Extensible Markup Language (XML) 1.0 (Second Edition)
W3C Recommendation 6 October 2000
- This version:
(XHTML, XML, PDF, XHTML review
version with color-coded revision indicators)
- Latest version:
- Previous versions:
- Tim Bray, Textuality and Netscape mailto:firstname.lastname@example.org
- Jean Paoli, Microsoft mailto:email@example.com
- C. M. Sperberg-McQueen, University of Illinois at Chicago and Text
Encoding Initiative mailto:firstname.lastname@example.org
- Eve Maler, Sun Microsystems, Inc. mailto:email@example.com - Second Edition
Copyright © 2000 W3C® (MIT, INRIA,
Keio), All Rights Reserved. W3C liability,
use, and software
licensing rules apply.
The Extensible Markup Language (XML) is a subset of SGML that is completely
described in this document. Its goal is to enable generic SGML to be served,
received, and processed on the Web in the way that is now possible with HTML.
XML has been designed for ease of implementation and for interoperability with
both SGML and HTML.
This document has been reviewed by W3C Members and other interested parties
and has been endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited as a normative reference
from another document. W3C's role in making the Recommendation is to draw
attention to the specification and to promote its widespread deployment. This
enhances the functionality and interoperability of the Web.
This document specifies a syntax created by subsetting an existing, widely
used international text processing standard (Standard Generalized Markup
Language, ISO 8879:1986(E) as amended and corrected) for use on the World Wide
Web. It is a product of the W3C XML Activity, details of which can be found at
http://www.w3.org/XML/. The English version
of this specification is the only normative version. However, for translations
of this document, see http://www.w3.org/XML/#trans. A list of
current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.
This second edition is not a new version of XML (first published 10
February 1998); it merely incorporates the changes dictated by the first-edition
errata (available at http://www.w3.org/XML/xml-19980210-errata)
as a convenience to readers. The errata list for this second edition is
available at http://www.w3.org/XML/xml-V10-2e-errata.
Please report errors in this document to firstname.lastname@example.org; archives are
C. M. Sperberg-McQueen's affiliation has changed since the publication of the
first edition. He is now at the World Wide Web Consortium, and can be contacted
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
[Definition: A software module
called an XML processor is used to read XML documents and provide access
to their content and structure.] [Definition: It is assumed that an XML processor is doing its
work on behalf of another module, called the application.] This
specification describes the required behavior of an XML processor in terms of
how it must read XML data and the information it must provide to the
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:
XML shall be straightforwardly usable over the Internet.
XML shall support a wide variety of applications.
XML shall be compatible with SGML.
It shall be easy to write programs which process XML documents.
The number of optional features in XML is to be kept to the absolute
minimum, ideally zero.
XML documents should be human-legible and reasonably clear.
The XML design should be prepared quickly.
The design of XML shall be formal and concise.
XML documents shall be easy to create.
Terseness in XML markup is of minimal importance.
This specification, together with associated standards (Unicode and ISO/IEC
10646 for characters, Internet RFC 1766 for language identification tags, ISO
639 for language name codes, and ISO 3166 for country name codes), provides all
the information necessary to understand XML Version 1.0 and construct computer
programs to process it.
This version of the XML specification may be distributed freely, as long as
all text and legal notices remain intact.
The terminology used to describe XML documents is defined in the body of this
specification. The terms defined in the following list are used in building
those definitions and in describing the actions of an XML processor:
[Definition: Conforming documents and XML
processors are permitted to but need not behave as described.]
[Definition: Conforming documents and XML
processors are required to behave as described; otherwise they are in error.
[Definition: 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
[Definition: An error which a
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
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
- validity constraint
[Definition: A rule which
applies to all valid XML
documents. Violations of validity constraints are errors; they must, at user
option, be reported by validating XML
- well-formedness constraint
[Definition: A rule
which applies to all well-formed
XML documents. Violations of well-formedness constraints are fatal
[Definition: (Of strings or names:) Two
strings or names being compared must be identical. Characters with multiple
possible representations in ISO/IEC 10646 (e.g. characters with both
precomposed and base+diacritic forms) match only if they have the same
representation in both strings. No case folding is performed. (Of strings and
rules in the grammar:) A string matches a grammatical production if it belongs
to the language generated by that production. (Of content and content models:)
An element matches its declaration when it conforms in the fashion described
in the constraint [VC: Element
- for compatibility
[Definition: Marks a
sentence describing a feature of XML included solely to ensure that XML
remains compatible with SGML.]
- for interoperability
[Definition: Marks a
sentence describing a non-binding recommendation included to increase the
chances that XML documents can be processed by the existing installed base of
SGML processors which predate the WebSGML Adaptations Annex to ISO
[Definition: A data object is an
XML document if it is well-formed,
as defined in this specification. A well-formed XML document may in addition be
valid if it meets
certain further constraints.]
Each XML document has both a logical and a physical structure. Physically,
the document is composed of units called entities. An
entity may refer to other
entities to cause their inclusion in the document. A document begins in a "root"
or document entity.
Logically, the document is composed of declarations, elements, comments,
character references, and processing instructions, all of which are indicated in
the document by explicit markup. The logical and physical structures must nest
properly, as described in 4.3.2
Well-Formed Parsed Entities.
2.1 Well-Formed XML Documents
[Definition: A textual object is
a well-formed XML document if:]
Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this
Each of the parsed
entities which is referenced directly or indirectly within the document is
Matching the document
production implies that:
It contains one or more elements.
[Definition: There is exactly one
element, called the root, or document element, no part of which appears
in the content of
any other element.] For all other elements, if the start-tag is in
the content of another element, the end-tag is in
the content of the same element. More simply stated, the elements, delimited
by start- and end-tags, nest properly within each other.
[Definition: As a consequence
of this, for each non-root element
C in the document, there is one
P in the document such that
C is in the
P, but is not in the content of any other element that
is in the content of
P is referred to as the
C as a child of
[Definition: A parsed entity contains
text, a sequence of characters,
which may represent markup or character data.] [Definition: A character is an atomic unit of text
as specified by ISO/IEC 10646 [ISO/IEC 10646]
(see also [ISO/IEC
10646-2000]). Legal characters are tab, carriage return, line feed, and the
legal characters of Unicode and ISO/IEC 10646. The versions of these standards
cited in A.1
Normative References were current at the time this document was
prepared. New characters may be added to these standards by amendments or new
editions. Consequently, XML processors must accept any character in the range
specified for Char. The use of
"compatibility characters", as defined in section 6.8 of [Unicode] (see
also D21 in section 3.6 of [Unicode3]), is
#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] |
|/* any Unicode character, excluding the surrogate blocks, FFFE, and
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,
(#x20 | #x9 | #xD | #xA)+
Characters are classified for convenience as letters, digits, or other
characters. A letter consists of an alphabetic or syllabic base character or an
ideographic character. Full definitions of the specific characters in each class
are given in B Character
[Definition: A Name is a token
beginning with a letter or one of a few punctuation characters, and continuing
with letters, digits, hyphens, underscores, colons, or full stops, together
known as name characters.] Names beginning with the string "
or any string which would match
(('X'|'x') ('M'|'m') ('L'|'l')),
are reserved for standardization in this or future versions of this
The Namespaces in XML Recommendation [XML Names]
assigns a meaning to names containing colon characters. Therefore, authors
should not use the colon in XML names except for namespace purposes, but XML
processors must accept the colon as a name character.
An Nmtoken (name
token) is any mixture of name characters.
Names and Tokens
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.
'"' ([^%&"] | PEReference
| "'" ([^%&'] | PEReference
'"' ([^<&"] | Reference)*
| "'" ([^<&'] | Reference)*
('"' [^"]* '"') | ("'" [^']* "'")
'"' | "'" (PubidChar
- "'")* "'"
#x20 | #xD | #xA | [a-zA-Z0-9]
Although the EntityValue
production allows the definition of an entity consisting of a single explicit
< in the literal (e.g.,
"<">), it is strongly advised to avoid this practice since any
reference to that entity will cause a well-formedness error.
2.4 Character Data and Markup
Text consists of
intermingled character data
and markup. [Definition: Markup takes
the form of start-tags, end-tags, empty-element
references, comments, CDATA section
delimiters, document type
declarations, and any white space that is at the top level of the document
entity (that is, outside the document element and not inside any other
[Definition: All text that is
not markup constitutes the character data of the document.]
The ampersand character (&) and the left angle bracket (<) may appear
in their literal form only when used as markup delimiters, or within a
comment, a processing
instruction, or a CDATA
section. If they are needed elsewhere, they must be escaped using
either numeric character
references or the strings "
<" 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
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
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
[^<&]* - ([^<&]* ']]>'
[Definition: Comments may appear
anywhere in a document outside other markup; in
addition, they may appear within the document type declaration at places allowed
by the grammar. They are not part of the document's character
data; an XML processor may, but need not, make it possible for an
application to retrieve the text of comments. For
compatibility, the string "
--" (double-hyphen) must not occur
within comments.] Parameter entity references are not recognized within
'<!--' ((Char - '-')
| ('-' (Char -
An example of a comment:
<!-- declarations for <head> & <body> -->
Note that the grammar does not allow a comment ending in
--->. The following example is not well-formed.
2.6 Processing Instructions
instructions (PIs) allow documents to contain instructions for
(S (Char* - (Char*
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
xml", and so on are reserved for
standardization in this or future versions of this specification. The XML Notation
mechanism may be used for formal declaration of PI targets. Parameter entity
references are not recognized within processing instructions.
2.7 CDATA Sections
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 "
end with the string "
Within a CDATA section, only the CDEnd string is
recognized as markup, so that left angle brackets and ampersands may occur in
their literal form; they need not (and cannot) be escaped using
<" and "
&". CDATA sections cannot
An example of a CDATA section, in which "
</greeting>" are recognized as character
data, not markup:
2.8 Prolog and Document Type Declaration
[Definition: XML documents
should begin with an XML declaration which specifies the version of XML
being used.] For example, the following is a complete XML document, well-formed
but not valid:
<?xml version="1.0"?> <greeting>Hello, world!</greeting>
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
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
The function of the markup in an XML document is to describe its storage and
logical structure and to associate attribute-value pairs with its logical
structures. XML provides a mechanism, the document type
declaration, to define constraints on the logical structure and to support
the use of predefined storage units. [Definition: An XML document is valid if it has an
associated document type declaration and if the document complies with the
constraints expressed in it.]
The document type declaration must appear before the first element in the
[Definition: The XML
document type declaration contains or points to markup
declarations that provide a grammar for a class of documents. This grammar
is known as a document type definition, or DTD. The document type
declaration can point to an external subset (a special kind of external entity)
containing markup declarations, or can contain the markup declarations directly
in an internal subset, or can do both. The DTD for a document consists of both
subsets taken together.]
[Definition: A markup
declaration is an element type
declaration, an attribute-list
declaration, an entity
declaration, or a notation
declaration.] These declarations may be contained in whole or in part within
as described in the well-formedness and validity constraints below. For further
information, see 4
Document Type Definition
Note that it is possible to construct a well-formed document containing a doctypedecl
that neither points to an external subset nor contains an internal subset.
The markup declarations may be made up in whole or in part of the replacement
text of parameter entities.
The productions later in this specification for individual nonterminals (elementdecl,
and so on) describe the declarations after all the parameter entities
have been included.
Parameter entity references are recognized anywhere in the DTD (internal and
external subsets and external parameter entities), except in literals,
processing instructions, comments, and the contents of ignored conditional
sections (see 3.4
Conditional Sections). They are also recognized in entity value
literals. The use of parameter entities in the internal subset is restricted as
Validity constraint: Root Element
The Name in
the document type declaration must match the element type of the root
Validity constraint: Proper
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.
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.)
Well-formedness constraint: External
The external subset, if any, must match the production for extSubset.
Well-formedness constraint: PE
The replacement text of a parameter entity reference in a DeclSep must
match the production extSubsetDecl.
Like the internal subset, the external subset and any external parameter
entities referenced in a DeclSep must
consist of a series of complete markup declarations of the types allowed by the
non-terminal symbol markupdecl,
interspersed with white space or parameter-entity
references. However, portions of the contents of the external subset or of
these external parameter entities may conditionally be ignored by using the conditional
section construct; this is not allowed in the internal subset.
The external subset and external parameter entities also differ from the
internal subset in that in them, parameter-entity
references are permitted within markup declarations, not only
between markup declarations.
An example of an XML document with a document type declaration:
<?xml version="1.0"?> <!DOCTYPE greeting SYSTEM "hello.dtd"> <greeting>Hello, world!</greeting>
The system identifier
hello.dtd" gives the address (a URI reference) of a DTD for the
The declarations can also be given locally, as in this example:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE greeting [
<!ELEMENT greeting (#PCDATA)>
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
to an application; examples are attribute defaults and entity declarations. The
standalone document declaration, which may appear as a component of the XML
declaration, signals whether or not there are such declarations which appear
external to the document entity
or in parameter entities. [Definition: An external markup declaration is
defined as a markup declaration occurring in the external subset or in a
parameter entity (external or internal, the latter being included because
non-validating processors are not required to read them).]
Standalone Document Declaration
In a standalone document declaration, the value "yes" indicates that there
are no external markup
declarations which affect the information passed from the XML processor to
the application. The value "no" indicates that there are or may be such external
markup declarations. Note that the standalone document declaration only denotes
the presence of external declarations; the presence, in a document, of
references to external entities, when those entities are internally
declared, does not change its standalone status.
If there are no external markup declarations, the standalone document
declaration has no meaning. If there are external markup declarations but there
is no standalone document declaration, the value "no" is assumed.
Any XML document for which
standalone="no" holds can be
converted algorithmically to a standalone document, which may be desirable for
some network delivery applications.
Validity constraint: Standalone
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
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
An example XML declaration with a standalone document declaration:
<?xml version="1.0" standalone='yes'?>
2.10 White Space Handling
In editing XML documents, it is often convenient to use "white space"
(spaces, tabs, and blank lines) to set apart the markup for greater readability.
Such white space is typically not intended for inclusion in the delivered
version of the document. On the other hand, "significant" white space that
should be preserved in the delivered version is common, for example in poetry
and source code.
An XML processor
must always pass all characters in a document that are not markup through to the
application. A validating XML
processor must also inform the application which of these characters
constitute white space appearing in element
A special attribute named
xml:space may be attached to an element to signal an intention that
in that element, white space should be preserved by applications. In valid
documents, this attribute, like any other, must be declared if it
is used. When declared, it must be given as an enumerated
type whose values are one or both of "default" and "preserve". For
<!ATTLIST poem xml:space (default|preserve) 'preserve'>
<!ATTLIST pre xml:space (preserve) #FIXED 'preserve'>
The value "default" signals that applications' default white-space processing
modes are acceptable for this element; the value "preserve" indicates the intent
that applications preserve all the white space. This declared intent is
considered to apply to all elements within the content of the element where it
is specified, unless overriden with another instance of the
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
entities are often stored in computer files which, for editing convenience,
are organized into lines. These lines are typically separated by some
combination of the characters carriage-return (#xD) and line-feed (#xA).
To simplify the tasks of applications, the
characters passed to an application by the XML processor
must be as if the XML processor normalized all line breaks in external parsed
entities (including the document entity) on input, before parsing, by
translating both the two-character sequence #xD #xA and any #xD that is not
followed by #xA to a single #xA character.
2.12 Language Identification
In document processing, it is often useful to identify the natural or formal
language in which the content is written. A special attribute named
xml:lang may be inserted in documents to specify the language used
in the contents and attribute values of any element in an XML document. In valid
documents, this attribute, like any other, must be declared if it
is used. The values of the attribute are language identifiers as defined by [IETF RFC 1766],
Tags for the Identification of Languages, or its successor on the
IETF Standards Track.
1766] tags are constructed from two-letter language codes as defined by [ISO 639], from
two-letter country codes as defined by [ISO 3166], or
from language identifiers registered with the Internet Assigned Numbers
It is expected that the successor to [IETF RFC 1766]
will introduce three-letter language codes for languages not presently covered
(Productions 33 through 38 have been removed.)
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p>
<p xml:lang="en-GB">What colour is it?</p>
<p xml:lang="en-US">What color is it?</p>
<sp who="Faust" desc='leise' xml:lang="de">
<l>Habe nun, ach! Philosophie,</l>
<l>Juristerei, und Medizin</l>
<l>und leider auch Theologie</l>
<l>durchaus studiert mit heißem Bemüh'n.</l>
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
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
xml:lang attribute might be declared this way:
<!ATTLIST poem xml:lang NMTOKEN 'fr'>
<!ATTLIST gloss xml:lang NMTOKEN 'en'>
<!ATTLIST note xml:lang NMTOKEN 'en'>
3 Logical Structures
[Definition: Each XML document
contains one or more elements, the boundaries of which are either
delimited by start-tags and end-tags, or, for
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
(('X'|'x')('M'|'m')('L'|'l')) are reserved for
standardization in this or future versions of this specification.
Well-formedness constraint: Element Type
The Name in
an element's end-tag must match the element type in the start-tag.
Validity constraint: Element
An element is valid if there is a declaration matching elementdecl
where the Name
matches the element type, and one of the following holds:
The declaration matches EMPTY and the element has no content.
The declaration matches children and
the sequence of child
elements belongs to the language generated by the regular expression in
the content model, with optional white space (characters matching the
between the start-tag and the first child element, between child elements, or
between the last child element and the end-tag. Note that a CDATA section
containing only white space does not match the nonterminal S, and hence cannot
appear in these positions.
The declaration matches Mixed and the
content consists of character
data and child
elements whose types match names in the content model.
The declaration matches ANY, and the types of any child
elements have been declared.
3.1 Start-Tags, End-Tags, and Empty-Element
[Definition: 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. [Definition: The Name-AttValue pairs
are referred to as the attribute specifications of the element], [Definition: with the Name in each pair
referred to as the attribute name] and [Definition: the content of the AttValue (the
text between the
" delimiters) as the
attribute value.]Note that the order of attribute specifications in a
start-tag or empty-element tag is not significant.
Well-formedness constraint: Unique
No attribute name may appear more than once in the same start-tag or
Validity constraint: Attribute Value
The attribute must have been declared; the value must be of the type declared
for it. (For attribute types, see 3.3 Attribute-List
Well-formedness constraint: No
External Entity References
Attribute values cannot contain direct or indirect entity references to
Well-formedness constraint: No
< in Attribute Values
text of any entity referred to directly or indirectly in an attribute value
must not contain a
An example of a start-tag:
<termdef id="dt-dog" term="dog">
[Definition: The end of every element
that begins with a start-tag must be marked by an end-tag containing a
name that echoes the element's type as given in the start-tag:]
'</' Name S?
An example of an end-tag:
[Definition: The text between the
start-tag and end-tag is called the element's content:]
Content of Elements
[Definition: An element with no content is
said to be empty.] The representation of an empty element is either a
start-tag immediately followed by an end-tag, or an empty-element tag. [Definition: An empty-element
tag takes a special form:]
Tags for Empty Elements
Empty-element tags may be used for any element which has no content, whether
or not it is declared using the keyword EMPTY. For
interoperability, the empty-element tag should be used, and should only be
used, for elements which are declared EMPTY.
Examples of empty elements:
3.2 Element Type Declarations
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
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.
element type declaration takes the form:]
Element Type Declaration
where the Name gives the
element type being declared.
Validity constraint: Unique Element Type
No element type may be declared more than once.
Examples of element type declarations:
<!ELEMENT br EMPTY>
<!ELEMENT p (#PCDATA|emph)* >
<!ELEMENT %name.para; %content.para; >
<!ELEMENT container ANY>
3.2.1 Element Content
[Definition: An element type has
element content when elements of that type must contain only child
elements (no character data), optionally separated by white space (characters
matching the nonterminal S).][Definition: In this case, the
constraint includes a content model, a simple grammar governing the
allowed types of the child elements and the order in which they are allowed to
appear.] The grammar is built on content particles (cps), which consist
of names, choice lists of content particles, or sequence lists of content
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
Validity constraint: Proper Group/PE
text must be properly nested with parenthesized 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.
interoperability, if a parameter-entity reference appears in a choice, seq, or Mixed construct,
its replacement text should contain at least one non-blank character, and
neither the first nor last non-blank character of the replacement text should be
a connector (
Examples of element-content models:
<!ELEMENT spec (front, body, back?)>
<!ELEMENT div1 (head, (p | list | note)*, div2*)>
<!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*>
3.2.2 Mixed Content
[Definition: An element type has mixed
content when elements of that type may contain character data, optionally
interspersed with child
elements.] In this case, the types of the child elements may be constrained, but
not their order or their number of occurrences:
where the Names give the
types of elements that may appear as children. The keyword #PCDATA
derives historically from the term "parsed character data."
Validity constraint: No
The same name must not appear more than once in a single mixed-content
Examples of mixed content declarations:
<!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
<!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* >
<!ELEMENT b (#PCDATA)>
3.3 Attribute-List Declarations
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:]
The Name in
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
interoperability, writers of DTDs may choose to provide at most one
attribute-list declaration for a given element type, at most one attribute
definition for a given attribute name in an attribute-list declaration, and at
least one attribute definition in each attribute-list declaration. For
interoperability, an XML processor may at user option issue a warning when more
than one attribute-list declaration is provided for a given element type, or
more than one attribute definition is provided for a given attribute, but this
is not an error.
3.3.1 Attribute Types
XML attribute types are of three kinds: a string type, a set of tokenized
types, and enumerated types. The string type may take any literal string as a
value; the tokenized types have varying lexical and semantic constraints. The
validity constraints noted in the grammar are applied after the attribute value
has been normalized as described in 3.3 Attribute-List
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
No element type may have more than one ID attribute specified.
Validity constraint: ID Attribute
An ID attribute must have a declared default of #IMPLIED or
Validity constraint: IDREF
Values of type IDREF must match the Name production,
and values of type IDREFS must match Names; each Name must match
the value of an ID attribute on some element in the XML document; i.e.
IDREF values must match the value of some ID attribute.
Validity constraint: Entity Name
Values of type ENTITY must match the Name production,
values of type ENTITIES must match Names; each Name must match
the name of an unparsed
entity declared in the DTD.
Validity constraint: Name Token
Values of type NMTOKEN must match the Nmtoken
production; values of type NMTOKENS must match Nmtokens.
Enumerated attributes can take one of a list of values provided in the
declaration]. There are two kinds of enumerated types:
Enumerated Attribute Types
A NOTATION attribute identifies a notation,
declared in the DTD with associated system and/or public identifiers, to be used
in interpreting the element to which the attribute is attached.
Validity constraint: Notation
Values of this type must match one of the notation
names included in the declaration; all notation names in the declaration must be
Validity constraint: One Notation
Per Element Type
No element type may have more than one NOTATION attribute
Validity constraint: No Notation
on Empty Element
compatibility, an attribute of type NOTATION must not be declared on
an element declared EMPTY.
Validity constraint: Enumeration
Values of this type must match one of the Nmtoken tokens
in the declaration.
interoperability, the same Nmtoken should
not occur more than once in the enumerated attribute types of a single element
3.3.2 Attribute Defaults
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. [Definition: If the declaration is
neither #REQUIRED nor #IMPLIED, then the AttValue value
contains the declared default value; the #FIXED keyword states
that the attribute must always have the default value. If a default value is
declared, when an XML processor encounters an omitted attribute, it is to behave
as though the attribute were present with the declared default value.]
Validity constraint: Required
If the default declaration is the keyword #REQUIRED, then the
attribute must be specified for all elements of the type in the attribute-list
Validity constraint: Attribute
The declared default value must meet the lexical constraints of the declared
Validity constraint: Fixed Attribute
If an attribute has a default value declared with the #FIXED keyword,
instances of that attribute must match the default value.
Examples of attribute-list declarations:
id ID #REQUIRED
name CDATA #IMPLIED>
type (bullets|ordered|glossary) "ordered">
method CDATA #FIXED "POST">
3.3.3 Attribute-Value Normalization
Before the value of an attribute is passed to the application or checked for
validity, the XML processor must normalize the attribute value by applying the
algorithm below, or by using some other method such that the value passed to the
application is the same as that produced by the algorithm.
All line breaks must have been normalized on input to #xA as described in
End-of-Line Handling, so the rest of this algorithm operates on text
normalized in this way.
Begin with a normalized value consisting of the empty string.
For each character, entity reference, or character reference in the
unnormalized attribute value, beginning with the first and continuing to the
last, do the following:
For a character reference, append the referenced character to the
For an entity reference, recursively apply step 3 of this algorithm to
the replacement text of the entity.
For a white space character (#x20, #xD, #xA, #x9), append a space
character (#x20) to the normalized value.
For another character, append the character to the normalized
If the attribute type is not CDATA, then the XML processor must further
process the normalized attribute value by discarding any leading and trailing
space (#x20) characters, and by replacing sequences of space (#x20) characters
by a single space (#x20) character.
Note that if the unnormalized attribute value contains a character reference
to a white space character other than space (#x20), the normalized value
contains the referenced character itself (#xD, #xA or #x9). This contrasts with
the case where the unnormalized value contains a white space character (not a
reference), which is replaced with a space character (#x20) in the normalized
value and also contrasts with the case where the unnormalized value contains an
entity reference whose replacement text contains a white space character; being
recursively processed, the white space character is replaced with a space
character (#x20) in the normalized value.
All attributes for which no declaration has been read should be treated by a
non-validating processor as if declared CDATA.
Following are examples of attribute normalization. Given the following
<!ENTITY d "
<!ENTITY a "
<!ENTITY da "
the attribute specifications in the left column below would be normalized to
the character sequences of the middle column if the attribute
declared NMTOKENS and to those of the right columns if
||a is NMTOKENS
||a is CDATA|
x y z
#x20 #x20 x y z
A #x20 B
#x20 #x20 A #x20 #x20 B #x20 #x20
#xD #xD A #xA #xA B #xD #xA
#xD #xD A #xA #xA B #xD #xD
Note that the last example is invalid (but well-formed) if
declared to be of type NMTOKENS.
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.]
Validity constraint: Proper
Conditional Section/PE Nesting
If any of the "
]]>" of a conditional section is contained in the replacement
text for a parameter-entity reference, all of them must be contained in the same
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. 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. The contents of an ignored conditional section are parsed by ignoring
all characters after the "
[" following the keyword, except
conditional section starts "
<![" and ends "
until the matching conditional section end is found. Parameter entity references
are not recognized in this process.
If the keyword of the conditional section is a parameter-entity reference,
the parameter entity must be replaced by its content before the processor
decides whether to include or ignore the conditional section.
<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >
<!ELEMENT book (comments*, title, body, supplements?)>
<!ELEMENT book (title, body, supplements?)>
4 Physical Structures
[Definition: An XML document may consist
of one or many storage units. These are called entities; they all have
content and are all (except for the document entity
and the external DTD
subset) identified by entity name.] Each XML document has one entity
called the document entity,
which serves as the starting point for the XML processor
and may contain the whole document.
Entities may be either parsed or unparsed. [Definition: A parsed entity's contents are referred
to as its replacement
text; this text is considered
an integral part of the document.]
[Definition: An unparsed
entity is a resource whose contents may or may not be text, and if text,
may be other than XML. Each unparsed entity has an associated notation,
identified by name. Beyond a requirement that an XML processor make the
identifiers for the entity and notation available to the application, XML places
no constraints on the contents of unparsed entities.]
Parsed entities are invoked by name using entity references; unparsed
entities by name, given in the value of ENTITY or ENTITIES
entities are entities for use within the document content. In this
specification, general entities are sometimes referred to with the unqualified
term entity when this leads to no ambiguity.] [Definition: Parameter entities
are parsed entities for use within the DTD.] These two types of entities use
different forms of reference and are recognized in different contexts.
Furthermore, they occupy different namespaces; a parameter entity and a general
entity with the same name are two distinct entities.
4.1 Character and Entity References
character reference refers to a specific character in the ISO/IEC 10646
character set, for example one not directly accessible from available input
Well-formedness constraint: Legal
Characters referred to using character references must match the production
If the character reference begins with "
&#x", the digits and
letters up to the terminating
; provide a hexadecimal
representation of the character's code point in ISO/IEC 10646. If it begins just
&#", the digits up to the terminating
provide a decimal representation of the character's code point.
[Definition: An entity
reference refers to the content of a named entity.] [Definition: References to
parsed general entities use ampersand (
&) and semicolon
;) as delimiters.] [Definition: Parameter-entity references use
%) and semicolon (
;) as delimiters.]
Well-formedness constraint: Entity
In a document without any DTD, a document with only an internal DTD subset
which contains no parameter entity references, or a document with
standalone='yes'", for an entity reference that does not occur
within the external subset or a parameter entity, the Name given in the
entity reference must match that in an
declaration that does not occur within the external subset or a
parameter entity, except that well-formed documents need not declare any of the
quot. The declaration of a general entity must
precede any reference to it which appears in a default value in an
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
Validity constraint: Entity
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
declaration. For interoperability, valid documents should declare the
quot, in the form specified in 4.6
Predefined Entities. The declaration of a parameter entity must precede
any reference to it. Similarly, the declaration of a general entity must precede
any attribute-list declaration containing a default value with a direct or
indirect reference to that general entity.
Well-formedness constraint: Parsed
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
A parsed entity must not contain a recursive reference to itself, either
directly or indirectly.
Well-formedness constraint: In DTD
Parameter-entity references may only appear in the DTD.
Examples of character and entity references:
Type <key>less-than</key> (<) to save options.
This document was prepared on &docdate; and
is classified &security-level;.
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... -->
<!ENTITY % ISOLat2
SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >
<!-- ... now reference it. -->
4.2 Entity Declarations
[Definition: Entities are
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.
4.2.2 External Entities
[Definition: If the entity is
not internal, it is an external entity, declared as follows:]
External Entity Declaration
If the NDataDecl is
present, this is a general unparsed
entity; otherwise it is a parsed entity.
Validity constraint: Notation
must match the declared name of a notation.
[Definition: The SystemLiteral
is called the entity's system identifier. It is a URI reference (as
defined in [IETF
RFC 2396], updated by [IETF RFC 2732]),
meant to be dereferenced to obtain input for the XML processor to construct the
entity's replacement text.] It is an error for a fragment identifier (beginning
# character) to be part of a system identifier. Unless
otherwise provided by information outside the scope of this specification (e.g.
a special XML element type defined by a particular DTD, or a processing
instruction defined by a particular application specification), relative URIs
are relative to the location of the resource within which the entity declaration
occurs. A URI might thus be relative to the document entity,
to the entity containing the external DTD
subset, or to some other external parameter
URI references require encoding and escaping of certain characters. The
disallowed characters include all non-ASCII characters, plus the excluded
characters listed in Section 2.4 of [IETF RFC 2396],
except for the number sign (
#) and percent sign (
characters and the square bracket characters re-allowed in [IETF RFC 2732].
Disallowed characters must be escaped as follows:
Each disallowed character is converted to UTF-8 [IETF RFC 2279]
as one or more bytes.
Any octets corresponding to a disallowed character are escaped with the URI
escaping mechanism (that is, converted to
HH is the hexadecimal notation of the byte value).
The original character is replaced by the resulting character
[Definition: In addition to a
system identifier, an external identifier may include a public
identifier.] An XML processor attempting to retrieve the entity's content
may use the public identifier to try to generate an alternative URI reference.
If the processor is unable to do so, it must use the URI reference specified in
the system literal. Before a match is attempted, all strings of white space in
the public identifier must be normalized to single space characters (#x20), and
leading and trailing white space must be removed.
Examples of external entity declarations:
PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
NDATA gif >
4.3 Parsed Entities
4.3.1 The Text Declaration
External parsed entities should each begin with a text
The text declaration must be provided literally, not by reference to a parsed
entity. No text declaration may appear at any position other than the beginning
of an external parsed entity. The text declaration in an external parsed entity
is not considered part of its replacement
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
All external parameter entities are well-formed by definition.
Well-Formed External Parsed Entity
An internal general parsed entity is well-formed if its replacement text
matches the production labeled content. All
internal parameter entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and physical
structures in an XML document are properly nested; no start-tag, end-tag, empty-element
tag, element, comment, processing
reference, or entity reference
can begin in one entity and end in another.
4.3.3 Character Encoding in Entities
Each external parsed entity in an XML document may use a different encoding
for its characters. All XML processors must be able to read entities in both the
UTF-8 and UTF-16 encodings. The terms "UTF-8" and "UTF-16" in this specification
do not apply to character encodings with any other labels, even if the encodings
or labels are very similar to UTF-8 or UTF-16.
Entities encoded in UTF-16 must begin with the Byte Order Mark described by
Annex F of [ISO/IEC 10646],
Annex H of [ISO/IEC
10646-2000], section 2.4 of [Unicode], and
section 2.7 of [Unicode3] (the
ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not
part of either the markup or the character data of the XML document. XML
processors must be able to use this character to differentiate between UTF-8 and
UTF-16 encoded documents.
Although an XML processor is required to read only entities in the UTF-8 and
UTF-16 encodings, it is recognized that other encodings are used around the
world, and it may be desired for XML processors to read entities that use them.
In the absence of external character encoding information (such as MIME
headers), parsed entities which are stored in an encoding other than UTF-8 or
UTF-16 must begin with a text declaration (see 4.3.1 The Text
Declaration) containing an encoding declaration:
'encoding' Eq ('"' EncName
'"' | "'" 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 "
ISO-10646-UCS-4" should be used for the various encodings and
transformations of Unicode / ISO/IEC 10646, the values
ISO-8859-n" (where n is the part number)
should be used for the parts of ISO 8859, and the values
Shift_JIS", and "
should be used for the various encoded forms of JIS X-0208-1997. It is
recommended that character encodings registered (as charsets) with the
Internet Assigned Numbers Authority [IANA-CHARSETS],
other than those just listed, be referred to using their registered names; other
encodings should use names starting with an "x-" prefix. XML processors should
match character encoding names in a case-insensitive way and should either
interpret an IANA-registered name as the encoding registered at IANA for that
name or treat it as unknown (processors are, of course, not required to support
all IANA-registered encodings).
In the absence of information provided by an external transport protocol
(e.g. HTTP or MIME), it is 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, or for an entity which
begins with neither a Byte Order Mark nor an encoding declaration to use an
encoding other than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary
ASCII entities do not strictly need an encoding declaration.
It is a fatal error for a TextDecl to
occur other than at the beginning of an external entity.
It is a fatal error when
an XML processor encounters an entity with an encoding that it is unable to
process. It is a fatal error if an XML entity is determined (via default,
encoding declaration, or higher-level protocol) to be in a certain encoding but
contains octet sequences that are not legal in that encoding. It is also a fatal
error if an XML entity contains no encoding declaration and its content is not
legal UTF-8 or UTF-16.
Examples of text declarations containing encoding declarations:
4.4 XML Processor Treatment of Entities and
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
- 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,
PI, Comment, SystemLiteral,
or the contents of an ignored conditional section (see 3.4
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.
[Definition: An entity is
included when its replacement
text is retrieved and processed, in place of the reference itself, as though
it were part of the document at the location the reference was recognized.] The
replacement text may contain both character data
and (except for parameter entities) markup, which
must be recognized in the usual way. (The string "
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
4.4.3 Included If Validating
When an XML processor recognizes a reference to a parsed entity, in order to
document, the processor must include its
replacement text. If the entity is external, and the processor is not attempting
to validate the XML document, the processor may, but need not,
include the entity's replacement text. If a non-validating processor does not
include the replacement text, it must inform the application that it recognized,
but did not read, the entity.
This rule is based on the recognition that the automatic inclusion provided
by the SGML and XML entity mechanism, primarily designed to support modularity
in authoring, is not necessarily appropriate for other applications, in
particular document browsing. Browsers, for example, when encountering an
external parsed entity reference, might choose to provide a visual indication of
the entity's presence and retrieve it for display only on demand.
The following are forbidden, and constitute fatal errors:
the appearance of a reference to an unparsed
the appearance of any character or general-entity reference in the DTD
except within an EntityValue
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
<!ENTITY % YN '"Yes"' >
<!ENTITY WhatHeSaid "He said %YN;" >
while this is not:
<!ENTITY EndAttr "27'" >
When the name of an unparsed
entity appears as a token in the value of an attribute of declared type
ENTITY or ENTITIES, a validating processor must inform the
application of the system and public (if any)
identifiers for both the entity and its associated notation.
When a general entity reference appears in the EntityValue
in an entity declaration, it is bypassed and left as is.
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. This
behavior does not apply to parameter entity references within entity values;
these are described in 4.4.5 Included in
4.5 Construction of Internal Entity
In discussing the treatment of internal entities, it is useful to distinguish
two forms of the entity's value. [Definition: The literal entity value is the quoted
string actually present in the entity declaration, corresponding to the
[Definition: The replacement
text is the content of the entity, after replacement of character references
and parameter-entity references.]
The literal entity value as given in an internal entity declaration (EntityValue)
may contain character, parameter-entity, and general-entity references. Such
references must be contained entirely within the literal entity value. The
actual replacement text that is included as
described above must contain the replacement text of any parameter
entities referred to, and must contain the character referred to, in place of
any character references in the literal entity value; however, general-entity
references must be left as-is, unexpanded. For example, given the following
<!ENTITY % pub "Éditions Gallimard" >
<!ENTITY rights "All rights reserved" >
<!ENTITY book "La Peste: Albert Camus,
© 1947 %pub;. &rights;" >
then the replacement text for the entity "
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
[Definition: Entity and character
references can both be used to escape the left angle bracket, ampersand,
and other delimiters. A set of general entities (
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 "
&" may be used to escape
& when they occur in character data.]
All XML processors must recognize these entities whether they are declared or
interoperability, valid XML documents should declare these entities, like
any others, before using them. If the entities
amp are declared, they must be declared as internal entities whose
replacement text is a character reference to the respective character (less-than
sign or ampersand) being escaped; the double escaping is required for these
entities so that references to them produce a well-formed result. If the
quot are declared,
they must be declared as internal entities whose replacement text is the single
character being escaped (or a character reference to that character; the double
escaping here is unnecessary but harmless). For example:
<!ENTITY lt "&#60;">
<!ENTITY gt ">">
<!ENTITY amp "&#38;">
<!ENTITY apos "'">
<!ENTITY quot """>
4.7 Notation Declarations
[Definition: Notations identify
by name the format of unparsed
entities, the format of elements which bear a notation attribute, or the
application to which a processing
instruction is addressed.]
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
Validity constraint: Unique
Only one notation declaration can declare a given Name.
XML processors must provide applications with the name and external
identifier(s) of any notation declared and referred to in an attribute value,
attribute definition, or entity declaration. They may additionally resolve the
external identifier into the system
identifier, file name, or other information needed to allow the application
to call a processor for data in the notation described. (It is not an error,
however, for XML documents to declare and refer to notations for which
notation-specific applications are not available on the system where the XML
processor or application is running.)
4.8 Document Entity
[Definition: The document
entity serves as the root of the entity tree and a starting-point for an XML
processor.] This specification does not specify how the document entity is
to be located by an XML processor; unlike other entities, the document entity
has no name and might well appear on a processor input stream without any
identification at all.
5.1 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, at user option, report violations of the
constraints expressed by the declarations in the DTD, and
failures to fulfill the validity constraints given in this specification.] To
accomplish this, validating XML processors must read and process the entire DTD
and all external parsed entities referenced in the document.
Non-validating processors are required to check only the document entity,
including the entire internal DTD subset, for well-formedness. [Definition: While they are
not required to check the document for validity, they are required to
process all the declarations they read in the internal DTD subset and in
any parameter entity that they read, up to the first reference to a parameter
entity that they do not read; that is to say, they must use the
information in those declarations to normalize
attribute values, include
the replacement text of internal entities, and supply default
attribute values.] Except when
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
Entity, and No
Recursion, as well as some of the cases described as forbidden
in 4.4 XML
Processor Treatment of Entities and References.
The information passed from the processor to the application may vary,
depending on whether the processor reads parameter and external entities. For
example, a non-validating processor may not normalize
attribute values, include
the replacement text of internal entities, or supply default
attribute values, where doing so depends on having read
declarations in external or parameter entities.
For maximum reliability in interoperating between different XML processors,
applications which use non-validating processors should not rely on any
behaviors not required of such processors. Applications which require facilities
such as the use of default attributes or internal entities which are declared in
external entities should use validating XML processors.
The formal grammar of XML is given in this specification using a simple
Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines one
symbol, in the form
Symbols are written with an initial capital letter if they are the start
symbol of a regular language, otherwise with an initial lower case letter.
Literal strings are quoted.
Within the expression on the right-hand side of a rule, the following
expressions are used to match strings of one or more characters:
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.
matches any Char with a
value in the range(s) indicated (inclusive).
matches any Char with a
value among the characters enumerated. Enumerations and ranges can be mixed in
one set of brackets.
matches any Char with a
value outside the range indicated.
matches any Char with a
value not among the characters given. Enumerations and ranges of forbidden
values can be mixed in one set of brackets.
matches a literal string matching that
given inside the double quotes.
matches a literal string matching that
given inside the single quotes.
These symbols may be combined to match more complex patterns as follows,
B represent simple expressions:
expression is treated as a unit and may be combined as
described in this list.
A or nothing; optional
A followed by
B. This operator has higher
precedence than alternation; thus
A B | C D is identical to
(A B) | (C D).
A | B
B but not both.
A - B
matches any string that matches
A but does not match
matches one or more occurrences of
A.Concatenation has higher
precedence than alternation; thus
A+ | B+ is identical to
(A+) | (B+).
matches zero or more occurrences of
A. Concatenation has
higher precedence than alternation; thus
A* | B* is identical to
(A*) | (B*).
Other notations used in the productions are:
/* ... */
[ wfc: ... ]
well-formedness constraint; this identifies by name a constraint on well-formed
documents associated with a production.
[ vc: ... ]
validity constraint; this identifies by name a constraint on valid documents
associated with a production.
A.1 Normative References
- (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. (See http://www.ietf.org/rfc/rfc1766.txt.)
- 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).
- ISO/IEC 10646-2000
- ISO (International Organization for Standardization). ISO/IEC
10646-1:2000. Information technology -- Universal Multiple-Octet Coded
Character Set (UCS) -- Part 1: Architecture and Basic Multilingual
Plane. [Geneva]: International Organization for Standardization, 2000.
- The Unicode Consortium. The Unicode Standard, Version 2.0.
Reading, Mass.: Addison-Wesley Developers Press, 1996.
- The Unicode Consortium. The Unicode Standard, Version 3.0.
Reading, Mass.: Addison-Wesley Developers Press, 2000. ISBN 0-201-61633-5.
A.2 Other References
- Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. Compilers:
Principles, Techniques, and Tools. Reading: Addison-Wesley, 1986, rpt.
- 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, Anne. Formal Models in Document Processing.
Habilitationsschrift. Faculty of Mathematics at the University of Freiburg,
1993. (See ftp://ftp.informatik.uni-freiburg.de/documents/papers/brueggem/habil.ps.)
- Brüggemann-Klein and Wood
- Brüggemann-Klein, Anne, and Derick Wood. Deterministic Regular
Languages. Universität Freiburg, Institut für Informatik, Bericht 38,
Oktober 1991. Extended abstract in A. Finkel, M. Jantzen, Hrsg., STACS 1992,
S. 173-184. Springer-Verlag, Berlin 1992. Lecture Notes in Computer Science
577. Full version titled One-Unambiguous Regular Languages in
Information and Computation 140 (2): 229-253, February 1998.
- James Clark. Comparison of SGML and XML. See http://www.w3.org/TR/NOTE-sgml-xml-971215.
- (Internet Assigned Numbers Authority) Registry of Language
Tags, ed. Keld Simonsen et al. (See http://www.isi.edu/in-notes/iana/assignments/languages/.)
- IETF RFC2141
- IETF (Internet Engineering Task Force). RFC 2141: URN Syntax, ed.
R. Moats. 1997. (See http://www.ietf.org/rfc/rfc2141.txt.)
- IETF RFC 2279
- IETF (Internet Engineering Task Force). RFC 2279: UTF-8, a
transformation format of ISO 10646, ed. F. Yergeau, 1998. (See http://www.ietf.org/rfc/rfc2279.txt.)
- IETF RFC 2376
- IETF (Internet Engineering Task Force). RFC 2376: XML Media
Types. ed. E. Whitehead, M. Murata. 1998. (See http://www.ietf.org/rfc/rfc2376.txt.)
- IETF RFC 2396
- IETF (Internet Engineering Task Force). RFC 2396: Uniform Resource
Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L.
Masinter. 1998. (See http://www.ietf.org/rfc/rfc2396.txt.)
- IETF RFC 2732
- IETF (Internet Engineering Task Force). RFC 2732: Format for Literal
IPv6 Addresses in URL's. R. Hinden, B. Carpenter, L. Masinter. 1999.
- IETF RFC 2781
- IETF (Internet Engineering Task Force). RFC 2781: UTF-16, an encoding
of ISO 10646, ed. P. Hoffman, F. Yergeau. 2000. (See http://www.ietf.org/rfc/rfc2781.txt.)
- ISO 639
- (International Organization for Standardization). ISO 639:1988 (E).
Code for the representation of names of languages. [Geneva]:
International Organization for Standardization, 1988.
- ISO 3166
- (International Organization for Standardization). ISO 3166-1:1997
(E). Codes for the representation of names of countries and their subdivisions
-- Part 1: Country codes [Geneva]: International Organization for
- 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.
- ISO (International Organization for Standardization). ISO 8879:1986
TC2. Information technology -- Document Description and Processing Languages.
[Geneva]: International Organization for Standardization, 1998. (See http://www.sgmlsource.com/8879rev/n0029.htm.)
- XML Names
- Tim Bray, Dave Hollander, and Andrew Layman, editors. Namespaces in
XML. Textuality, Hewlett-Packard, and Microsoft. World Wide Web
Consortium, 1999. (See http://www.w3.org/TR/REC-xml-names/.)
B Character Classes
Following the characteristics defined in the Unicode standard, characters are
classed as base characters (among others, these contain the alphabetic
characters of the Latin alphabet), ideographic characters, and combining
characters (among others, this class contains most diacritics) Digits and
extenders are also distinguished.
[#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]
[#x4E00-#x9FA5] | #x3007 | [#x3021-#x3029]
[#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
[#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]
#x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640
| #x0E46 | #x0EC6 | #x3005 | [#x3031-#x3035]
| [#x309D-#x309E] | [#x30FC-#x30FE]
The character classes defined here can be derived from the Unicode 2.0
character database as follows:
Name start characters must have one of the categories Ll, Lu, Lo, Lt,
Name characters other than Name-start characters must have one of the
categories Mc, Me, Mn, Lm, or Nd.
Characters in the compatibility area (i.e. with character code greater than
#xF900 and less than #xFFFE) are not allowed in XML names.
Characters which have a font or compatibility decomposition (i.e. those
with a "compatibility formatting tag" in field 5 of the database -- marked by
field 5 beginning with a "<") are not allowed.
The following characters are treated as name-start characters rather than
name characters, because the property file classifies them as Alphabetic:
[#x02BB-#x02C1], #x0559, #x06E5, #x06E6.
Characters #x20DD-#x20E0 are excluded (in accordance with Unicode 2.0,
Character #x00B7 is classified as an extender, because the property list so
Character #x0387 is added as a name character, because #x00B7 is its
Characters ':' and '_' are allowed as name-start characters.
Characters '-' and '.' are allowed as name characters.
C XML and SGML (Non-Normative)
XML is designed to be a subset of SGML, in that every XML document should
also be a conforming SGML document. For a detailed comparison of the additional
restrictions that XML places on documents beyond those of SGML, see [Clark].
D Expansion of Entity and Character References
This appendix contains some examples illustrating the sequence of entity- and
character-reference recognition and expansion, as specified in 4.4 XML Processor
Treatment of Entities and References.
If the DTD contains the declaration
<!ENTITY example "<p>An ampersand (&#38;) may be escaped
numerically (&#38;#38;) or with a general entity
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 "
<p>An ampersand (&) may be escaped
numerically (&#38;) or with a general entity
A reference in the document to "
&example;" will cause the
text to be reparsed, at which time the start- and end-tags of the
element will be recognized and the three references will be recognized and
expanded, resulting in a
p element with the following content (all
data, no delimiters or markup):
An ampersand (&) may be escaped
numerically (&) or with a general entity
A more complex example will illustrate the rules and their effects fully. In
the following example, the line numbers are solely for reference.
1 <?xml version='1.0'?>
2 <!DOCTYPE test [
3 <!ELEMENT test (#PCDATA) >
4 <!ENTITY % xx '%zz;'>
5 <!ENTITY % zz '<!ENTITY tricky "error-prone" >' >
8 <test>This sample shows a &tricky; method.</test>
This produces the following:
in line 4, the reference to character 37 is expanded immediately, and the
parameter entity "
xx" is stored in the symbol table with the
%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
in line 5, the character reference "
<" is expanded
immediately and the parameter entity "
zz" is stored with the
replacement text "
<!ENTITY tricky "error-prone" >", which
is a well-formed entity declaration.
in line 6, the reference to "
xx" is recognized, and the
replacement text of "
xx" (namely "
%zz;") is parsed.
The reference to "
zz" is recognized in its turn, and its
replacement text ("
<!ENTITY tricky "error-prone" >") is
parsed. The general entity "
tricky" has now been declared, with
the replacement text "
in line 8, the reference to the general entity "
recognized, and it is expanded, so the full content of the
element is the self-describing (and ungrammatical) string This sample
shows a error-prone method.
E Deterministic Content Models (Non-Normative)
As noted in 3.2.1
Element Content, it is required that content models in element type
declarations be deterministic. This requirement is for
compatibility with SGML (which calls deterministic content models
"unambiguous"); XML processors built using SGML systems may flag
non-deterministic content models as errors.
For example, the content model
((b, c) | (b, d)) is
non-deterministic, because given an initial
b the XML processor
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
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 processor doesn't need to
look ahead to see what follows; either
d would be
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
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
F.1 Detection Without External Encoding
Because each XML entity not accompanied by external encoding information and
not in UTF-8 or UTF-16 encoding must begin with an XML encoding
declaration, in which the first characters must be '
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 "
the Byte Order Mark required of UTF-16 data streams is "
The notation ## is used to denote any byte value except that two
consecutive ##s cannot be both 00.
With a Byte Order Mark:
00 00 FE FF
|UCS-4, big-endian machine (1234 order)|
FF FE 00 00
|UCS-4, little-endian machine (4321 order)|
00 00 FF FE
|UCS-4, unusual octet order (2143)|
FE FF 00 00
|UCS-4, unusual octet order (3412)|
FE FF ## ##
FF FE ## ##
EF BB BF
Without a Byte Order Mark:
00 00 00 3C
|UCS-4 or other encoding with a 32-bit code unit and ASCII
characters encoded as ASCII values, in respectively big-endian (1234),
little-endian (4321) and two unusual byte orders (2143 and 3412). The
encoding declaration must be read to determine which of UCS-4 or other
supported 32-bit encodings applies.|
3C 00 00 00
00 00 3C 00
00 3C 00 00
00 3C 00 3F
|UTF-16BE or big-endian ISO-10646-UCS-2 or other encoding with a 16-bit
code unit in big-endian order and ASCII characters encoded as ASCII values
(the encoding declaration must be read to determine which)|
3C 00 3F 00
|UTF-16LE or little-endian ISO-10646-UCS-2 or other encoding with a
16-bit code unit in little-endian order and ASCII characters encoded as
ASCII values (the encoding declaration must be read to determine
3C 3F 78 6D
|UTF-8, ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any
other 7-bit, 8-bit, or mixed-width encoding which ensures that the
characters of ASCII have their normal positions, width, and values; the
actual encoding declaration must be read to detect which of these applies,
but since all of these encodings use the same bit patterns for the
relevant ASCII characters, the encoding declaration itself may be read
4C 6F A7 94
|EBCDIC (in some flavor; the full encoding declaration must be read to
tell which code page is in use)|
||UTF-8 without an encoding declaration, or else the data stream is
mislabeled (lacking a required encoding declaration), corrupt,
fragmentary, or enclosed in a wrapper of some kind|
In cases above which do not require reading the encoding declaration to
determine the encoding, section 4.3.3 still requires that the encoding
declaration, if present, be read and that the encoding name be checked to match
the actual encoding of the entity. Also, it is possible that new character
encodings will be invented that will make it necessary to use the encoding
declaration to determine the encoding, in cases where this is not required at
This level of autodetection is enough to read the XML encoding declaration
and parse the character-encoding identifier, which is still necessary to
distinguish the individual members of each family of encodings (e.g. to tell
UTF-8 from 8859, and the parts of 8859 from each other, or to distinguish the
specific EBCDIC code page in use, and so on).
Because the contents of the encoding declaration are restricted to characters
from the ASCII repertoire (however encoded), a processor can reliably read the
entire encoding declaration as soon as it has detected which family of encodings
is in use. Since in practice, all widely used character encodings fall into one
of the categories above, the XML encoding declaration allows reasonably reliable
in-band labeling of character encodings, even when external sources of
information at the operating-system or transport-protocol level are unreliable.
Character encodings such as UTF-7 that make overloaded usage of ASCII-valued
bytes may fail to be reliably detected.
Once the processor has detected the character encoding in use, it can act
appropriately, whether by invoking a separate input routine for each case, or by
calling the proper conversion function on each character of input.
Like any self-labeling system, the XML encoding declaration will not work if
any software changes the entity's character set or encoding without updating the
encoding declaration. Implementors of character-encoding routines should be
careful to ensure the accuracy of the internal and external information used to
label the entity.
F.2 Priorities in the Presence of
External Encoding Information
The second possible case occurs when the XML entity is accompanied by
encoding information, as in some file systems and some network protocols. When
multiple sources of information are available, their relative priority and the
preferred method of handling conflict should be specified as part of the
higher-level protocol used to deliver XML. In particular, please refer to [IETF RFC 2376] or
its successor, which defines the
application/xml MIME types and provides some useful guidance. In
the interests of interoperability, however, the following rule is
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, SoftQuad, Grif SA, Muzmo and Veo Systems
- MURATA Makoto (FAMILY Given), Fuji Xerox Information Systems
- Joel Nava, Adobe
- Conleth O'Connell, Vignette
- Peter Sharpe, SoftQuad
- John Tigue, DataChannel
H W3C XML Core Group (Non-Normative)
The second edition of this specification was prepared by the W3C XML Core
Working Group (WG). The members of the WG at the time of publication of this
- Paula Angerstein, Vignette
- Daniel Austin, Ask Jeeves
- Tim Boland
- Allen Brown, Microsoft
- Dan Connolly, W3C (Staff Contact)
- John Cowan, Reuters Limited
- John Evdemon, XMLSolutions Corporation
- Paul Grosso, Arbortext (Co-Chair)
- Arnaud Le Hors, IBM (Co-Chair)
- Eve Maler, Sun Microsystems (Second Edition Editor)
- Jonathan Marsh, Microsoft
- MURATA Makoto (FAMILY Given), IBM
- Mark Needleman, Data Research Associates
- David Orchard, Jamcracker
- Lew Shannon, NCR
- Richard Tobin, University of Edinburgh
- Daniel Veillard, W3C
- Dan Vint, Lexica
- Norman Walsh, Sun Microsystems
- François Yergeau, Alis Technologies (Errata List Editor)
- Kongyi Zhou, Oracle