Extensible Markup Language (XML) 1.0
W3C Recommendation 10-February-1998
This version:
http://www.w3.org/TR/1998/REC-xml-19980210
http://www.w3.org/TR/1998/REC-xml-19980210.xml
http://www.w3.org/TR/1998/REC-xml-19980210.html
http://www.w3.org/TR/1998/REC-xml-19980210.pdf
http://www.w3.org/TR/1998/REC-xml-19980210.ps
Latest version:
http://www.w3.org/TR/REC-xml
Previous version:
http://www.w3.org/TR/PR-xml-971208
Editors:
Tim Bray (Textuality and Netscape)
Jean Paoli (Microsoft)
C. M. Sperberg-McQueen (University of Illinois at Chicago)
Abstract
The Extensible Markup Language (XML) is a subset of SGML that is
completely described in this document. Its goal is to enable generic
SGML to be served, received, and processed on the Web in the way that
is now possible with HTML. XML has been designed for ease of
implementation and for interoperability with both SGML and HTML.
Status of this document
This document has been reviewed by W3C Members and other interested
parties and has been endorsed by the Director as a W3C Recommendation.
It is a stable document and may be used as reference material or cited
as a normative reference from another document. W3C's role in making
the Recommendation is to draw attention to the specification and to
promote its widespread deployment. This enhances the functionality and
interoperability of the Web.
This document specifies a syntax created by subsetting an existing,
widely used international text processing standard (Standard
Generalized Markup Language, ISO 8879:1986(E) as amended and corrected)
for use on the World Wide Web. It is a product of the W3C XML Activity,
details of which can be found at http://www.w3.org/XML. A list of
current W3C Recommendations and other technical documents can be found
at http://www.w3.org/TR.
This specification uses the term URI, which is defined by [Berners-Lee
et al.], a work in progress expected to update [IETF RFC1738] and [IETF
RFC1808].
The list of known errors in this specification is available at
http://www.w3.org/XML/xml-19980210-errata.
Please report errors in this document to xml-editor@w3.org.
Extensible Markup Language (XML) 1.0
Table of Contents
1. Introduction
1.1 Origin and Goals
1.2 Terminology
2. Documents
2.1 Well-Formed XML Documents
2.2 Characters
2.3 Common Syntactic Constructs
2.4 Character Data and Markup
2.5 Comments
2.6 Processing Instructions
2.7 CDATA Sections
2.8 Prolog and Document Type Declaration
2.9 Standalone Document Declaration
2.10 White Space Handling
2.11 End-of-Line Handling
2.12 Language Identification
3. Logical Structures
3.1 Start-Tags, End-Tags, and Empty-Element Tags
3.2 Element Type Declarations
3.2.1 Element Content
3.2.2 Mixed Content
3.3 Attribute-List Declarations
3.3.1 Attribute Types
3.3.2 Attribute Defaults
3.3.3 Attribute-Value Normalization
3.4 Conditional Sections
4. Physical Structures
4.1 Character and Entity References
4.2 Entity Declarations
4.2.1 Internal Entities
4.2.2 External Entities
4.3 Parsed Entities
4.3.1 The Text Declaration
4.3.2 Well-Formed Parsed Entities
4.3.3 Character Encoding in Entities
4.4 XML Processor Treatment of Entities and References
4.4.1 Not Recognized
4.4.2 Included
4.4.3 Included If Validating
4.4.4 Forbidden
4.4.5 Included in Literal
4.4.6 Notify
4.4.7 Bypassed
4.4.8 Included as PE
4.5 Construction of Internal Entity Replacement Text
4.6 Predefined Entities
4.7 Notation Declarations
4.8 Document Entity
5. Conformance
5.1 Validating and Non-Validating Processors
5.2 Using XML Processors
6. Notation
Appendices
A. References
A.1 Normative References
A.2 Other References
B. Character Classes
C. XML and SGML (Non-Normative)
D. Expansion of Entity and Character References (Non-Normative)
E. Deterministic Content Models (Non-Normative)
F. Autodetection of Character Encodings (Non-Normative)
G. W3C XML Working Group (Non-Normative)
__________________________________________________________________
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:
1. XML shall be straightforwardly usable over the Internet.
2. XML shall support a wide variety of applications.
3. XML shall be compatible with SGML.
4. It shall be easy to write programs which process XML documents.
5. The number of optional features in XML is to be kept to the
absolute minimum, ideally zero.
6. XML documents should be human-legible and reasonably clear.
7. The XML design should be prepared quickly.
8. The design of XML shall be formal and concise.
9. XML documents shall be easy to create.
10. Terseness in XML markup is of minimal importance.
This specification, together with associated standards (Unicode and
ISO/IEC 10646 for characters, Internet RFC 1766 for language
identification tags, ISO 639 for language name codes, and ISO 3166 for
country name codes), provides all the information necessary to
understand XML Version 1.0 and construct computer programs to process
it.
This version of the XML specification may be distributed freely, as
long as all text and legal notices remain intact.
1.2 Terminology
The terminology used to describe XML documents is defined in the body
of this specification. The terms defined in the following list are used
in building those definitions and in describing the actions of an XML
processor:
may
Conforming documents and XML processors are permitted to but
need not behave as described.
must
Conforming documents and XML processors are required to behave
as described; otherwise they are in error.
error
A violation of the rules of this specification; results are
undefined. Conforming software may detect and report an error
and may recover from it.
fatal error
An error which a conforming XML processor must detect and report
to the application. After encountering a fatal error, the
processor may continue processing the data to search for further
errors and may report such errors to the application. In order
to support correction of errors, the processor may make
unprocessed data from the document (with intermingled character
data and markup) available to the application. Once a fatal
error is detected, however, the processor must not continue
normal processing (i.e., it must not continue to pass character
data and information about the document's logical structure to
the application in the normal way).
at user option
Conforming software may or must (depending on the modal verb in
the sentence) behave as described; if it does, it must provide
users a means to enable or disable the behavior described.
validity constraint
A rule which applies to all valid XML documents. Violations of
validity constraints are errors; they must, at user option, be
reported by validating XML processors.
well-formedness constraint
A rule which applies to all well-formed XML documents.
Violations of well-formedness constraints are fatal errors.
match
(Of strings or names:) Two strings or names being compared must
be identical. Characters with multiple possible representations
in ISO/IEC 10646 (e.g. characters with both precomposed and
base+diacritic forms) match only if they have the same
representation in both strings. At user option, processors may
normalize such characters to some canonical form. No case
folding is performed. (Of strings and rules in the grammar:) A
string matches a grammatical production if it belongs to the
language generated by that production. (Of content and content
models:) An element matches its declaration when it conforms in
the fashion described in the constraint "Element Valid".
for compatibility
A feature of XML included solely to ensure that XML remains
compatible with SGML.
for interoperability
A non-binding recommendation included to increase the chances
that XML documents can be processed by the existing installed
base of SGML processors which predate the WebSGML Adaptations
Annex to ISO 8879.
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:
1. Taken as a whole, it matches the production labeled document.
2. It meets all the well-formedness constraints given in this
specification.
3. Each of the parsed entities which is referenced directly or
indirectly within the document is well-formed.
Document
[1] document ::= prolog element Misc*
Matching the document production implies that:
1. It contains one or more elements.
2. There is exactly one element, called the root, or document element,
no part of which appears in the content of any other element. For
all other elements, if the start-tag is in the content of another
element, the end-tag is in the content of the same element. More
simply stated, the elements, delimited by start- and end-tags, nest
properly within each other.
As a consequence of this, for each non-root element C in the document,
there is one other element P in the document such that C is in the
content of P, but is not in the content of any other element that is in
the content of P. P is referred to as the parent of C, and C as a child
of P.
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.
Character Range
[2] Char ::= #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD]
| [#x10000-#x10FFFF] /* any Unicode character, excluding the surrogate
blocks, FFFE, and FFFF. */
The mechanism for encoding character code points into bit patterns may
vary from entity to entity. All XML processors must accept the UTF-8
and UTF-16 encodings of 10646; the mechanisms for signaling which of
the two is in use, or for bringing other encodings into play, are
discussed later, in "4.3.3 Character Encoding in Entities".
2.3 Common Syntactic Constructs
This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x20) characters,
carriage returns, line feeds, or tabs.
White Space
[3] S ::= (#x20 | #x9 | #xD | #xA)+
Characters are classified for convenience as letters, digits, or other
characters. Letters consist of an alphabetic or syllabic base character
possibly followed by one or more combining characters, or of an
ideographic character. Full definitions of the specific characters in
each class are given in "B. Character Classes".
A Name is a token beginning with a letter or one of a few punctuation
characters, and continuing with letters, digits, hyphens, underscores,
colons, or full stops, together known as name characters. Names
beginning with the string "xml", or any string which would match
(('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in
this or future versions of this specification.
Note: The colon character within XML names is reserved for
experimentation with name spaces. Its meaning is expected to be
standardized at some future point, at which point those documents using
the colon for experimental purposes may need to be updated. (There is
no guarantee that any name-space mechanism adopted for XML will in fact
use the colon as a name-space delimiter.) In practice, this means that
authors should not use the colon in XML names except as part of
name-space experiments, but that XML processors should accept the colon
as a name character.
An Nmtoken (name token) is any mixture of name characters.
Names and Tokens
[4] NameChar ::= Letter | Digit | '.' | '-' | '_' | ':'
| CombiningChar | Extender
[5] Name ::= (Letter | '_' | ':') (NameChar)*
[6] Names ::= Name (S Name)*
[7] Nmtoken ::= (NameChar)+
[8] Nmtokens ::= Nmtoken (S Nmtoken)*
Literal data is any quoted string not containing the quotation mark
used as a delimiter for that string. Literals are used for specifying
the content of internal entities (EntityValue), the values of
attributes (AttValue), and external identifiers (SystemLiteral). Note
that a SystemLiteral can be parsed without scanning for markup.
Literals
[9] EntityValue ::= '"' ([^%&"] | PEReference | Reference)* '"'
| "'" ([^%&'] | PEReference | Reference)* "'"
[10] AttValue ::= '"' ([^<&"] | Reference)* '"'
| "'" ([^<&'] | Reference)* "'"
[11] SystemLiteral ::= ('"' [^"]* '"') | ("'" [^']* "'")
[12] PubidLiteral ::= '"' PubidChar* '"' | "'" (PubidChar - "'")* "'"
[13] PubidChar ::= #x20 | #xD | #xA | [a-zA-Z0-9]
| [-'()+,./:=?;!*#@$_%]
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.
All text that is not markup constitutes the character data of the
document.
The ampersand character (&) and the left angle bracket (<) may appear
in their literal form only when used as markup delimiters, or within a
comment, a processing instruction, or a CDATA section. They are also
legal within the literal entity value of an internal entity
declaration; see "4.3.2 Well-Formed Parsed Entities". If they are
needed elsewhere, they must be escaped using either numeric character
references or the strings "&" and "<" respectively. The right
angle bracket (>) may be represented using the string ">", and must,
for compatibility, be escaped using ">" or a character reference
when it appears in the string "]]>" in content, when that string is not
marking the end of a CDATA section.
In the content of elements, character data is any string of characters
which does not contain the start-delimiter of any markup. In a CDATA
section, character data is any string of characters not including the
CDATA-section-close delimiter, "]]>".
To allow attribute values to contain both single and double quotes, the
apostrophe or single-quote character (') may be represented as
"'", and the double-quote character (") as """.
Character Data
[14] CharData ::= [^<&]* - ([^<&]* ']]>' [^<&]*)
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.
Comments
[15] Comment ::= ''
An example of a comment:
2.6 Processing Instructions
Processing instructions (PIs) allow documents to contain instructions
for applications.
Processing Instructions
[16] PI ::= '' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
[17] PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))
PIs are not part of the document's character data, but must be passed
through to the application. The PI begins with a target (PITarget) used
to identify the application to which the instruction is directed. The
target names "XML", "xml", and so on are reserved for standardization
in this or future versions of this specification. The XML Notation
mechanism may be used for formal declaration of PI targets.
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 Sections
[18] CDSect ::= CDStart CData CDEnd
[19] CDStart ::= '' Char*))
[21] CDEnd ::= ']]>'
Within a CDATA section, only the CDEnd string is recognized as markup,
so that left angle brackets and ampersands may occur in their literal
form; they need not (and cannot) be escaped using "<" and "&".
CDATA sections cannot nest.
An example of a CDATA section, in which "" and ""
are recognized as character data, not markup:
Hello, world!]]>
2.8 Prolog and Document Type Declaration
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:
Hello, world!
and so is this:
Hello, world!
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.
Prolog
[22] prolog ::= XMLDecl? Misc* (doctypedecl Misc*)?
[23] XMLDecl ::= ''
[24] VersionInfo ::= S 'version' Eq (' VersionNum ' | " VersionNum ")
[25] Eq ::= S? '=' S?
[26] VersionNum ::= ([a-zA-Z0-9_.:] | '-')+
[27] Misc ::= Comment | PI | S
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".
Document Type Definition
[28] doctypedecl ::= '' [ VC: Root Element Type ]
[29] markupdecl ::= elementdecl | AttlistDecl | EntityDecl
| NotationDecl | PI | Comment [ VC: Proper Declaration/PE Nesting ]
[ WFC: PEs in Internal Subset ]
The markup declarations may be made up in whole or in part of the
replacement text of parameter entities. The productions later in this
specification for individual nonterminals (elementdecl, AttlistDecl,
and so on) describe the declarations after all the parameter entities
have been included.
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
[30] extSubset ::= TextDecl? extSubsetDecl
[31] extSubsetDecl ::= ( markupdecl | conditionalSect | PEReference
| S )*
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:
Hello, world!
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:
]>
Hello, world!
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.
Standalone Document Declaration
[32] SDDecl ::= S 'standalone' Eq (("'" ('yes' | 'no') "'") | ('"'
('yes' | 'no') '"')) [ VC: 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:
* attributes with default values, if elements to which these
attributes apply appear in the document without specifications of
values for these attributes, or
* entities (other than amp, lt, gt, apos, quot), if references to
those entities appear in the document, or
* attributes with values subject to normalization, where the
attribute appears in the document with a value which will change as
a result of normalization, or
* element types with element content, if white space occurs directly
within any instance of those types.
An example XML declaration with a standalone document declaration:
2.10 White Space Handling
In editing XML documents, it is often convenient to use "white space"
(spaces, tabs, and blank lines, 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":
Language Identification
[33] LanguageID ::= Langcode ('-' Subcode)*
[34] Langcode ::= ISO639Code | IanaCode | UserCode
[35] ISO639Code ::= ([a-z] | [A-Z]) ([a-z] | [A-Z])
[36] IanaCode ::= ('i' | 'I') '-' ([a-z] | [A-Z])+
[37] UserCode ::= ('x' | 'X') '-' ([a-z] | [A-Z])+
[38] Subcode ::= ([a-z] | [A-Z])+
The Langcode may be any of the following:
* a two-letter language code as defined by [ISO 639], "Codes for the
representation of names of languages"
* a language identifier registered with the Internet Assigned Numbers
Authority [IANA]; these begin with the prefix "i-" (or "I-")
* a language identifier assigned by the user, or agreed on between
parties in private use; these must begin with the prefix "x-" or
"X-" in order to ensure that they do not conflict with names later
standardized or registered with IANA
There may be any number of Subcode segments; if the first subcode
segment exists and the Subcode consists of two letters, then it must be
a country code from [ISO 3166], "Codes for the representation of names
of countries." If the first subcode consists of more than two letters,
it must be a subcode for the language in question registered with IANA,
unless the Langcode begins with the prefix "x-" or "X-".
It is customary to give the language code in lower case, and the
country code (if any) in upper case. Note that these values, unlike
other names in XML documents, are case insensitive.
For example:
The quick brown fox jumps over the lazy dog.
What colour is it?
What color is it?
Habe nun, ach! Philosophie,
Juristerei, und Medizin
und leider auch Theologie
durchaus studiert mit heißem Bemüh'n.
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:
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.
Element
[39] element ::= EmptyElemTag
| STag content ETag [ WFC: Element Type Match ]
[ VC: Element Valid ]
This specification does not constrain the semantics, use, or (beyond
syntax) names of the element types and attributes, except that names
beginning with a match to (('X'|'x')('M'|'m')('L'|'l')) are reserved
for standardization in this or future versions of this specification.
Well-Formedness Constraint: Element Type Match
The Name in an element's end-tag must match the element type in the
start-tag.
Validity Constraint: Element Valid
An element is valid if there is a declaration matching elementdecl
where the Name matches the element type, and one of the following
holds:
1. The declaration matches EMPTY and the element has no content.
2. The declaration matches children and the sequence of child elements
belongs to the language generated by the regular expression in the
content model, with optional white space (characters matching the
nonterminal S) between each pair of child elements.
3. The declaration matches Mixed and the content consists of character
data and child elements whose types match names in the content
model.
4. The declaration matches ANY, and the types of any child elements
have been declared.
3.1 Start-Tags, End-Tags, and Empty-Element Tags
The beginning of every non-empty XML element is marked by a start-tag.
Start-tag
[40] STag ::= '<' Name (S Attribute)* S? '>' [ WFC: Unique Att Spec ]
[41] Attribute ::= Name Eq AttValue [ VC: Attribute Value Type ]
[ WFC: No External Entity References ]
[ WFC: No < in Attribute Values ]
The Name in the start- and end-tags gives the element's type. 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:
End-tag
[42] ETag ::= '' Name S? '>'
An example of an end-tag:
The text between the start-tag and end-tag is called the element's
content:
Content of Elements
[43] content ::= (element | CharData | Reference | CDSect | PI
| Comment)*
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:
Tags for Empty Elements
[44] EmptyElemTag ::= '<' Name (S Attribute)* S? '/>' [ WFC: Unique
Att Spec ]
Empty-element tags may be used for any element which has no content,
whether or not it is declared using the keyword EMPTY. For
interoperability, the empty-element tag 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:
Element Type Declaration
[45] elementdecl ::= '' [ VC:
Unique Element Type Declaration ]
[46] contentspec ::= 'EMPTY' | 'ANY' | Mixed | children
where the Name gives the element type being declared.
Validity Constraint: Unique Element Type Declaration
No element type may be declared more than once.
Examples of element type declarations:
3.2.1 Element Content
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
[47] children ::= (choice | seq) ('?' | '*' | '+')?
[48] cp ::= (Name | choice | seq) ('?' | '*' | '+')?
[49] choice ::= '(' S? cp ( S? '|' S? cp )* S? ')' [ VC: Proper
Group/PE Nesting ]
[50] seq ::= '(' S? cp ( S? ',' S? cp )* S? ')' [ VC: Proper Group/PE
Nesting ]
where each Name is the type of an element which may appear as a child.
Any content particle in a choice list may appear in the element content
at the location where the choice list appears in the grammar; content
particles occurring in a sequence list must each appear in the element
content in the order given in the list. The optional character
following a name or list governs whether the element or the content
particles in the list may occur one or more (+), zero or more (*), or
zero or one times (?). The absence of such an operator means that the
element or content particle must appear exactly once. This syntax and
meaning are identical to those used in the productions in this
specification.
The content of an element matches a content model if and only if it is
possible to trace out a path through the content model, obeying the
sequence, choice, and repetition operators and matching each element in
the content against an element type in the content model. For
compatibility, it is an error if an element in the document can match
more than one occurrence of an element type in the content model. For
more information, see "E. Deterministic Content Models".
Validity Constraint: Proper Group/PE Nesting
Parameter-entity replacement text must be properly nested with
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:
Mixed-content Declaration
[51] Mixed ::= '(' S? '#PCDATA' (S? '|' S? Name)* S? ')*'
| '(' S? '#PCDATA' S? ')' [ VC: Proper Group/PE Nesting ]
[ VC: No Duplicate Types ]
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:
* To define the set of attributes pertaining to a given element type.
* To establish type constraints for these attributes.
* To provide default values for attributes.
Attribute-list declarations specify the name, data type, and default
value (if any) of each attribute associated with a given element type:
Attribute-list Declaration
[52] AttlistDecl ::= ''
[53] AttDef ::= S Name S AttType S DefaultDecl
The Name in the AttlistDecl rule is the type of an element. At user
option, an XML processor may issue a warning if attributes are declared
for an element type not itself declared, but this is not an error. The
Name in the AttDef rule is the name of the attribute.
When more than one AttlistDecl is provided for a given element type,
the contents of all those provided are merged. When more than one
definition is provided for the same attribute of a given element type,
the first declaration is binding and later declarations are ignored.
For interoperability, writers of DTDs may choose to provide at most one
attribute-list declaration for a given element type, at most one
attribute definition for a given attribute name, 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:
Attribute Types
[54] AttType ::= StringType | TokenizedType | EnumeratedType
[55] StringType ::= 'CDATA'
[56] TokenizedType ::= 'ID' [ VC: ID ]
[ VC: One ID per Element Type ]
[ VC: ID Attribute Default ]
| 'IDREF' [ VC: IDREF ]
| 'IDREFS' [ VC: IDREF ]
| 'ENTITY' [ VC: Entity Name ]
| 'ENTITIES' [ VC: Entity Name ]
| 'NMTOKEN' [ VC: Name Token ]
| 'NMTOKENS' [ VC: Name Token ]
Validity Constraint: ID
Values of type ID must match the Name production. A name must not
appear more than once in an XML document as a value of this type; i.e.,
ID values must uniquely identify the elements which bear them.
Validity Constraint: One ID per Element Type
No element type may have more than one ID attribute specified.
Validity Constraint: ID Attribute Default
An ID attribute must have a declared default of #IMPLIED or #REQUIRED.
Validity Constraint: IDREF
Values of type IDREF must match the Name production, and values of type
IDREFS must match Names; each Name must match the value of an ID
attribute on some element in the XML document; i.e. IDREF values must
match the value of some ID attribute.
Validity Constraint: Entity Name
Values of type ENTITY must match the Name production, values of type
ENTITIES must match Names; each Name must match the name of an unparsed
entity declared in the DTD.
Validity Constraint: Name Token
Values of type NMTOKEN must match the Nmtoken production; values of
type NMTOKENS must match Nmtokens.
Enumerated attributes can take one of a list of values provided in the
declaration. There are two kinds of enumerated types:
Enumerated Attribute Types
[57] EnumeratedType ::= NotationType | Enumeration
[58] NotationType ::= 'NOTATION' S '(' S? Name (S? '|' S? Name)* S?
')' [ VC: Notation Attributes ]
[59] Enumeration ::= '(' S? Nmtoken (S? '|' S? Nmtoken)* S? ')' [ VC:
Enumeration ]
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.
3.3.2 Attribute Defaults
An attribute declaration provides information on whether the
attribute's presence is required, and if not, how an XML processor
should react if a declared attribute is absent in a document.
Attribute Defaults
[60] DefaultDecl ::= '#REQUIRED' | '#IMPLIED'
| (('#FIXED' S)? AttValue) [ VC: Required Attribute ]
[ VC: Attribute Default Legal ]
[ WFC: No < in Attribute Values ]
[ VC: Fixed Attribute Default ]
In an attribute declaration, #REQUIRED means that the attribute must
always be provided, #IMPLIED that no default value is provided. 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:
* a character reference is processed by appending the referenced
character to the attribute value
* an entity reference is processed by recursively processing the
replacement text of the entity
* a whitespace character (#x20, #xD, #xA, #x9) is processed by
appending #x20 to the normalized value, except that only a single
#x20 is appended for a "#xD#xA" sequence that is part of an
external parsed entity or the literal entity value of an internal
parsed entity
* other characters are processed by appending them to the normalized
value
If the declared value is not CDATA, then the XML processor must further
process the normalized attribute value by discarding any leading and
trailing space (#x20) characters, and by replacing sequences of space
(#x20) characters by a single space (#x20) character.
All attributes for which no declaration has been read should be treated
by a non-validating parser as if declared CDATA.
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.
Conditional Section
[61] conditionalSect ::= includeSect | ignoreSect
[62] includeSect ::= ''
[63] ignoreSect ::= ''
[64] ignoreSectContents ::= Ignore (''
Ignore)*
[65] Ignore ::= Char* - (Char* ('') Char*)
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.
Character Reference
[66] CharRef ::= '' [0-9]+ ';'
| '' [0-9a-fA-F]+ ';' [ WFC: Legal Character ]
Well-Formedness Constraint: Legal Character
Characters referred to using character references must match the
production for Char.
If the character reference begins with "", the digits and letters up
to the terminating ; provide a hexadecimal representation of the
character's code point in ISO/IEC 10646. If it begins just with "",
the digits up to the terminating ; provide a decimal representation of
the character's code point.
An entity reference refers to the content of a named entity. References
to parsed general entities use ampersand (&) and semicolon (;) as
delimiters. Parameter-entity references use percent-sign (%) and
semicolon (;) as delimiters.
Entity Reference
[67] Reference ::= EntityRef | CharRef
[68] EntityRef ::= '&' Name ';' [ WFC: Entity Declared ]
[ VC: Entity Declared ]
[ WFC: Parsed Entity ]
[ WFC: No Recursion ]
[69] PEReference ::= '%' Name ';' [ VC: Entity Declared ]
[ WFC: No Recursion ]
[ WFC: In DTD ]
Well-Formedness Constraint: Entity Declared
In a document without any DTD, a document with only an internal DTD
subset which contains no parameter entity references, or a document
with "standalone='yes'", 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 less-than (<) to save options.
This document was prepared on &docdate; and
is classified &security-level;.
Example of a parameter-entity reference:
%ISOLat2;
4.2 Entity Declarations
Entities are declared thus:
Entity Declaration
[70] EntityDecl ::= GEDecl | PEDecl
[71] GEDecl ::= ''
[72] PEDecl ::= ''
[73] EntityDef ::= EntityValue | (ExternalID NDataDecl?)
[74] PEDef ::= EntityValue | ExternalID
The Name identifies the entity in an entity reference or, in the case
of an unparsed entity, in the value of an ENTITY or ENTITIES attribute.
If the same entity is declared more than once, the first declaration
encountered is binding; at user option, an XML processor may issue a
warning if entities are declared multiple times.
4.2.1 Internal Entities
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:
External Entity Declaration
[75] ExternalID ::= 'SYSTEM' S SystemLiteral
| 'PUBLIC' S PubidLiteral S SystemLiteral
[76] NDataDecl ::= S 'NDATA' S Name [ VC: Notation Declared ]
If the NDataDecl is present, this is a general unparsed entity;
otherwise it is a parsed entity.
Validity Constraint: Notation Declared
The Name must match the declared name of a notation.
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.
Text Declaration
[77] TextDecl ::= ''
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.
Well-Formed External Parsed Entity
[78] extParsedEnt ::= TextDecl? content
[79] extPE ::= TextDecl? extSubsetDecl
An internal general parsed entity is well-formed if its replacement
text matches the production labeled content. All internal parameter
entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and
physical structures in an XML document are properly nested; no
start-tag, end-tag, empty-element tag, element, comment, processing
instruction, character reference, or entity reference can begin in one
entity and end in another.
4.3.3 Character Encoding in Entities
Each external parsed entity in an XML document may use a different
encoding for its characters. All XML processors must be able to read
entities in either UTF-8 or UTF-16.
Entities encoded in UTF-16 must begin with the Byte Order Mark
described by ISO/IEC 10646 Annex E and Unicode Appendix B (the ZERO
WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature,
not part of either the markup or the character data of the XML
document. XML processors must be able to use this character to
differentiate between UTF-8 and UTF-16 encoded documents.
Although an XML processor is required to read only entities in the
UTF-8 and UTF-16 encodings, it is recognized that other encodings are
used around the world, and it may be desired for XML processors to read
entities that use them. Parsed entities which are stored in an encoding
other than UTF-8 or UTF-16 must begin with a text declaration
containing an encoding declaration:
Encoding Declaration
[80] EncodingDecl ::= S 'encoding' Eq ('"' EncName '"' | "'" EncName
"'" )
[81] EncName ::= [A-Za-z] ([A-Za-z0-9._] | '-')* /* Encoding name
contains only Latin characters */
In the document entity, the encoding declaration is part of the XML
declaration. The EncName is the name of the encoding used.
In an encoding declaration, the values "UTF-8", "UTF-16",
"ISO-10646-UCS-2", and "ISO-10646-UCS-4" should be used for the various
encodings and transformations of Unicode / ISO/IEC 10646, the values
"ISO-8859-1", "ISO-8859-2", ... "ISO-8859-9" should be used for the
parts of ISO 8859, and the values "ISO-2022-JP", "Shift_JIS", and
"EUC-JP" should be used for the various encoded forms of JIS
X-0208-1997. XML processors may recognize other encodings; it is
recommended that character encodings registered (as charsets) with the
Internet Assigned Numbers Authority [IANA], other than those just
listed, should be referred to using their registered names. Note that
these registered names are defined to be case-insensitive, so
processors wishing to match against them should do so in a
case-insensitive way.
In the absence of information provided by an external transport
protocol (e.g. HTTP or MIME), it is an error for an entity including an
encoding declaration to be presented to the XML processor in an
encoding other than that named in the declaration, for an encoding
declaration to occur other than at the beginning of an external entity,
or for an entity which begins with neither a Byte Order Mark nor an
encoding declaration to use an encoding other than UTF-8. Note that
since ASCII is a subset of UTF-8, ordinary ASCII entities do not
strictly need an encoding declaration.
It is a fatal error when an XML processor encounters an entity with an
encoding that it is unable to process.
Examples of encoding declarations:
4.4 XML Processor Treatment of Entities and References
The table below summarizes the contexts in which character references,
entity references, and invocations of unparsed entities might appear
and the required behavior of an XML processor in each case. The labels
in the leftmost column describe the recognition context:
Reference in Content
as a reference anywhere after the start-tag and before the
end-tag of an element; corresponds to the nonterminal content.
Reference in Attribute Value
as a reference within either the value of an attribute in a
start-tag, or a default value in an attribute declaration;
corresponds to the nonterminal AttValue.
Occurs as Attribute Value
as a Name, not a reference, appearing either as the value of an
attribute which has been declared as type ENTITY, or as one of
the space-separated tokens in the value of an attribute which
has been declared as type ENTITIES.
Reference in Entity Value
as a reference within a parameter or internal entity's literal
entity value in the entity's declaration; corresponds to the
nonterminal EntityValue.
Reference in DTD
as a reference within either the internal or external subsets of
the DTD, but outside of an EntityValue 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
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:
* the appearance of a reference to an unparsed entity.
* the appearance of any character or general-entity reference in the
DTD except within an EntityValue or AttValue.
* a reference to an external entity in an attribute value.
4.4.5 Included in Literal
When an entity reference appears in an attribute value, or a parameter
entity reference appears in a literal entity value, its replacement
text is processed in place of the reference itself as though it were
part of the document at the location the reference was recognized,
except that a single or double quote character in the replacement text
is always treated as a normal data character and will not terminate the
literal. For example, this is well-formed:
while this is not:
then the replacement text for the entity "book" is:
La Peste: Albert Camus,
© 1947 Éditions Gallimard. &rights;
The general-entity reference "&rights;" would be expanded should the
reference "&book;" appear in the document's content or an attribute
value.
These simple rules may have complex interactions; for a detailed
discussion of a difficult example, see "D. Expansion of Entity and
Character References".
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.
Notation Declarations
[82] NotationDecl ::= ''
[83] PublicID ::= 'PUBLIC' S PubidLiteral
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:
* Certain well-formedness errors, specifically those that require
reading external entities, may not be detected by a non-validating
processor. Examples include the constraints entitled Entity
Declared, Parsed Entity, and No Recursion, as well as some of the
cases described as forbidden in "4.4 XML Processor Treatment of
Entities and References".
* The information passed from the processor to the application may
vary, depending on whether the processor reads parameter and
external entities. For example, a non-validating processor may not
normalize attribute values, include the replacement text of
internal entities, or supply default attribute values, where doing
so depends on having read declarations in external or parameter
entities.
For maximum reliability in interoperating between different XML
processors, applications which use non-validating processors should not
rely on any behaviors not required of such processors. Applications
which require facilities such as the use of default attributes or
internal entities which are declared in external entities should use
validating XML processors.
6. Notation
The formal grammar of XML is given in this specification using a simple
Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar
defines one symbol, in the form
symbol ::= expression
Symbols are written with an initial capital letter if they are defined
by a regular expression, or with an initial lower case letter
otherwise. Literal strings are quoted.
Within the expression on the right-hand side of a rule, the following
expressions are used to match strings of one or more characters:
#xN
where N is a hexadecimal integer, the expression matches the
character in ISO/IEC 10646 whose canonical (UCS-4) code value,
when interpreted as an unsigned binary number, has the value
indicated. The number of leading zeros in the #xN form is
insignificant; the number of leading zeros in the corresponding
code value is governed by the character encoding in use and is
not significant for XML.
[a-zA-Z], [#xN-#xN]
matches any character with a value in the range(s) indicated
(inclusive).
[^a-z], [^#xN-#xN]
matches any character with a value outside the range indicated.
[^abc], [^#xN#xN#xN]
matches any character with a value not among the characters
given.
"string"
matches a literal string matching that given inside the double
quotes.
'string'
matches a literal string matching that given inside the single
quotes.
These symbols may be combined to match more complex patterns as
follows, where A and B represent simple expressions:
(expression)
expression is treated as a unit and may be combined as described
in this list.
A?
matches A or nothing; optional A.
A B
matches A followed by B.
A | B
matches A or B but not both.
A - B
matches any string that matches A but does not match B.
A+
matches one or more occurrences of A.
A*
matches zero or more occurrences of A.
Other notations used in the productions are:
/* ... */
comment.
[ wfc: ... ]
well-formedness constraint; this identifies by name a constraint
on well-formed documents associated with a production.
[ vc: ... ]
validity constraint; this identifies by name a constraint on
valid documents associated with a production.
__________________________________________________________________
Appendices
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.
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.
Characters
[84] Letter ::= BaseChar | Ideographic
[85] BaseChar ::= [#x0041-#x005A] | [#x0061-#x007A] | [#x00C0-#x00D6]
| [#x00D8-#x00F6] | [#x00F8-#x00FF] | [#x0100-#x0131] | [#x0134-#x013E]
| [#x0141-#x0148] | [#x014A-#x017E] | [#x0180-#x01C3] | [#x01CD-#x01F0]
| [#x01F4-#x01F5] | [#x01FA-#x0217] | [#x0250-#x02A8] | [#x02BB-#x02C1]
| #x0386 | [#x0388-#x038A] | #x038C | [#x038E-#x03A1] | [#x03A3-#x03CE]
| [#x03D0-#x03D6] | #x03DA | #x03DC | #x03DE | #x03E0 | [#x03E2-#x03F3]
| [#x0401-#x040C] | [#x040E-#x044F] | [#x0451-#x045C] | [#x045E-#x0481]
| [#x0490-#x04C4] | [#x04C7-#x04C8] | [#x04CB-#x04CC] | [#x04D0-#x04EB]
| [#x04EE-#x04F5] | [#x04F8-#x04F9] | [#x0531-#x0556] | #x0559
| [#x0561-#x0586] | [#x05D0-#x05EA] | [#x05F0-#x05F2] | [#x0621-#x063A]
| [#x0641-#x064A] | [#x0671-#x06B7] | [#x06BA-#x06BE] | [#x06C0-#x06CE]
| [#x06D0-#x06D3] | #x06D5 | [#x06E5-#x06E6] | [#x0905-#x0939] | #x093D
| [#x0958-#x0961] | [#x0985-#x098C] | [#x098F-#x0990] | [#x0993-#x09A8]
| [#x09AA-#x09B0] | #x09B2 | [#x09B6-#x09B9] | [#x09DC-#x09DD]
| [#x09DF-#x09E1] | [#x09F0-#x09F1] | [#x0A05-#x0A0A] | [#x0A0F-#x0A10]
| [#x0A13-#x0A28] | [#x0A2A-#x0A30] | [#x0A32-#x0A33] | [#x0A35-#x0A36]
| [#x0A38-#x0A39] | [#x0A59-#x0A5C] | #x0A5E | [#x0A72-#x0A74]
| [#x0A85-#x0A8B] | #x0A8D | [#x0A8F-#x0A91] | [#x0A93-#x0AA8]
| [#x0AAA-#x0AB0] | [#x0AB2-#x0AB3] | [#x0AB5-#x0AB9] | #x0ABD | #x0AE0
| [#x0B05-#x0B0C] | [#x0B0F-#x0B10] | [#x0B13-#x0B28] | [#x0B2A-#x0B30]
| [#x0B32-#x0B33] | [#x0B36-#x0B39] | #x0B3D | [#x0B5C-#x0B5D]
| [#x0B5F-#x0B61] | [#x0B85-#x0B8A] | [#x0B8E-#x0B90] | [#x0B92-#x0B95]
| [#x0B99-#x0B9A] | #x0B9C | [#x0B9E-#x0B9F] | [#x0BA3-#x0BA4]
| [#x0BA8-#x0BAA] | [#x0BAE-#x0BB5] | [#x0BB7-#x0BB9] | [#x0C05-#x0C0C]
| [#x0C0E-#x0C10] | [#x0C12-#x0C28] | [#x0C2A-#x0C33] | [#x0C35-#x0C39]
| [#x0C60-#x0C61] | [#x0C85-#x0C8C] | [#x0C8E-#x0C90] | [#x0C92-#x0CA8]
| [#x0CAA-#x0CB3] | [#x0CB5-#x0CB9] | #x0CDE | [#x0CE0-#x0CE1]
| [#x0D05-#x0D0C] | [#x0D0E-#x0D10] | [#x0D12-#x0D28] | [#x0D2A-#x0D39]
| [#x0D60-#x0D61] | [#x0E01-#x0E2E] | #x0E30 | [#x0E32-#x0E33]
| [#x0E40-#x0E45] | [#x0E81-#x0E82] | #x0E84 | [#x0E87-#x0E88] | #x0E8A
| #x0E8D | [#x0E94-#x0E97] | [#x0E99-#x0E9F] | [#x0EA1-#x0EA3] | #x0EA5
| #x0EA7 | [#x0EAA-#x0EAB] | [#x0EAD-#x0EAE] | #x0EB0 | [#x0EB2-#x0EB3]
| #x0EBD | [#x0EC0-#x0EC4] | [#x0F40-#x0F47] | [#x0F49-#x0F69]
| [#x10A0-#x10C5] | [#x10D0-#x10F6] | #x1100 | [#x1102-#x1103]
| [#x1105-#x1107] | #x1109 | [#x110B-#x110C] | [#x110E-#x1112] | #x113C
| #x113E | #x1140 | #x114C | #x114E | #x1150 | [#x1154-#x1155] | #x1159
| [#x115F-#x1161] | #x1163 | #x1165 | #x1167 | #x1169 | [#x116D-#x116E]
| [#x1172-#x1173] | #x1175 | #x119E | #x11A8 | #x11AB | [#x11AE-#x11AF]
| [#x11B7-#x11B8] | #x11BA | [#x11BC-#x11C2] | #x11EB | #x11F0 | #x11F9
| [#x1E00-#x1E9B] | [#x1EA0-#x1EF9] | [#x1F00-#x1F15] | [#x1F18-#x1F1D]
| [#x1F20-#x1F45] | [#x1F48-#x1F4D] | [#x1F50-#x1F57] | #x1F59 | #x1F5B
| #x1F5D | [#x1F5F-#x1F7D] | [#x1F80-#x1FB4] | [#x1FB6-#x1FBC] | #x1FBE
| [#x1FC2-#x1FC4] | [#x1FC6-#x1FCC] | [#x1FD0-#x1FD3] | [#x1FD6-#x1FDB]
| [#x1FE0-#x1FEC] | [#x1FF2-#x1FF4] | [#x1FF6-#x1FFC] | #x2126
| [#x212A-#x212B] | #x212E | [#x2180-#x2182] | [#x3041-#x3094]
| [#x30A1-#x30FA] | [#x3105-#x312C] | [#xAC00-#xD7A3]
[86] Ideographic ::= [#x4E00-#x9FA5] | #x3007 | [#x3021-#x3029]
[87] CombiningChar ::= [#x0300-#x0345] | [#x0360-#x0361]
| [#x0483-#x0486] | [#x0591-#x05A1] | [#x05A3-#x05B9] | [#x05BB-#x05BD]
| #x05BF | [#x05C1-#x05C2] | #x05C4 | [#x064B-#x0652] | #x0670
| [#x06D6-#x06DC] | [#x06DD-#x06DF] | [#x06E0-#x06E4] | [#x06E7-#x06E8]
| [#x06EA-#x06ED] | [#x0901-#x0903] | #x093C | [#x093E-#x094C] | #x094D
| [#x0951-#x0954] | [#x0962-#x0963] | [#x0981-#x0983] | #x09BC | #x09BE
| #x09BF | [#x09C0-#x09C4] | [#x09C7-#x09C8] | [#x09CB-#x09CD] | #x09D7
| [#x09E2-#x09E3] | #x0A02 | #x0A3C | #x0A3E | #x0A3F | [#x0A40-#x0A42]
| [#x0A47-#x0A48] | [#x0A4B-#x0A4D] | [#x0A70-#x0A71] | [#x0A81-#x0A83]
| #x0ABC | [#x0ABE-#x0AC5] | [#x0AC7-#x0AC9] | [#x0ACB-#x0ACD]
| [#x0B01-#x0B03] | #x0B3C | [#x0B3E-#x0B43] | [#x0B47-#x0B48]
| [#x0B4B-#x0B4D] | [#x0B56-#x0B57] | [#x0B82-#x0B83] | [#x0BBE-#x0BC2]
| [#x0BC6-#x0BC8] | [#x0BCA-#x0BCD] | #x0BD7 | [#x0C01-#x0C03]
| [#x0C3E-#x0C44] | [#x0C46-#x0C48] | [#x0C4A-#x0C4D] | [#x0C55-#x0C56]
| [#x0C82-#x0C83] | [#x0CBE-#x0CC4] | [#x0CC6-#x0CC8] | [#x0CCA-#x0CCD]
| [#x0CD5-#x0CD6] | [#x0D02-#x0D03] | [#x0D3E-#x0D43] | [#x0D46-#x0D48]
| [#x0D4A-#x0D4D] | #x0D57 | #x0E31 | [#x0E34-#x0E3A] | [#x0E47-#x0E4E]
| #x0EB1 | [#x0EB4-#x0EB9] | [#x0EBB-#x0EBC] | [#x0EC8-#x0ECD]
| [#x0F18-#x0F19] | #x0F35 | #x0F37 | #x0F39 | #x0F3E | #x0F3F
| [#x0F71-#x0F84] | [#x0F86-#x0F8B] | [#x0F90-#x0F95] | #x0F97
| [#x0F99-#x0FAD] | [#x0FB1-#x0FB7] | #x0FB9 | [#x20D0-#x20DC] | #x20E1
| [#x302A-#x302F] | #x3099 | #x309A
[88] Digit ::= [#x0030-#x0039] | [#x0660-#x0669] | [#x06F0-#x06F9]
| [#x0966-#x096F] | [#x09E6-#x09EF] | [#x0A66-#x0A6F] | [#x0AE6-#x0AEF]
| [#x0B66-#x0B6F] | [#x0BE7-#x0BEF] | [#x0C66-#x0C6F] | [#x0CE6-#x0CEF]
| [#x0D66-#x0D6F] | [#x0E50-#x0E59] | [#x0ED0-#x0ED9] | [#x0F20-#x0F29]
[89] Extender ::= #x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46
| #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309D-#x309E] | [#x30FC-#x30FE]
The character classes defined here can be derived from the Unicode
character database as follows:
* Name start characters must have one of the categories Ll, Lu, Lo,
Lt, Nl.
* Name characters other than Name-start characters must have one of
the categories Mc, Me, Mn, Lm, or Nd.
* Characters in the compatibility area (i.e. with character code
greater than #xF900 and less than #xFFFE) are not allowed in XML
names.
* Characters which have a font or compatibility decomposition (i.e.
those with a "compatibility formatting tag" in field 5 of the
database -- marked by field 5 beginning with a "<") are not
allowed.
* The following characters are treated as name-start characters
rather than name characters, because the property file classifies
them as Alphabetic: [#x02BB-#x02C1], #x0559, #x06E5, #x06E6.
* Characters #x20DD-#x20E0 are excluded (in accordance with Unicode,
section 5.14).
* Character #x00B7 is classified as an extender, because the property
list so identifies it.
* Character #x0387 is added as a name character, because #x00B7 is
its canonical equivalent.
* Characters ':' and '_' are allowed as name-start characters.
* Characters '-' and '.' are allowed as name characters.
C. XML and SGML (Non-Normative)
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].
D. Expansion of Entity and Character References (Non-Normative)
This appendix contains some examples illustrating the sequence of
entity- and character-reference recognition and expansion, as specified
in "4.4 XML Processor Treatment of Entities and References".
If the DTD contains the declaration
An ampersand (&) may be escaped
numerically (&#38;) or with a general entity
(&).
" >
then the XML processor will recognize the character references when it
parses the entity declaration, and resolve them before storing the
following string as the value of the entity "example":
An ampersand (&) may be escaped
numerically (&) or with a general entity
(&).
A reference in the document to "&example;" will cause the text to be
reparsed, at which time the start- and end-tags of the "p" element will
be recognized and the three references will be recognized and expanded,
resulting in a "p" element with the following content (all data, no
delimiters or markup):
An ampersand (&) may be escaped
numerically (&) or with a general entity
(&).
A more complex example will illustrate the rules and their effects
fully. In the following example, the line numbers are solely for
reference.
1
2
4
5 ' >
6 %xx;
7 ]>
8 This sample shows a &tricky; method.
This produces the following:
* in line 4, the reference to character 37 is expanded immediately,
and the parameter entity "xx" is stored in the symbol table with
the value "%zz;". Since the replacement text is not rescanned, the
reference to parameter entity "zz" is not recognized. (And it would
be an error if it were, since "zz" is not yet declared.)
* in line 5, the character reference "<" is expanded immediately
and the parameter entity "zz" is stored with the replacement text
"", which is a well-formed entity
declaration.
* in line 6, the reference to "xx" is recognized, and the replacement
text of "xx" (namely "%zz;") is parsed. The reference to "zz" is
recognized in its turn, and its replacement text ("") is parsed. The general entity "tricky" has now
been declared, with the replacement text "error-prone".
* in line 8, the reference to the general entity "tricky" is
recognized, and it is expanded, so the full content of the "test"
element is the self-describing (and ungrammatical) string This
sample shows a error-prone method.
E. Deterministic Content Models (Non-Normative)
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].
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
'