Extensible Markup Language (XML) 1.0
W3C Recommendation 10 February 1998
This version:
Latest version:
Previous version:
Editors:
Tim Bray
(Textuality and Netscape)
<tbray@textuality.com>
Jean Paoli
(Microsoft)
<jeanpa@microsoft.com>
C. M. Sperberg-McQueen
(University of Illinois at Chicago)
<cmsmcq@uic.edu>
Copyright © 1999 W3C (MIT, INRIA, Keio),
All Rights Reserved. W3C liability, trademark,
document use and software licensing rules apply.
Abstract
The Extensible Markup Language (XML) is a subset of
SGML that is completely described in this document. Its goal is to
enable generic SGML to be served, received, and processed on the Web
in the way that is now possible with HTML. XML has been designed for
ease of implementation and for interoperability with both SGML and
HTML.
Status of this document
This 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.
Table of Contents
1. Introduction
1.1. Origin and Goals
1.2. Terminology
2. Documents
2.2. Characters
2.5. Comments
2.7. CDATA Sections
2.10. White Space Handling
2.11. End-of-Line Handling
2.12. Language Identification
3.2.1. Element Content
3.2.2. Mixed Content
3.3.1. Attribute Types
3.3.2. Attribute Defaults
3.4. Conditional Sections
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
4.4.1. 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
4.6. Predefined Entities
4.8. Document Entity
5. Conformance
5.2. Using XML Processors
6. Notation
Appendices
A. References
A.1. Normative References
A.2. Other References
1. Introduction
Extensible Markup Language, abbreviated XML, describes a class of
data objects called XML documents and
partially describes the behavior of
computer programs which process them. XML is an application profile or
restricted form of SGML, the Standard Generalized Markup
Language [ISO 8879].
By construction, XML documents
are conforming SGML documents.
XML documents are made up of storage units called entities, which contain either parsed
or unparsed data.
Parsed data is made up of characters,
some
of which form character data,
and some of which form markup.
Markup encodes a description of the document's storage layout and
logical structure. XML provides a mechanism to impose constraints on
the storage layout and logical structure.
A software module
called an XML processor is used to read XML documents
and provide access to their content and structure. It is assumed that an XML processor is
doing its work on behalf of another module, called the
application. This specification describes the
required behavior of an XML processor in terms of how it must read XML
data and the information it must provide to the application.
1.1. Origin and Goals
XML was developed by an XML Working Group (originally known as the
SGML Editorial Review Board) formed under the auspices of the World
Wide Web Consortium (W3C) in 1996.
It was chaired by Jon Bosak of Sun
Microsystems with the active participation of an XML Special
Interest Group (previously known as the SGML Working Group) also
organized by the W3C. The membership of the XML Working Group is given
in an appendix. Dan Connolly served as the WG's contact with the W3C.
The design goals for XML are:
This specification,
together with associated standards
(Unicode and ISO/IEC 10646 for characters,
Internet RFC 1766 for language identification tags,
ISO 639 for language name codes, and
ISO 3166 for country name codes),
provides all the information necessary to understand
XML Version 1.0
and construct computer programs to process it.
This version of the XML specification
may be distributed freely, as long as
all text and legal notices remain intact.
1.2. Terminology
The terminology used to describe XML documents is defined in the body of
this specification.
The terms defined in the following list are used in building those
definitions and in describing the actions of an XML processor:
may
must
Conforming documents and XML processors
are required to behave as described; otherwise they are in error.
error
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
for interoperability
2. Documents
A data object is an
XML document if it is
well-formed, as
defined in this specification.
A well-formed XML document may in addition be
valid if it meets certain further
constraints.
Each XML document has both a logical and a physical structure.
Physically, the document is composed of units called entities. An entity may refer to other entities to cause their
inclusion in the document. A document begins in a "root" or document entity.
Logically, the document is composed of declarations, elements,
comments,
character references, and
processing
instructions, all of which are indicated in the document by explicit
markup.
The logical and physical structures must nest properly, as described
in [4.3.2. Well-Formed Parsed Entities].
2.1. Well-Formed XML Documents
A textual object is
a well-formed XML document if:
Matching the document production
implies that:
2.2. Characters
A parsed entity contains
text, a sequence of
characters,
which may represent markup or character data.
A character
is an atomic unit of text as specified by
ISO/IEC 10646 [ISO/IEC 10646].
Legal characters are tab, carriage return, line feed, and the legal
graphic characters of Unicode and ISO/IEC 10646.
The use of "compatibility characters", as defined in section 6.8
of [Unicode], is discouraged.
The mechanism for encoding character code points into bit patterns may
vary from entity to entity. All XML processors must accept the UTF-8
and UTF-16 encodings of 10646; the mechanisms for signaling which of
the two is in use, or for bringing other encodings into play, are
discussed later, in [4.3.3. Character Encoding in Entities].
2.3. Common Syntactic Constructs
This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x20)
characters, carriage returns, line feeds, or tabs.
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 [Appendix 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.
2.4. Character Data and Markup
Text consists of intermingled
character
data and markup.
Markup takes the form of
start-tags,
end-tags,
empty-element tags,
entity references,
character references,
comments,
CDATA section delimiters,
document type declarations, and
processing instructions.
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
""".
2.5. Comments
Comments may
appear anywhere in a document outside other
markup; in addition,
they may appear within the document type declaration
at places allowed by the grammar.
They are not part of the document's character
data; an XML
processor may, but need not, make it possible for an application to
retrieve the text of comments.
For compatibility, the string
"--" (double-hyphen) must not occur within
comments.
An example of a comment:
2.6. Processing Instructions
Processing
instructions (PIs) allow documents to contain instructions
for applications.
PIs are not part of the document's character
data, but must be passed through to the application. The
PI begins with a target (PITarget) used
to identify the application to which the instruction is directed.
The target names "XML", "xml", and so on are
reserved for standardization in this or future versions of this
specification.
The
XML Notation mechanism
may be used for
formal declaration of PI targets.
2.7. CDATA Sections
CDATA sections
may occur
anywhere character data may occur; they are
used to escape blocks of text containing characters which would
otherwise be recognized as markup. CDATA sections begin with the
string "<![CDATA[" and end with the string
"]]>":
An example of a CDATA section, in which "<greeting>" and
"</greeting>"
are recognized as character data, not
markup:
2.8. Prolog and Document Type Declaration
XML documents
may, and should,
begin with an XML declaration which specifies
the version of
XML being used.
For example, the following is a complete XML document, well-formed but not
valid:
and so is this:
The version number "1.0" should be used to indicate
conformance to this version of this specification; it is an error
for a document to use the value "1.0"
if it does not conform to this version of this specification.
It is the intent
of the XML working group to give later versions of this specification
numbers other than "1.0", but this intent does not
indicate a
commitment to produce any future versions of XML, nor if any are produced, to
use any particular numbering scheme.
Since future versions are not ruled out, this construct is provided
as a means to allow the possibility of automatic version recognition, should
it become necessary.
Processors may signal an error if they receive documents labeled with
versions they do not support.
The function of the markup in an XML document is to describe its
storage and logical structure and to associate attribute-value pairs
with its logical structures. XML provides a mechanism, the document type declaration, to define
constraints on the logical structure and to support the use of
predefined storage units.
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:
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:
If both the external and internal subsets are used, the internal subset is considered to occur before the external subset. This has the effect that entity and attribute-list declarations in the internal subset take precedence over those in the external subset. 2.9. Standalone Document Declaration
Markup declarations can affect the content of the document,
as passed from an XML processor
to an application; examples are attribute defaults and entity
declarations.
The standalone document declaration,
which may appear as a component of the XML declaration, signals
whether or not there are such declarations which appear external to
the document entity.
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:
An example XML declaration with a standalone document declaration:
2.10. White Space Handling
In editing XML documents, it is often convenient to use "white space"
(spaces, tabs, and blank lines, 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:
The value "default" signals that applications'
default white-space processing modes are acceptable for this element; the
value "preserve" indicates the intent that applications preserve
all the white space.
This declared intent is considered to apply to all elements within the content
of the element where it is specified, unless overriden with another instance
of the xml:space attribute.
The root element of any document
is considered to have signaled no intentions as regards application space
handling, unless it provides a value for
this attribute or the attribute is declared with a default value.
2.11. End-of-Line Handling
XML parsed entities are often stored in
computer files which, for editing convenience, are organized into lines.
These lines are typically separated by some combination of the characters
carriage-return (#xD) and line-feed (#xA).
To simplify the tasks of applications,
wherever an external parsed entity or the literal entity value
of an internal parsed entity contains either the literal
two-character sequence "#xD#xA" or a standalone literal
#xD, an XML processor must
pass to the application the single character #xA.
(This behavior can
conveniently be produced by normalizing all
line breaks to #xA on input, before parsing.)
2.12. Language Identification
In document processing, it is often useful to
identify the natural or formal language
in which the content is
written.
A special attribute named
xml:lang may be inserted in
documents to specify the
language used in the contents and attribute values
of any element in an XML document.
In valid documents, this attribute, like any other, must be
declared if it is used.
The values of the attribute are language identifiers as defined
by [IETF RFC 1766], "Tags for the Identification of Languages":
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:
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
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:
3. Logical Structures
Each XML document contains one or more
elements, the boundaries of which are
either delimited by start-tags
and end-tags, or, for empty elements, by an empty-element tag. Each element has a type,
identified by name, sometimes called its "generic
identifier" (GI), and may have a set of
attribute specifications. Each attribute specification
has a name and a value.
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:
3.1. Start-Tags, End-Tags, and Empty-Element Tags
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:
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:
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:
3.2. Element Type Declarations
The element structure of an
XML document may, for
validation purposes,
be constrained
using element type and attribute-list declarations.
An element type declaration constrains the element's
content.
Element type declarations often constrain which element types can
appear as children of the element.
At user option, an XML processor may issue a warning
when a declaration mentions an element type for which no declaration
is provided, but this is not an error.
An element
type declaration takes the form:
Validity Constraint: Unique Element Type Declaration
No element type may be declared more than once.
Examples of element type declarations:
3.2.1. Element Content
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:
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 [Appendix 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:
3.2.2. Mixed Content
An element
type has
mixed content when elements of that type may contain
character data, optionally interspersed with
child elements.
In this case, the types of the child elements
may be constrained, but not their order or their number of occurrences:
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:
3.3. Attribute-List Declarations
Attributes are used to associate
name-value pairs with elements.
Attribute specifications may appear only within start-tags
and empty-element tags;
thus, the productions used to
recognize them appear in [3.1. Start-Tags, End-Tags, and Empty-Element Tags].
Attribute-list
declarations may be used:
Attribute-list declarations specify the name, data type, and default
value (if any) of each attribute associated with a given element type:
When more than one AttlistDecl is provided for a
given element type, the contents of all those provided are merged. When
more than one definition is provided for the same attribute of a
given element type, the first declaration is binding and later
declarations are ignored.
For interoperability, writers of DTDs
may choose to provide at most one attribute-list declaration
for a given element type, at most one attribute definition
for a given attribute name, and at least one attribute definition
in each attribute-list declaration.
For interoperability, an XML processor may at user option
issue a warning when more than one attribute-list declaration is
provided for a given element type, or more than one attribute definition
is provided
for a given attribute, but this is not an error.
3.3.1. Attribute Types
XML attribute types are of three kinds: a string type, a
set of tokenized types, and enumerated types. The string type may take
any literal string as a value; the tokenized types have varying lexical
and semantic constraints, as noted:
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
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
Enumerated attributes can take one
of a list of values provided in the declaration. There are two
kinds of enumerated types:
Validity Constraint: Notation Attributes
Values of this type must match
one of the notation names included in
the declaration; all notation names in the declaration must
be declared.
Validity Constraint: Enumeration
Values of this type
must match one of the Nmtoken tokens in the
declaration.
For interoperability, the same
Nmtoken should not occur more than once in the
enumerated attribute types of a single element type.
3.3.2. Attribute Defaults
An attribute declaration provides
information on whether
the attribute's presence is required, and if not, how an XML processor should
react if a declared attribute is absent in a document.
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:
3.3.3. Attribute-Value Normalization
Before the value of an attribute is passed to the application
or checked for validity, the
XML processor must normalize it as follows:
If the declared value is not CDATA, then the XML processor must
further process the normalized attribute value by discarding any
leading and trailing space (#x20) characters, and by replacing
sequences of space (#x20) characters by a single space (#x20)
character.
All attributes for which no declaration has been read should be treated
by a non-validating parser as if declared
CDATA.
3.4. Conditional Sections
Conditional sections are portions of the
document type declaration external subset
which are
included in, or excluded from, the logical structure of the DTD based on
the keyword which governs them.
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:
4. Physical Structures
An XML document may consist
of one or many storage units. These are called
entities; they all have content and are all
(except for the document entity, see below, and
the external DTD subset)
identified by name.
Each XML document has one entity
called the document entity, which serves
as the starting point for the XML
processor and may contain the whole document.
Entities may be either parsed or unparsed.
A parsed entity's
contents are referred to as its
replacement text;
this text is considered an
integral part of the document.
An
unparsed entity
is a resource whose contents may or may not be
text, and if text, may not be XML.
Each unparsed entity
has an associated notation, identified by name.
Beyond a requirement
that an XML processor make the identifiers for the entity and
notation available to the application,
XML places no constraints on the contents of unparsed entities.
Parsed entities are invoked by name using entity references;
unparsed entities by name, given in the value of ENTITY
or ENTITIES
attributes.
General entities
are entities for use within the document content.
In this specification, general entities are sometimes referred
to with the unqualified term entity when this leads
to no ambiguity.
Parameter entities
are parsed entities for use within the DTD.
These two types of entities use different forms of reference and
are recognized in different contexts.
Furthermore, they occupy different namespaces; a parameter entity and
a general entity with the same name are two distinct entities.
4.1. Character and Entity References
A character reference refers to a specific character in the
ISO/IEC 10646 character set, for example one not directly accessible from
available input devices.
Well-Formedness Constraint: Legal Character
Characters referred to using character references must
match the production for
Char.
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:
Example of a parameter-entity reference:
4.2. Entity Declarations
Entities are declared thus:
4.2.1. Internal Entities
If
the entity definition is an
EntityValue,
the defined entity is called an internal entity.
There is no separate physical
storage object, and the content of the entity is given in the
declaration.
Note that some processing of entity and character references in the
literal entity value may be required to
produce the correct replacement
text: see [4.5. Construction of Internal Entity Replacement Text].
An internal entity is a parsed
entity.
Example of an internal entity declaration:
4.2.2. External Entities
If the entity is not
internal, it is an external
entity, declared as follows:
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:
4.3. Parsed Entities
4.3.1. The Text Declaration
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.
4.3.2. Well-Formed Parsed Entities
The document entity is well-formed if it matches the production labeled
document.
An external general
parsed entity is well-formed if it matches the production labeled
extParsedEnt.
An external parameter
entity is well-formed if it matches the production labeled
extPE.
A consequence of well-formedness in entities is that the logical
and physical structures in an XML document are properly nested; no
start-tag,
end-tag,
empty-element tag,
element,
comment,
processing instruction,
character
reference, or
entity reference
can begin in one entity and end in another.
4.3.3. Character Encoding in Entities
Each external parsed entity in an XML document may use a different
encoding for its characters. All XML processors must be able to read
entities in either UTF-8 or UTF-16.
Entities encoded in UTF-16 must
begin with the Byte Order Mark described by ISO/IEC 10646 Annex E and
Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF).
This is an encoding signature, not part of either the markup or the
character data of the XML document.
XML processors must be able to use this character to
differentiate between UTF-8 and UTF-16 encoded documents.
Although an XML processor is required to read only entities in
the UTF-8 and UTF-16 encodings, it is recognized that other encodings are
used around the world, and it may be desired for XML processors
to read entities that use them.
Parsed entities which are stored in an encoding other than
UTF-8 or UTF-16 must begin with a text
declaration containing an encoding declaration:
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:
4.4. XML Processor Treatment of Entities and References
The table below summarizes the contexts in which character references,
entity references, and invocations of unparsed entities might appear and the
required behavior of an XML processor in
each case.
The labels in the leftmost column describe the recognition context:
Reference in Content
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.
4.4.1. Not Recognized
Outside the DTD, the % character has no
special significance; thus, what would be parameter entity references in the
DTD are not recognized as markup in content.
Similarly, the names of unparsed entities are not recognized except
when they appear in the value of an appropriately declared attribute.
4.4.2. Included
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.
4.4.3. Included If Validating
When an XML processor recognizes a reference to a parsed entity, in order
to validate
the document, the processor must
include its
replacement text.
If the entity is external, and the processor is not
attempting to validate the XML document, the
processor may, but need not,
include the entity's replacement text.
If a non-validating 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.
4.4.4. Forbidden
The following are forbidden, and constitute
fatal errors:
4.4.5. Included in Literal
When an entity reference appears in an
attribute value, or a parameter entity reference appears in a literal entity
value, its replacement text is
processed in place of the reference itself as though it
were part of the document at the location the reference was recognized,
except that a single or double quote character in the replacement text
is always treated as a normal data character and will not terminate the
literal.
For example, this is well-formed:
while this is not:
4.4.6. Notify
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.
4.4.7. Bypassed
When a general entity reference appears in the
EntityValue in an entity declaration,
it is bypassed and left as is.
4.4.8. Included as PE
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.
4.5. Construction of Internal Entity Replacement Text
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:
then the replacement text for the entity "book" is: 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
[Appendix D. Expansion of Entity and Character References].
4.6. Predefined Entities
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.
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. 4.7. Notation Declarations
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.)
4.8. Document Entity
The document
entity serves as the root of the entity
tree and a starting-point for an XML
processor.
This specification does
not specify how the document entity is to be located by an XML
processor; unlike other entities, the document entity has no name and might
well appear on a processor input stream
without any identification at all.
5. Conformance
5.1. Validating and Non-Validating Processors
Conforming XML processors fall into two
classes: validating and non-validating.
Validating and non-validating processors alike must report
violations of this specification's well-formedness constraints
in the content of the
document entity and any
other parsed entities that
they read.
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.
5.2. Using XML Processors
The behavior of a validating XML processor is highly predictable; it
must read every piece of a document and report all well-formedness and
validity violations.
Less is required of a non-validating processor; it need not read any
part of the document other than the document entity.
This has two effects that may be important to users of XML processors:
For maximum reliability in interoperating between different XML
processors, applications which use non-validating processors should not
rely on any behaviors not required of such processors.
Applications which require facilities such as the use of default
attributes or internal entities which are declared in external
entities should use validating XML processors.
6. Notation
The formal grammar of XML is given in this specification using a simple
Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines
one symbol, in the form
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.
(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.
/* ... */
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.
Appendix A. References
A.1. Normative References
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.
A.2. Other References
Aho/Ullman
Aho, Alfred V.,
Ravi Sethi, and Jeffrey D. Ullman.
Compilers: Principles, Techniques, and Tools.
Reading: Addison-Wesley, 1986, rpt. corr. 1988.
Berners-Lee et al.
Berners-Lee, T., R. Fielding, and L. Masinter.
Uniform Resource Identifiers (URI): Generic Syntax and
Semantics.
1997.
(Work in progress; see updates to RFC1738.)
Brüggemann-Klein
Brüggemann-Klein, Anne.
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.
Appendix B. Character Classes
Following the characteristics defined in the Unicode standard,
characters are classed as base characters (among others, these
contain the alphabetic characters of the Latin alphabet, 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.
The character classes defined here can be derived from the
Unicode character database as follows:
Appendix C. XML and SGML (Non-Normative)
Appendix D. Expansion of Entity and Character References (Non-Normative)
This appendix contains some examples illustrating the
sequence of entity- and character-reference recognition and
expansion, as specified in [4.4. XML Processor Treatment of Entities and References].
If the DTD contains the declaration
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": 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):
A more complex example will illustrate the rules and their
effects fully. In the following example, the line numbers are
solely for reference.
This produces the following:
Appendix E. Deterministic Content Models (Non-Normative)
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].
Appendix F. Autodetection of Character Encodings (Non-Normative)
The XML encoding declaration functions as an internal label on each
entity, indicating which character encoding is in use. Before an XML
processor can read the internal label, however, it apparently has to
know what character encoding is in use--which is what the internal label
is trying to indicate. In the general case, this is a hopeless
situation. It is not entirely hopeless in XML, however, because XML
limits the general case in two ways: each implementation is assumed
to support only a finite set of character encodings, and the XML
encoding declaration is restricted in position and content in order to
make it feasible to autodetect the character encoding in use in each
entity in normal cases. Also, in many cases other sources of information
are available in addition to the XML data stream itself.
Two cases may be distinguished,
depending on whether the XML entity is presented to the
processor without, or with, any accompanying
(external) information. We consider the first case first.
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".
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.
Appendix G. W3C XML Working Group (Non-Normative)
This specification was prepared and approved for publication by the
W3C XML Working Group (WG). WG approval of this specification does
not necessarily imply that all WG members voted for its approval.
The current and former members of the XML WG are:
Jon Bosak, Sun (Chair); James Clark (Technical Lead); Tim Bray, Textuality and Netscape (XML Co-editor); Jean Paoli, Microsoft (XML Co-editor); C. M. Sperberg-McQueen, U. of Ill. (XML
Co-editor); Dan Connolly, W3C (W3C Liaison); Paula Angerstein, Texcel; Steve DeRose, INSO; Dave Hollander, HP; Eliot Kimber, ISOGEN; Eve Maler, ArborText; Tom Magliery, NCSA; Murray Maloney, Muzmo and Grif; Makoto Murata, Fuji Xerox Information Systems; Joel Nava, Adobe; Conleth O'Connell, Vignette; Peter Sharpe, SoftQuad; John Tigue, DataChannel
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