.g. a special XML element type defined by a
particular DTD, or a processing instruction defined by a particular application
specification), relative URIs are relative to the location of the resource
within which the entity declaration occurs. A URI might thus be relative to the
document
entity, to the entity containing the external DTD
subset, or to some other external parameter
entity.
An XML processor should handle a non-ASCII character in a URI by representing
the character in UTF-8 as one or more bytes, and then escaping these bytes with
the URI escaping mechanism (i.e., by converting each byte to %HH, where HH is
the hexadecimal notation of the byte value).
In addition to a system identifier, an external
identifier may include a public identifier. An XML processor attempting
to retrieve the entity's content may use the public identifier to try to
generate an alternative URI. If the processor is unable to do so, it must use
the URI specified in the system literal. Before a match is attempted, all
strings of white space in the public identifier must be normalized to single
space characters (#x20), and leading and trailing white space must be
removed.
Examples of external entity declarations:
<!ENTITY open-hatch SYSTEM "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY open-hatch PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN" "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY hatch-pic SYSTEM "../grafix/OpenHatch.gif" NDATA gif > |
External parsed entities may each begin with a text declaration.
The text declaration must be provided literally, not by reference to a parsed
entity. No text declaration may appear at any position other than the beginning
of an external parsed entity.
The document entity is well-formed if it matches the production labeled
document .
An external general parsed entity is well-formed if it matches the production
labeled extParsedEnt .
An external parameter entity is well-formed if it matches the production labeled
extPE .
Well-Formed External Parsed Entity |
|
An internal general parsed entity is well-formed if its replacement text
matches the production labeled content .
All internal parameter entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and physical
structures in an XML document are properly nested; no start-tag, end-tag, empty-element
tag, element, comment, processing
instruction, character
reference, or entity reference
can begin in one entity and end in another.
Each external parsed entity in an XML document may use a different encoding
for its characters. All XML processors must be able to read entities in either
UTF-8 or UTF-16.
Entities encoded in UTF-16 must begin with the Byte Order Mark described by
ISO/IEC 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE
character, #xFEFF). This is an encoding signature, not part of either the markup
or the character data of the XML document. XML processors must be able to use
this character to differentiate between UTF-8 and UTF-16 encoded documents.
Although an XML processor is required to read only entities in the UTF-8 and
UTF-16 encodings, it is recognized that other encodings are used around the
world, and it may be desired for XML processors to read entities that use them.
Parsed entities which are stored in an encoding other than UTF-8 or UTF-16 must
begin with a text
declaration containing an encoding declaration:
Encoding Declaration |
[80] |
EncodingDecl |
::= |
S
'encoding' Eq ('"'
EncName
'"' | "'" EncName
"'" ) |
[81] |
EncName |
::= |
[A-Za-z] ([A-Za-z0-9._]
| '-')* |
/* |
Encoding name contains only Latin
characters */ | |
In the document entity,
the encoding declaration is part of the XML
declaration. The EncName
is the name of the encoding used.
In an encoding declaration, the values "UTF-8 ",
"UTF-16 ", "ISO-10646-UCS-2 ", and
"ISO-10646-UCS-4 " should be used for the various encodings and
transformations of Unicode / ISO/IEC 10646, the values
"ISO-8859-1 ", "ISO-8859-2 ", ...
"ISO-8859-9 " should be used for the parts of ISO 8859, and the
values "ISO-2022-JP ", "Shift_JIS ", and
"EUC-JP " should be used for the various encoded forms of JIS
X-0208-1997. XML processors may recognize other encodings; it is recommended
that character encodings registered (as charsets) with the Internet
Assigned Numbers Authority [IANA], other than
those just listed, should be referred to using their registered names. Note that
these registered names are defined to be case-insensitive, so processors wishing
to match against them should do so in a case-insensitive way.
In the absence of information provided by an external transport protocol
(e.g. HTTP or MIME), it is an error for an
entity including an encoding declaration to be presented to the XML processor in
an encoding other than that named in the declaration, for an encoding
declaration to occur other than at the beginning of an external entity, 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 when an XML processor encounters an entity with an encoding that it is
unable to process.
Examples of encoding declarations:
<?xml encoding='UTF-8'?> <?xml encoding='EUC-JP'?> |
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
or AttValue .
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.
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, except that the replacement text of
entities used to escape markup delimiters (the entities amp ,
lt , gt , apos , quot ) is
always treated as data. (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.
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 parser 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.
The following are forbidden, and constitute fatal errors:
- the appearance of a reference to an unparsed
entity.
- the appearance of any character or general-entity reference in the DTD
except within an
EntityValue
or AttValue .
- a reference to an external entity in an attribute value.
When an entity
reference appears in an attribute value, or a parameter entity reference
appears in a literal entity value, its replacement
text is processed in place of the reference itself as though it were part of
the document at the location the reference was recognized, except that a single
or double quote character in the replacement text is always treated as a normal
data character and will not terminate the literal. For example, this is
well-formed:
<!ENTITY % YN '"Yes"' > <!ENTITY WhatHeSaid "He said &YN;" > |
while this is not:
<!ENTITY EndAttr "27'" > <element attribute='a-&EndAttr;> |
When the name of an unparsed
entity appears as a token in the value of an attribute of declared type
ENTITY or ENTITIES , a validating processor must inform
the application of the system and public (if any)
identifiers for both the entity and its associated notation.
When a general entity reference appears in the EntityValue
in an entity declaration, it is bypassed and left as is.
Just as with external parsed entities, parameter entities need only be included if
validating. When a parameter-entity reference is recognized in the DTD and
included, its replacement
text is enlarged by the attachment of one leading and one following space
(#x20) character; the intent is to constrain the replacement text of parameter
entities to contain an integral number of grammatical tokens in the DTD.
In discussing the treatment of internal entities, it is useful to distinguish
two forms of the entity's value. The literal entity
value is the quoted string actually present in the entity declaration,
corresponding to the non-terminal EntityValue .
The replacement text is the content of the
entity, after replacement of character references and parameter-entity
references.
The literal entity value as given in an internal entity declaration (EntityValue )
may contain character, parameter-entity, and general-entity references. Such
references must be contained entirely within the literal entity value. The
actual replacement text that is included as
described above must contain the replacement text of any parameter
entities referred to, and must contain the character referred to, in place of
any character references in the literal entity value; however, general-entity
references must be left as-is, unexpanded. For example, given the following
declarations:
<!ENTITY % pub "Éditions Gallimard" > <!ENTITY rights "All rights reserved" > <!ENTITY book "La Peste: Albert Camus, © 1947 %pub;. &rights;" > |
then the replacement text for the entity "book " is:
La Peste: Albert Camus, © 1947 Éditions Gallimard. &rights; |
The general-entity reference "&rights; " would be expanded
should the reference "&book; " appear in the document's content
or an attribute value.
These simple rules may have complex interactions; for a detailed discussion
of a difficult example, see "D. Expansion
of Entity and Character References".
Entity and character references can both be used to
escape the left angle bracket, ampersand, and other delimiters. A set of
general entities (amp , lt , gt ,
apos , quot ) is specified for this purpose. Numeric
character references may also be used; they are expanded immediately when
recognized and must be treated as character data, so the numeric character
references "< " and "& " may be used to
escape < and & when they occur in character
data.
All XML processors must recognize these entities whether they are declared or
not. For
interoperability, valid XML documents should declare these entities, like
any others, before using them. If the entities in question 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, as shown
below.
<!ENTITY lt "&#60;"> <!ENTITY gt ">"> <!ENTITY amp "&#38;"> <!ENTITY apos "'"> <!ENTITY quot """>
|
Note that the < and & characters in the
declarations of "lt " and "amp " are doubly escaped to
meet the requirement that entity replacement be well-formed.
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.
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.
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.)
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.
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.
Validating processors must 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. 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. 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.
The behavior of a validating XML processor is highly predictable; it must
read every piece of a document and report all well-formedness and validity
violations. Less is required of a non-validating processor; it need not read any
part of the document other than the document entity. This has two effects that
may be important to users of XML processors:
- Certain well-formedness errors, specifically those that require reading
external entities, may not be detected by a non-validating processor. Examples
include the constraints entitled Entity
Declared, Parsed
Entity, and No
Recursion, as well as some of the cases described as forbidden in
"4.4 XML
Processor Treatment of Entities and References".
- The information passed from the processor to the application may vary,
depending on whether the processor reads parameter and external entities. For
example, a non-validating processor may not normalize
attribute values, include the
replacement text of internal entities, or supply default
attribute values, where doing so depends on having read declarations in
external or parameter entities.
For maximum reliability in interoperating between different XML processors,
applications which use non-validating processors should not rely on any
behaviors not required of such processors. Applications which require facilities
such as the use of default attributes or internal entities which are declared in
external entities should use validating XML processors.
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
Symbols are written with an initial capital letter if they are defined by a
regular expression, or with an initial lower case letter otherwise. 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
N is a hexadecimal integer, the expression matches the
character in ISO/IEC 10646 whose canonical (UCS-4) code value, when
interpreted as an unsigned binary number, has the value indicated. The number
of leading zeros in the #xN form is insignificant; the number of
leading zeros in the corresponding code value is governed by the character
encoding in use and is not significant for XML.
[a-zA-Z] , [#xN-#xN]
- matches any character
with a value in the range(s) indicated (inclusive).
[^a-z] , [^#xN-#xN]
- matches any character
with a value outside the range indicated.
[^abc] , [^#xN#xN#xN]
- matches any character
with a value not among the characters given.
"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 A and B
represent simple expressions:
- (
expression )
expression is treated as a unit and may be combined as
described in this list.
A?
- matches
A or nothing; optional A .
A B
- matches
A followed by B .
A | B
- matches
A or B but not both.
A - B
- matches any string that matches
A but does not match
B .
A+
- matches one or more occurrences of
A .
A*
- matches zero or more occurrences of
A . 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.
Appendices
- IANA
- (Internet Assigned Numbers Authority) Official Names for Character
Sets, ed. Keld Simonsen et al. See ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets.
- IETF RFC 1766
- IETF (Internet Engineering Task Force). RFC 1766: Tags for the
Identification of Languages, ed. H. Alvestrand. 1995.
- 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/IEC 10646
- ISO (International Organization for Standardization). ISO/IEC
10646-1993 (E). Information technology -- Universal Multiple-Octet Coded
Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane.
[Geneva]: International Organization for Standardization, 1993 (plus
amendments AM 1 through AM 7).
- Unicode
- The Unicode Consortium. The Unicode Standard, Version 2.0.
Reading, Mass.: Addison-Wesley Developers Press, 1996.
- Aho/Ullman
- Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. Compilers:
Principles, Techniques, and Tools. Reading: Addison-Wesley, 1986, rpt.
corr. 1988.
- Berners-Lee et al.
- Berners-Lee, T., R. Fielding, and L. Masinter. Uniform Resource
Identifiers (URI): Generic Syntax and Semantics. 1997. (Work in progress;
see updates to RFC1738.)
- Brüggemann-Klein
- Brüggemann-Klein, Anne. Regular Expressions into Finite Automata.
Extended abstract in I. Simon, Hrsg., LATIN 1992, S. 97-98. Springer-Verlag,
Berlin 1992. Full Version in Theoretical Computer Science 120: 197-213, 1993.
- Brüggemann-Klein and Wood
- Brüggemann-Klein, Anne, and Derick Wood. Deterministic Regular
Languages. Universität Freiburg, Institut für Informatik, Bericht 38,
Oktober 1991.
- Clark
- James Clark. Comparison of SGML and XML. See http://www.w3.org/TR/NOTE-sgml-xml-971215.
- IETF RFC1738
- IETF (Internet Engineering Task Force). RFC 1738: Uniform Resource
Locators (URL), ed. T. Berners-Lee, L. Masinter, M. McCahill. 1994.
- IETF RFC1808
- IETF (Internet Engineering Task Force). RFC 1808: Relative Uniform
Resource Locators, ed. R. Fielding. 1995.
- IETF RFC2141
- IETF (Internet Engineering Task Force). RFC 2141: URN Syntax, ed.
R. Moats. 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.
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, without diacritics), ideographic characters,
and combining characters (among others, this class contains most diacritics);
these classes combine to form the class of letters. 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 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, 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.
XML is designed to be a subset of SGML, in that every valid XML
document should also be a conformant SGML document. For a detailed comparison of
the additional restrictions that XML places on documents beyond those of SGML,
see [Clark].
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
<!ENTITY example "<p>An ampersand (&#38;) may be escaped numerically (&#38;#38;) or with a general entity (&amp;).</p>" >
|
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 ":
<p>An ampersand (&) may be escaped numerically (&#38;) or with a general entity (&amp;).</p>
|
A reference in the document to "&example; " will cause the
text to be reparsed, at which time the start- and end-tags of the
"p " element will be recognized and the three references will be
recognized and expanded, resulting in a "p " 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 <?xml version='1.0'?> 2 <!DOCTYPE test [ 3 <!ELEMENT test (#PCDATA) > 4 <!ENTITY % xx '%zz;'> 5 <!ENTITY % zz '<!ENTITY tricky "error-prone" >' > 6 %xx; 7 ]> 8 <test>This sample shows a &tricky; method.</test>
|
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 "<!ENTITY tricky "error-prone" > ", 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 ("<!ENTITY tricky "error-prone" > ") 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.
For
compatibility, it is required that content models in element type
declarations be deterministic.
SGML requires deterministic content models (it calls them "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 b the parser cannot
know which b in the model is being matched without looking ahead to
see which element follows the b . In this case, the two references
to b can be collapsed into a single reference, making the model
read (b, (c | d)) . An initial b now clearly matches
only a single name in the content model. The parser doesn't need to look ahead
to see what follows; either c or d 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].
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.
Because each XML entity not in UTF-8 or UTF-16 format must begin
with an XML encoding declaration, in which the first characters must be
'<?xml ', 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 ".
00 00 00 3C : UCS-4, big-endian machine (1234 order)
3C 00 00 00 : UCS-4, little-endian machine (4321 order)
00 00 3C 00 : UCS-4, unusual octet order (2143)
00 3C 00 00 : UCS-4, unusual octet order (3412)
FE FF : UTF-16, big-endian
FF FE : UTF-16, little-endian
00 3C 00 3F : UTF-16, big-endian, no Byte Order Mark (and
thus, strictly speaking, in error)
3C 00 3F 00 : UTF-16, little-endian, no Byte Order Mark (and
thus, strictly speaking, in error)
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
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
corrupt, fragmentary, or enclosed in a wrapper of some kind
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 ASCII
characters, 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.
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.
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. Rules for the relative priority of
the internal label and the MIME-type label in an external header, for example,
should be part of the RFC document defining the text/xml and application/xml
MIME types. In the interests of interoperability, however, the following rules
are recommended.
- If an XML entity is in a file, the Byte-Order Mark and
encoding-declaration PI are used (if present) to determine the character
encoding. All other heuristics and sources of information are solely for error
recovery.
- If an XML entity is delivered with a MIME type of text/xml, then the
charset parameter on the MIME type determines the character
encoding method; all other heuristics and sources of information are solely
for error recovery.
- If an XML entity is delivered with a MIME type of application/xml, then
the Byte-Order Mark and encoding-declaration PI are used (if present) to
determine the character encoding. All other heuristics and sources of
information are solely for error recovery.
These rules apply only in
the absence of protocol-level documentation; in particular, when the MIME types
text/xml and application/xml are defined, the recommendations of the relevant
RFC will supersede these rules.
This specification was prepared and approved for publication by the W3C XML
Working Group (WG). WG approval of this specification does not necessarily imply
that all WG members voted for its approval. The current and former members of
the XML WG are: Jon Bosak, Sun (Chair); James Clark (Technical Lead); Tim
Bray, Textuality and Netscape (XML Co-editor); Jean Paoli, Microsoft (XML
Co-editor); C. M. Sperberg-McQueen, U. of Ill. (XML Co-editor); Dan Connolly,
W3C (W3C Liaison); Paula Angerstein, Texcel; Steve DeRose, INSO; Dave Hollander,
HP; Eliot Kimber, ISOGEN; Eve Maler, ArborText; Tom Magliery, NCSA; Murray
Maloney, Muzmo and Grif; Makoto Murata, Fuji Xerox Information Systems; Joel
Nava, Adobe; Conleth O'Connell, Vignette; Peter Sharpe, SoftQuad; John Tigue,
DataChannel
Extensible Markup Language (XML) 1.0
W3C Recommendation 10-February-1998
- This version:
- http://www.w3.org/TR/1998/REC-xml-19980210
-
- http://www.w3.org/TR/1998/REC-xml-19980210.xml
- http://www.w3.org/TR/1998/REC-xml-19980210.html
- http://www.w3.org/TR/1998/REC-xml-19980210.pdf
- http://www.w3.org/TR/1998/REC-xml-19980210.ps
- Latest version:
- http://www.w3.org/TR/REC-xml
- Previous version:
- http://www.w3.org/TR/PR-xml-971208
- Editors:
- Tim Bray (Textuality and Netscape) mailto:tbray@textuality.com
- Jean Paoli (Microsoft) mailto:jeanpa@microsoft.com
- C. M. Sperberg-McQueen (University of Illinois at Chicago) mailto:cmsmcq@uic.edu
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