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.
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, 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.
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:
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:
behave as
described. (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).
in the
sentence) behave as described; if it does, it must
provide users a means to enable or disable the behavior
described..
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".
[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".
[Definition:] A textual object is a well-formed XML document if:
document.
| Document | ||||
|
Matching the document production
implies that:
[Definition:] As a consequence
of this,
for each non-root element
C in the document, there is one other element P
in the document such that
C is in the content of P, but is not in
the content of any other element that is in the content of
P.
P is referred to as the
parent of C, and C as a
child of P.
[Definition:] A parsed entity contains
text, a sequence of
characters,
which may represent markup or character data.
[Definition:] A character
is an atomic unit of text as specified by
ISO/IEC 10646 [ISO/IEC 10646].
Legal characters are tab, carriage return, line feed, and the legal
graphic characters of Unicode and ISO/IEC 10646.
The use of "compatibility characters", as defined in section 6.8
of [Unicode], is discouraged.
| Character Range | ||||||
|
The mechanism for encoding character code points into bit patterns may
vary from entity to entity. All XML processors must accept the UTF-8
and UTF-16 encodings of 10646; the mechanisms for signaling which of
the two is in use, or for bringing other encodings into play, are
discussed later, in "4.3.3 Character Encoding in Entities".
This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x20)
characters, carriage returns, line feeds, or tabs.
| White Space | ||||
|
Characters are classified for convenience as letters, digits, or other
characters. Letters consist of an alphabetic or syllabic
base character possibly
followed by one or more combining characters, or of an ideographic
character.
Full definitions of the specific characters in each class
are given in "B. Character Classes".
[Definition:] A Name is a token
beginning with a letter or one of a few punctuation characters, and continuing
with letters, digits, hyphens, underscores, colons, or full stops, together
known as name characters.
Names beginning with the string "xml", or any string
which would match (('X'|'x') ('M'|'m') ('L'|'l')), are
reserved for standardization in this or future versions of this
specification.
Note: The colon character within XML names is reserved for experimentation with
name spaces.
Its meaning is expected to be
standardized at some future point, at which point those documents
using the colon for experimental purposes may need to be updated.
(There is no guarantee that any name-space mechanism
adopted for XML will in fact use the colon as a name-space delimiter.)
In practice, this means that authors should not use the colon in XML
names except as part of name-space experiments, but that XML processors
should accept the colon as a name character.
An
Nmtoken (name token) is any mixture of
name characters.
| Names and Tokens | ||||||||||||||||||||
|
Literal data is any quoted string not containing
the quotation mark used as a delimiter for that string.
Literals are used
for specifying the content of internal entities
(EntityValue),
the values of attributes (AttValue),
and external identifiers
(SystemLiteral).
Note that a SystemLiteral
can be parsed without scanning for markup.
| Literals | ||||||||||||||||||||||||||||
|
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, and
processing instructions.
[Definition:] All text that is not markup constitutes the character data of the document.
The ampersand character (&) and the left angle bracket (<)
may appear in their literal form only when used as markup
delimiters, or within a comment, a
processing instruction,
or a CDATA section.
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 | ||||
|
[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.
| Comments | ||||
|
An example of a comment:
<!-- declarations for <head> & <body> --> |
[Definition:] Processing
instructions (PIs) allow documents to contain instructions
for applications.
| Processing Instructions | ||||||||
|
PIs are not part of the document's character
data, but must be passed through to the application. The
PI begins with a target (PITarget) used
to identify the application to which the instruction is directed.
The target names "XML", "xml", and so on are
reserved for standardization in this or future versions of this
specification.
The
XML Notation mechanism
may be used for
formal declaration of PI targets.
[Definition:] CDATA sections
may occur
anywhere character data may occur; they are
used to escape blocks of text containing characters which would
otherwise be recognized as markup. CDATA sections begin with the
string "<![CDATA[" and end with the string
"]]>":
| CDATA Sections | ||||||||||||||||
|
Within a CDATA section, only the CDEnd string is
recognized as markup, so that left angle brackets and ampersands may occur in
their literal form; they need not (and cannot) be escaped using
"<" and "&". CDATA sections
cannot nest.
An example of a CDATA section, in which "<greeting>" and
"</greeting>"
are recognized as character data, not
markup:
<![CDATA[<greeting>Hello, world!</greeting>]]> |
[Definition:] XML documents
may, and should,
begin with an XML declaration which specifies
the version of
XML being used.
For example, the following is a complete XML document, well-formed but not
valid:
<?xml version="1.0"?> |
and so is this:
<greeting>Hello, world!</greeting> |
The version number "1.0" should be used to indicate
conformance to this version of this specification; it is an error
for a document to use the value "1.0"
if it does not conform to this version of this specification.
It is the intent
of the XML working group to give later versions of this specification
numbers other than "1.0", but this intent does not
indicate a
commitment to produce any future versions of XML, nor if any are produced, to
use any particular numbering scheme.
Since future versions are not ruled out, this construct is provided
as a means to allow the possibility of automatic version recognition, should
it become necessary.
Processors may signal an error if they receive documents labeled with
versions they do not support.
The function of the markup in an XML document is to describe its
storage and logical structure and to associate attribute-value pairs
with its logical structures. XML provides a mechanism, the document type declaration, to define
constraints on the logical structure and to support the use of
predefined storage units.
[Definition:] An XML document is
valid if it has an associated document type
declaration and if the document
complies with the constraints expressed in it.
The document type declaration must appear before the first element in the document.
| Prolog | ||||||||||||||||||||||||
|
[Definition:] The XML
document type declaration
contains or points to
markup declarations
that provide a grammar for a
class of documents.
This grammar is known as a document type definition,
or DTD.
The document type declaration can point to an external subset (a
special kind of
external entity) containing markup
declarations, or can
contain the markup declarations directly in an internal subset, or can do
both.
The DTD for a document consists of both subsets taken
together.
[Definition:] A markup declaration is an element type declaration, an attribute-list declaration, an entity declaration, or a notation declaration. These declarations may be contained in whole or in part within parameter entities, as described in the well-formedness and validity constraints below. For fuller information, see "4. Physical Structures".
| Document Type Definition | ||||||||||||||||||
|
The markup declarations may be made up in whole or in part of
the replacement text of
parameter entities.
The productions later in this specification for
individual nonterminals (elementdecl,
AttlistDecl, and so on) describe
the declarations after all the parameter entities have been
included.
Validity Constraint:
Root Element Type
The Name in the document type declaration must
match the element type of the root element.
Validity Constraint:
Proper Declaration/PE Nesting
Parameter-entity
replacement text must be properly nested
with markup declarations.
That is to say, if either the first character
or the last character of a markup
declaration (markupdecl above)
is contained in the replacement text for a
parameter-entity reference,
both must be contained in the same replacement text.
Well-Formedness Constraint:
PEs in Internal Subset
In the internal DTD subset,
parameter-entity references
can occur only where markup declarations can occur, not
within markup declarations. (This does not apply to
references that occur in
external parameter entities or to the external subset.)
Like the internal subset, the external subset and
any external parameter entities referred to in the DTD
must consist of a series of complete markup declarations of the types
allowed by the non-terminal symbol
markupdecl, interspersed with white space
or parameter-entity references.
However, portions of the contents
of the
external subset or of external parameter entities may conditionally be ignored
by using
the conditional section
construct; this is not allowed in the internal subset.
| External Subset | ||||||||
|
The external subset and external parameter entities also differ from the internal subset in that in them, parameter-entity references are permitted within markup declarations, not only between markup declarations.
An example of an XML document with a document type declaration:
<?xml version="1.0"?> |
The system identifier
"hello.dtd" gives the URI of a DTD for the document.
The declarations can also be given locally, as in this
example:
<?xml version="1.0" encoding="UTF-8" ?> |
If both the external and internal subsets are used, the
internal subset is considered to occur before the external subset.
This has the effect that entity and attribute-list declarations in the
internal subset take precedence over those in the external subset.
Markup declarations can affect the content of the document,
as passed from an XML processor
to an application; examples are attribute defaults and entity
declarations.
The standalone document declaration,
which may appear as a component of the XML declaration, signals
whether or not there are such declarations which appear external to
the document entity.
| Standalone Document Declaration | ||||||
|
In a standalone document declaration, the value "yes" indicates
that there
are no markup declarations external to the document
entity (either in the DTD external subset, or in an
external parameter entity referenced from the internal subset)
which affect the information passed from the XML processor to
the application.
The value "no" indicates that there are or may be such
external markup declarations.
Note that the standalone document declaration only
denotes the presence of external declarations; the presence, in a
document, of
references to external entities, when those entities are
internally declared,
does not change its standalone status.
If there are no external markup declarations, the standalone document
declaration has no meaning.
If there are external markup declarations but there is no standalone
document declaration, the value "no" is assumed.
Any XML document for which standalone="no" holds can
be converted algorithmically to a standalone document,
which may be desirable for some network delivery applications.
Validity Constraint:
Standalone Document Declaration
The standalone document declaration must have
the value "no" if any external markup declarations
contain declarations of:
amp,
lt,
gt,
apos,
quot),
if references to those
entities appear in the document, or
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 | ||||||||||||||||||||||||
|
The Langcode may be any of the following:
i-" (or "I-")x-" or "X-" in order to ensure that they do not conflict
with names later standardized or registered with IANAThere may be any number of Subcode segments; if
the first
subcode segment exists and the Subcode consists of two
letters, then it must be a country code from
[ISO 3166], "Codes
for the representation of names of countries."
If the first
subcode consists of more than two letters, it must be
a subcode for the language in question registered with IANA,
unless the Langcode begins with the prefix
"x-" or
"X-".
It is customary to give the language code in lower case, and the country code (if any) in upper case. Note that these values, unlike other names in XML documents, are case insensitive.
For example:
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p> |
The intent declared with xml:lang is considered to apply to
all attributes and content of the element where it is specified,
unless overridden with an instance of xml:lang
on another element within that content.
A simple declaration for xml:lang might take
the form
xml:lang NMTOKEN #IMPLIED |
but specific default values may also be given, if appropriate. In a collection of French poems for English students, with glosses and notes in English, the xml:lang attribute might be declared this way:
<!ATTLIST poem xml:lang NMTOKEN 'fr'> |
[Definition:] Each XML document contains one or more
elements, the boundaries of which are
either delimited by start-tags
and end-tags, or, for empty elements, by an empty-element tag. Each element has a type,
identified by name, sometimes called its "generic
identifier" (GI), and may have a set of
attribute specifications. Each attribute specification
has a name and a value.
| Element | ||||||||||||||||
|
This specification does not constrain the semantics, use, or (beyond
syntax) names of the element types and attributes, except that names
beginning with a match to (('X'|'x')('M'|'m')('L'|'l'))
are reserved for standardization in this or future versions of this
specification.
Well-Formedness Constraint:
Element Type Match
The Name in an element's end-tag must match
the element type in
the start-tag.
Validity Constraint:
Element Valid
An element is
valid if
there is a declaration matching
elementdecl where the
Name matches the element type, and
one of the following holds:
EMPTY and the element has no
content.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.Mixed and
the content consists of character
data and child elements
whose types match names in the content model.ANY, and the types
of any child elements have
been declared.[Definition:] The beginning of every non-empty XML element is marked by a start-tag.
| Start-tag | ||||||||||||||||||||||||
|
The Name in
the start- and end-tags gives the
element's type.
[Definition:]
The Name-AttValue pairs are
referred to as
the attribute specifications of the element,
[Definition:] with the
Name in each pair
referred to as the attribute name and
[Definition:] the content of the
AttValue (the text between the
' or " delimiters)
as the attribute value.
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"> |
[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 | ||||
|
An example of an end-tag:
</termdef> |
[Definition:] The
text between the start-tag and
end-tag is called the element's
content:
| Content of Elements | ||||
|
[Definition:] 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.
[Definition:] An
empty-element tag takes a special form:
| Tags for Empty Elements | ||||||
|
Empty-element tags may be used for any element which has no
content, whether or not it is declared using the keyword
EMPTY.
For interoperability, the empty-element
tag must be used, and can only be used, for elements which are
declared EMPTY.
Examples of empty elements:
<IMG align="left" |
The element structure of an
XML document may, for
validation purposes,
be constrained
using element type and attribute-list declarations.
An element type declaration constrains the element's
content.
Element type declarations often constrain which element types can
appear as children of the element.
At user option, an XML processor may issue a warning
when a declaration mentions an element type for which no declaration
is provided, but this is not an error.
[Definition:] An element type declaration takes the form:
| Element Type Declaration | ||||||||||
|
where the Name gives the element type
being declared.
Validity Constraint:
Unique Element Type Declaration
No element type may be declared more than once.
Examples of element type declarations:
<!ELEMENT br EMPTY> |
[Definition:] An element type has
element content when elements of that
type must contain only child
elements (no character data), optionally separated by
white space (characters matching the nonterminal
S).
In this case, the
constraint includes a content model, a simple grammar governing
the allowed types of the child
elements and the order in which they are allowed to appear.
The grammar is built on
content particles (cps), which consist of names,
choice lists of content particles, or
sequence lists of content particles:
| Element-content Models | ||||||||||||||||||||
|
where each Name is the type of an element which may
appear as a child.
Any content
particle in a choice list may appear in the element content at the location where
the choice list appears in the grammar;
content particles occurring in a sequence list must each
appear in the element content in the
order given in the list.
The optional character following a name or list governs
whether the element or the content particles in the list may occur one
or more (+), zero or more (*), or zero or
one times (?).
The absence of such an operator means that the element or content particle
must appear exactly once.
This syntax
and meaning are identical to those used in the productions in this
specification.
The content of an element matches a content model if and only if it is
possible to trace out a path through the content model, obeying the
sequence, choice, and repetition operators and matching each element in
the content against an element type in the content model. For compatibility, it is an error
if an element in the document can
match more than one occurrence of an element type in the content model.
For more information, see "E. Deterministic Content Models".
Validity Constraint:
Proper Group/PE Nesting
Parameter-entity
replacement text must be properly nested
with parenthetized groups.
That is to say, if either of the opening or closing parentheses
in a choice, seq, or
Mixed construct
is contained in the replacement text for a
parameter entity,
both must be contained in the same replacement text.
For interoperability,
if a parameter-entity reference appears in a
choice, seq, or
Mixed construct, its replacement text
should not be empty, and
neither the first nor last non-blank
character of the replacement text should be a connector
(| or ,).
Examples of element-content models:
<!ELEMENT spec (front, body, back?)> |
[Definition:] An element
type has
mixed content when elements of that type may contain
character data, optionally interspersed with
child elements.
In this case, the types of the child elements
may be constrained, but not their order or their number of occurrences:
| Mixed-content Declaration | ||||||||||||||||
|
where the Names give the types of elements
that may appear as children.
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)*> |
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:
[Definition:]
Attribute-list declarations specify the name, data type, and default
value (if any) of each attribute associated with a given element type:
| Attribute-list Declaration | ||||||||
|
The Name in the
AttlistDecl rule is the type of an element. At
user option, an XML processor may issue a warning if attributes are
declared for an element type not itself declared, but this is not an
error. The Name in the
AttDef rule is
the name of the attribute.
When more than one AttlistDecl is provided for a
given element type, the contents of all those provided are merged. When
more than one definition is provided for the same attribute of a
given element type, the first declaration is binding and later
declarations are ignored.
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:
| Attribute Types | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Validity Constraint:
ID
Values of type ID must match the
Name production.
A name must not appear more than once in
an XML document as a value of this type; i.e., ID values must uniquely
identify the elements which bear them.
Validity Constraint:
One ID per Element Type
No element type may have more than one ID attribute specified.
Validity Constraint:
ID Attribute Default
An ID attribute must have a declared default of #IMPLIED or
#REQUIRED.
Validity Constraint:
IDREF
Values of type IDREF must match
the Name production, and
values of type IDREFS must match
Names;
each Name must match the value of an ID attribute on
some element in the XML document; i.e. IDREF values must
match the value of some ID attribute.
Validity Constraint:
Entity Name
Values of type ENTITY
must match the Name production,
values of type ENTITIES must match
Names;
each Name must
match the
name of an unparsed entity declared in the
DTD.
Validity Constraint:
Name Token
Values of type NMTOKEN must match the
Nmtoken production;
values of type NMTOKENS must
match Nmtokens.
[Definition:] Enumerated attributes can take one of a list of values provided in the declaration. There are two kinds of enumerated types:
| Enumerated Attribute Types | ||||||||||||||||
|
A NOTATION attribute identifies a
notation, declared in the
DTD with associated system and/or public identifiers, to
be used in interpreting the element to which the attribute
is attached.
Validity Constraint:
Notation Attributes
Values of this type must match
one of the notation names included in
the declaration; all notation names in the declaration must
be declared.
Validity Constraint:
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.
| Attribute Defaults | ||||||||||||||||||||||||||||
|
In an attribute declaration, #REQUIRED means that the
attribute must always be provided, #IMPLIED that no default
value is provided.
[Definition:] If the
declaration
is neither #REQUIRED nor #IMPLIED, then the
AttValue value contains the declared
default value; the #FIXED keyword states that
the attribute must always have the default value.
If a default value
is declared, when an XML processor encounters an omitted attribute, it
is to behave as though the attribute were present with
the declared default value.
Validity Constraint:
Required Attribute
If the default declaration is the keyword #REQUIRED, then
the attribute must be specified for
all elements of the type in the attribute-list declaration.
Validity Constraint:
Attribute Default Legal
The declared
default value must meet the lexical constraints of the declared attribute type.
Validity Constraint:
Fixed Attribute Default
If an attribute has a default value declared with the
#FIXED keyword, instances of that attribute must
match the default value.
Examples of attribute-list declarations:
<!ATTLIST termdef |
Before the value of an attribute is passed to the application
or checked for validity, the
XML processor must normalize it as follows:
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.
[Definition:]
Conditional sections are portions of the
document type declaration external subset
which are
included in, or excluded from, the logical structure of the DTD based on
the keyword which governs them.
| Conditional Section | ||||||||||||||||||||
|
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' > |
[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, 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.
[Definition:] A parsed entity's
contents are referred to as its
replacement text;
this text is considered an
integral part of the document.
[Definition:] An unparsed entity is a resource whose contents may or may not be text, and if text, 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.
[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.
[Definition:]
A character reference refers to a specific character in the
ISO/IEC 10646 character set, for example one not directly accessible from
available input devices.
| Character Reference | ||||||||||
|
Well-Formedness Constraint:
Legal Character
Characters referred to using character references must
match the production for
Char.
&#x", the digits and
letters up to the terminating ; provide a hexadecimal
representation of the character's code point in ISO/IEC 10646.
If it begins just with "&#", the digits up to the terminating
; provide a decimal representation of the character's
code point.
[Definition:] An entity
reference refers to the content of a named entity.
[Definition:] References to
parsed general entities
use ampersand (&) and semicolon (;) as
delimiters.
[Definition:]
Parameter-entity references use percent-sign (%) and
semicolon
(;) as delimiters.
| Entity Reference | ||||||||||||||||||||||||||||||||||||||||||||||
|
Well-Formedness Constraint:
Entity Declared
In a document without any DTD, a document with only an internal
DTD subset which contains no parameter entity references, or a document with
"standalone='yes'",
the Name given in the entity reference must
match that in an
entity declaration, except that
well-formed documents need not declare
any of the following entities: amp,
lt,
gt,
apos,
quot.
The declaration of a parameter entity must precede any reference to it.
Similarly, the declaration of a general entity must precede any
reference to it which appears in a default value in an attribute-list
declaration.
Note that if entities are declared in the external subset or in
external parameter entities, a non-validating processor is
not obligated to read
and process their declarations; for such documents, the rule that
an entity must be declared is a well-formedness constraint only
if standalone='yes'.
Validity Constraint:
Entity Declared
In a document with an external subset or external parameter
entities with "standalone='no'",
the Name given in the entity reference must match that in an
entity declaration.
For interoperability, valid documents should declare the entities
amp,
lt,
gt,
apos,
quot, in the form
specified in "4.6 Predefined Entities".
The declaration of a parameter entity must precede any reference to it.
Similarly, the declaration of a general entity must precede any
reference to it which appears in a default value in an attribute-list
declaration.
Well-Formedness Constraint:
Parsed Entity
An entity reference must not contain the name of an unparsed entity. Unparsed entities may be referred
to only in attribute values declared to
be of type ENTITY or ENTITIES.
Well-Formedness Constraint:
No Recursion
A parsed entity must not contain a recursive reference to itself,
either directly or indirectly.
Well-Formedness Constraint:
In DTD
Parameter-entity references may only appear in the
DTD.
Examples of character and entity references:
Type <key>less-than</key> (<) to save options. |
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... --> |
[Definition:] Entities are declared thus:
| Entity Declaration | ||||||||||||||||||||
|
The Name identifies the entity in an
entity reference or, in the case of an
unparsed entity, in the value of an ENTITY or ENTITIES
attribute.
If the same entity is declared more than once, the first declaration
encountered is binding; at user option, an XML processor may issue a
warning if entities are declared multiple times.
[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:
<!ENTITY Pub-Status "This is a pre-release of the |
[Definition:] If the entity is not
internal, it is an external
entity, declared as follows:
| External Entity Declaration | ||||||||||||||
|
If the NDataDecl is present, this is a
general unparsed
entity; otherwise it is a parsed entity.
Validity Constraint:
Notation Declared
The Name must match the declared name of a
notation.
[Definition:] The
SystemLiteral
is called the entity's system identifier. It is a 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).
[Definition:]
In addition to a system identifier, an external identifier may
include a public identifier.
An XML processor attempting to retrieve the entity's content may use the public
identifier to try to generate an alternative URI. 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 |
External parsed entities may each begin with a text
declaration.
| Text Declaration | ||||
|
The text declaration must be provided literally, not
by reference to a parsed entity.
No text declaration may appear at any position other than the beginning of
an external parsed entity.
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 | ||||||||||
|
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'?> |
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:
content.AttValue.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.
EntityValue.EntityValue or
AttValue.| Entity Type | Character | ||||
| Parameter | Internal General |
External Parsed General |
Unparsed | ||
| Reference in Content |
Not recognized | Included | Included if validating | Forbidden | Included |
| Reference in Attribute Value |
Not recognized | Included in literal | Forbidden | Forbidden | Included |
| Occurs as Attribute Value |
Not recognized | Forbidden | Forbidden | Notify | Not recognized |
| Reference in EntityValue |
Included in literal | Bypassed | Bypassed | Forbidden | Included |
| Reference in DTD |
Included as PE | Forbidden | Forbidden | Forbidden | Forbidden |
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.
[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, 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:
EntityValue or
AttValue.
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"' > |
while this is not:
<!ENTITY EndAttr "27'" > |
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.
[Definition:] The literal
entity value is the quoted string actually
present in the entity declaration, corresponding to the
non-terminal EntityValue.
[Definition:] The replacement
text is the content of the entity, after
replacement of character references and parameter-entity
references.
The literal entity value
as given in an internal entity declaration
(EntityValue) may contain character,
parameter-entity, and general-entity references.
Such references must be contained entirely within the
literal entity value.
The actual replacement text that is
included as described above
must contain the replacement text of any
parameter entities referred to, and must contain the character
referred to, in place of any character references in the
literal entity value; however,
general-entity references must be left as-is, unexpanded.
For example, given the following declarations:
<!ENTITY % pub "Éditions Gallimard" > |
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".
[Definition:] Entity and character
references can both be used to escape the left angle bracket,
ampersand, and other delimiters. A set of general entities
(amp,
lt,
gt,
apos,
quot) is specified for this purpose.
Numeric character references may also be used; they are
expanded immediately when recognized and must be treated as
character data, so the numeric character references
"<" and "&" may be used to
escape < and & when they occur
in character data.
All XML processors must recognize these entities whether they are declared or not. For interoperability, valid XML documents should declare these entities, like any others, before using them. If the entities 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;"> |
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.
[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 | ||||||||
|
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.)
[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.
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 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.
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:
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
symbol ::= expression |
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:
#xNN 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][^a-z], [^#xN-#xN][^abc], [^#xN#xN#xN]"string"'string'A and B represent simple expressions:
expression)expression is treated as a unit
and may be combined as described in this list.A?A or nothing; optional A.A BA followed by B.A | BA or B but not both.A - BA but does not match
B.
A+A.A*A./* ... */[ wfc: ... ][ vc: ... ]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 | ||||||||||||||||||||||||
|
The character classes defined here can be derived from the
Unicode character database as follows:
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 |
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 |
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 |
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'?> |
This produces the following:
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.)<" 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.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".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)
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.
charset parameter
on the MIME type determines the
character encoding method; all other heuristics and sources of
information are solely for error recovery.
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