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.
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 at firstname.lastname@example.org.
1.1 Origin and Goals
2.1 Well-Formed XML Documents
2.3 Common Syntactic Constructs
2.4 Character Data and Markup
2.6 Processing Instructions
2.7 CDATA Sections
2.8 Prolog and Document Type Declaration
2.9 Standalone Document Declaration
2.10 White Space Handling
2.11 End-of-Line Handling
2.12 Language Identification
3 Logical Structures
3.1 Start-Tags, End-Tags, and Empty-Element Tags
3.2 Element Type Declarations
3.2.1 Element Content
3.2.2 Mixed Content
3.3 Attribute-List Declarations
3.3.1 Attribute Types
3.3.2 Attribute Defaults
3.3.3 Attribute-Value Normalization
3.4 Conditional Sections
4 Physical Structures
4.1 Character and Entity References
4.2 Entity Declarations
4.2.1 Internal Entities
4.2.2 External Entities
4.3 Parsed Entities
4.3.1 The Text Declaration
4.3.2 Well-Formed Parsed Entities
4.3.3 Character Encoding in Entities
4.4 XML Processor Treatment of Entities and References
4.4.1 Not Recognized
4.4.3 Included If Validating
4.4.5 Included in Literal
4.4.8 Included as PE
4.5 Construction of Internal Entity Replacement Text
4.6 Predefined Entities
4.7 Notation Declarations
4.8 Document Entity
5.1 Validating and Non-Validating Processors
5.2 Using XML Processors
A.1 Normative References
A.2 Other References
B Character Classes
C XML and SGML (Non-Normative)
D Expansion of Entity and Character References (Non-Normative)
E Deterministic Content Models (Non-Normative)
F Autodetection of Character Encodings (Non-Normative)
F.1 Detection Without External Encoding Information
F.2 Priorities in the Presence of External Encoding Information
G W3C XML Working Group (Non-Normative)
H W3C XML Core Group (Non-Normative)
I Production Notes (Non-Normative)
Extensible Markup Language, abbreviated XML, describes a class of data objects called XML documents and partially describes the behavior of computer programs which process them. XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.
XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.
[Definition: A software module called an XML processor is used to read XML documents and provide access to their content and structure.] [Definition: It is assumed that an XML processor is doing its work on behalf of another module, called the application.] This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.
XML was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the WG's contact with the W3C.
The design goals for XML are:
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.]
[Definition: An error which a conforming XML processor must detect and report to the application. After encountering a fatal error, the processor may continue processing the data to search for further errors and may report such errors to the application. In order to support correction of errors, the processor may make unprocessed data from the document (with intermingled character data and markup) available to the application. Once a fatal error is detected, however, the processor must not continue normal processing (i.e., it must not continue to pass character data and information about the document's logical structure to the application in the normal way).]
[Definition: Conforming software may or must (depending on the modal verb in the sentence) behave as described; if it does, it must provide users a means to enable or disable the behavior described.]
[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 Valid].]
[Definition: Marks a sentence describing a feature of XML included solely to ensure that XML remains compatible with SGML.]
[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 8879.]
Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup. The logical and physical structures must nest properly, as described in 4.3.2 Well-Formed Parsed Entities.
[Definition: A textual object is a well-formed XML document if:]
Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this specification.
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 discouraged.]
||/* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */|
The mechanism for encoding character code points into bit patterns may vary from entity to entity. All XML processors must accept the UTF-8 and UTF-16 encodings of 10646; the mechanisms for signaling which of the two is in use, or for bringing other encodings into play, are discussed later, in 4.3.3 Character Encoding in Entities.
This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x20) characters, carriage returns, line feeds, or tabs.
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 Classes.
[Definition: A Name is a token
beginning with a letter or one of a few punctuation characters, and continuing
with letters, digits, hyphens, underscores, colons, or full stops, together
known as name characters.] Names beginning with the string "
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.
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.
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.
Text consists of intermingled character data and markup. [Definition: Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character references, comments, CDATA section delimiters, document type declarations, processing instructions, XML declarations, text 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 markup).]
[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
processing instruction, or a CDATA
section. If they are needed elsewhere, they must be escaped using
either numeric character references or the strings
&" and "
<" respectively. The right
angle bracket (>) may be represented using the string
>", and must, for
compatibility, be escaped using "
>" or a character
reference when it appears in the string "
]]>" in content, when
that string is not marking the end of a CDATA
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
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.
<!-- B+, B, or B--->
[Definition: Processing instructions (PIs) allow documents to contain instructions for applications.]
PIs are not part of the document's character
data, but must be passed through to the application. The PI begins with a
target (PITarget) used to
identify the application to which the instruction is directed. The target names
xml", 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.
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 "
&". CDATA sections cannot nest.
<?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 document.
[Definition: The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.]
[Definition: A markup declaration is an element type declaration, an attribute-list declaration, an entity declaration, or a notation declaration.] These declarations may be contained in whole or in part within parameter entities, as described in the well-formedness and validity constraints below. For further information, see 4 Physical Structures.
||[VC: Root Element Type]|
|[WFC: External Subset]|
||[WFC: PE Between Declarations]|
||[VC: Proper Declaration/PE Nesting]|
|[WFC: PEs in Internal Subset]|
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, AttlistDecl, 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 described below.
Parameter-entity replacement text must be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration (markupdecl above) is contained in the replacement text for a parameter-entity reference, both must be contained in the same replacement text.
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.)
The external subset, if any, must match the production for extSubset.
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 "
the address (a URI reference) of a DTD for the document.
The declarations can also be given locally, as in this example:
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE greeting [ <!ELEMENT greeting (#PCDATA)> ]> <greeting>Hello, world!</greeting>
If both the external and internal subsets are used, the internal subset is considered to occur before the external subset. This has the effect that entity and attribute-list declarations in the internal subset take precedence over those in the external subset.
Markup declarations can affect the content of the document, as passed from an XML processor to an application; examples are attribute defaults and entity declarations. The standalone document declaration, which may appear as a component of the XML declaration, signals whether or not there are such declarations which appear external to the document entity 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).]
||[VC: 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.
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
attributes with values subject to normalization, where the attribute appears in the document with a value which will change as a result of normalization, or
element types with element content, if white space occurs directly within any instance of those types.
An example XML declaration with a standalone document declaration:
<?xml version="1.0" standalone='yes'?>
In editing XML documents, it is often convenient to use "white space" (spaces, tabs, and blank lines) to set apart the markup for greater readability. Such white space is typically not intended for inclusion in the delivered version of the document. On the other hand, "significant" white space that should be preserved in the delivered version is common, for example in poetry and source code.
An XML processor must always pass all characters in a document that are not markup through to the application. A validating XML processor must also inform the application which of these characters constitute white space appearing in element content.
A special attribute named
xml:space may be attached to an
element to signal an intention that in that element, white space should be
preserved by applications. In valid documents, this attribute, like any other,
must be declared if it is used. When declared, it
must be given as an enumerated type whose values are one or
both of "default" and "preserve". For example:
<!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.
XML parsed entities are often stored in computer files which, for editing convenience, are organized into lines. These lines are typically separated by some combination of the characters carriage-return (#xD) and line-feed (#xA).
To simplify the tasks of applications, 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.
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
[IETF RFC 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 Authority [IANA-LANGCODES]. It is expected that the successor to [IETF RFC 1766] will introduce three-letter language codes for languages not presently covered by [ISO 639].
(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> </sp>
The intent declared with
xml:lang is considered to apply to all
attributes and content of the element where it is specified, unless overridden
with an instance of
xml:lang on another element within that
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'>
[Definition: Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. Each element has a type, identified by name, sometimes called its "generic identifier" (GI), and may have a set of attribute specifications.] Each attribute specification has a name and a value.
||[WFC: Element Type Match]|
|[VC: Element Valid]|
This specification does not constrain the semantics, use, or (beyond syntax)
names of the element types and attributes, except that names beginning with a
(('X'|'x')('M'|'m')('L'|'l')) are reserved for
standardization in this or future versions of this specification.
The Name in an element's end-tag must match the element type in the start-tag.
The declaration matches EMPTY and the element has no content.
The declaration matches children and the sequence of child elements belongs to the language generated by the regular expression in the content model, with optional white space (characters matching the nonterminal S) between 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 ANY, and the types of any child elements have been declared.
[Definition: The beginning of every non-empty XML element is marked by a start-tag.]
||[WFC: Unique Att Spec]|
||[VC: Attribute Value Type]|
|[WFC: No External Entity References]|
|[WFC: No < in Attribute Values]|
The Name in the start- and
end-tags gives the element's type. [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
" delimiters) as the attribute
value.]Note that the order of attribute specifications in a start-tag or
empty-element tag is not significant.
No attribute name may appear more than once in the same start-tag or empty-element tag.
The attribute must have been declared; the value must be of the type declared for it. (For attribute types, see 3.3 Attribute-List Declarations.)
Attribute values cannot contain direct or indirect entity references to external entities.
The replacement 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:]
An example of an end-tag:
[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:]
||[WFC: Unique Att Spec]|
Empty-element tags may be used for any element which has no content, whether or not it is declared using the keyword EMPTY. For interoperability, the empty-element tag should be used, and should only be used, for elements which are declared EMPTY.
Examples of empty elements:
<IMG align="left" src="http://www.w3.org/Icons/WWW/w3c_home" /> <br></br> <br/>
Element type declarations often constrain which element types can appear as children of the element. At user option, an XML processor may issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.
[Definition: An element type declaration takes the form:]
||[VC: Unique Element Type Declaration]|
where the Name gives the element type being declared.
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>
[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 particles:
|[VC: Proper Group/PE Nesting]|
|[VC: Proper Group/PE Nesting]|
where each Name is the
type of an element which may appear as a child.
Any content particle in a choice list may appear in the element content at the location where the choice
list appears in the grammar; content particles occurring in a sequence list must
each appear in the element content in the order given in the list. The
optional character following a name or list governs whether the element or the
content particles in the list may occur one or more (
+), zero or
*), 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
The content of an element matches a content model if and only if it is possible to trace out a path through the content model, obeying the sequence, choice, and repetition operators and matching each element in the content against an element type in the content model. For compatibility, it is an error if an element in the document can match more than one occurrence of an element type in the content model. For more information, see E Deterministic Content Models.
Parameter-entity replacement 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.
For 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
Examples of element-content models:
<!ELEMENT spec (front, body, back?)> <!ELEMENT div1 (head, (p | list | note)*, div2*)> <!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*>
[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:
||[VC: Proper Group/PE Nesting]|
|[VC: No Duplicate Types]|
where the Names give the types of elements that may appear as children. The keyword #PCDATA derives historically from the term "parsed character data."
The same name must not appear more than once in a single mixed-content declaration.
Examples of mixed content declarations:
<!ELEMENT p (#PCDATA|a|ul|b|i|em)*> <!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* > <!ELEMENT b (#PCDATA)>
Attributes are used to associate name-value pairs with elements. Attribute specifications may appear only within start-tags and empty-element tags; thus, the productions used to recognize them appear in 3.1 Start-Tags, End-Tags, and Empty-Element Tags. Attribute-list declarations may be used:
To define the set of attributes pertaining to a given element type.
To establish type constraints for these attributes.
To provide default values for attributes.
[Definition: 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 the AttlistDecl rule is the type of an element. At user option, an XML processor may issue a warning if attributes are declared for an element type not itself declared, but this is not an error. The Name in the AttDef rule is the name of the attribute.
When more than one AttlistDecl is provided for a given element type, the contents of all those provided are merged. When more than one definition is provided for the same attribute of a given element type, the first declaration is binding and later declarations are ignored. For interoperability, writers of DTDs may choose to provide at most one attribute-list declaration for a given element type, at most one attribute definition for a given attribute name 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.
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 Declarations.
|[VC: One ID per Element Type]|
|[VC: ID Attribute Default]|
||[VC: Entity Name]|
||[VC: Entity Name]|
||[VC: Name Token]|
||[VC: Name Token]|
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.
No element type may have more than one ID attribute specified.
An ID attribute must have a declared default of #IMPLIED or #REQUIRED.
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.
[Definition: Enumerated attributes can take one of a list of values provided in the declaration]. There are two kinds of enumerated types:
||[VC: Notation Attributes]|
|[VC: One Notation Per Element Type]|
|[VC: No Notation on Empty Element]|
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.
Values of this type must match one of the notation names included in the declaration; all notation names in the declaration must be declared.
No element type may have more than one NOTATION attribute specified.
For compatibility, an attribute of type NOTATION must not be declared on an element declared EMPTY.
Values of this type must match one of the Nmtoken tokens in the declaration.
An attribute declaration provides information on whether the attribute's presence is required, and if not, how an XML processor should react if a declared attribute is absent in a document.
||[VC: Required Attribute]|
|[VC: Attribute Default Legal]|
|[WFC: No < in Attribute Values]|
|[VC: Fixed Attribute Default]|
In an attribute declaration, #REQUIRED means that the attribute must always be provided, #IMPLIED that no default value is provided. [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.]
If the default declaration is the keyword #REQUIRED, then the attribute must be specified for all elements of the type in the attribute-list declaration.
The declared default value must meet the lexical constraints of the declared attribute type.
If an attribute has a default value declared with the #FIXED keyword, instances of that attribute must match the default value.
Examples of attribute-list declarations:
<!ATTLIST termdef id ID #REQUIRED name CDATA #IMPLIED> <!ATTLIST list type (bullets|ordered|glossary) "ordered"> <!ATTLIST form method CDATA #FIXED "POST">
Before the value of an attribute is passed to the application or checked for validity, the XML processor must normalize 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 2.11 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 normalized value.
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 value.
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 declarations:
<!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
|Attribute specification||a is NMTOKENS||a is CDATA|
Note that the last example is invalid (but well-formed) if
declared to be of type NMTOKENS.
[Definition: Conditional sections are portions of the document type declaration external subset which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.]
|[VC: Proper Conditional Section/PE Nesting]|
|[VC: 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' > <![%draft;[ <!ELEMENT book (comments*, title, body, supplements?)> ]]> <![%final;[ <!ELEMENT book (title, body, supplements?)> ]]>
[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.
[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 attributes.
[Definition: General entities are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity.] [Definition: Parameter entities are parsed entities for use within the DTD.] These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.
[Definition: A character reference refers to a specific character in the ISO/IEC 10646 character set, for example one not directly accessible from available input devices.]
||[WFC: Legal Character]|
Characters referred to using character references must match the production for Char.
If the character reference begins with "
&#x", the digits and
letters up to the terminating
; provide a hexadecimal
representation of the character's code point in ISO/IEC 10646. If it begins just
&#", 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 (
delimiters.] [Definition: Parameter-entity
references use percent-sign (
%) and semicolon (
||[WFC: Entity Declared]|
|[VC: Entity Declared]|
|[WFC: Parsed Entity]|
|[WFC: No Recursion]|
||[VC: Entity Declared]|
|[WFC: No Recursion]|
|[WFC: In DTD]|
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 entity
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 standalone='yes'.
In a document with an external subset or external parameter entities with
standalone='no'", the Name given in the entity
reference must match that in an entity
declaration. For interoperability, valid documents should declare the
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.
A parsed entity must not contain a recursive reference to itself, either directly or indirectly.
Parameter-entity references may only appear in the DTD.
Examples of character and entity references:
Type <key>less-than</key> (<) to save options. This document was prepared on &docdate; and is classified &security-level;.
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... --> <!ENTITY % ISOLat2 SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" > <!-- ... now reference it. --> %ISOLat2;
[Definition: Entities are declared thus:]
The Name identifies the entity in an entity reference or, in the case of an unparsed entity, in the value of an ENTITY or ENTITIES attribute. If the same entity is declared more than once, the first declaration encountered is binding; at user option, an XML processor may issue a warning if entities are declared multiple times.
[Definition: If the entity definition is an EntityValue, the defined entity is called an internal entity. There is no separate physical storage object, and the content of the entity is given in the declaration.] Note that some processing of entity and character references in the literal entity value may be required to produce the correct replacement text: see 4.5 Construction of Internal Entity Replacement Text.
An internal entity is a parsed entity.
Example of an internal entity declaration:
<!ENTITY Pub-Status "This is a pre-release of the specification.">
[Definition: If the entity is not internal, it is an external entity, declared as follows:]
||[VC: Notation Declared]|
[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 with a
# 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
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 sequence.
[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:
<!ENTITY open-hatch SYSTEM "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY open-hatch PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN" "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY hatch-pic SYSTEM "../grafix/OpenHatch.gif" NDATA gif >
External parsed entities should each begin with a text declaration.
The text declaration must be provided literally, not by reference to a parsed entity. No text declaration may appear at any position other than the beginning of an external parsed entity. The text declaration in an external parsed entity is not considered part of its replacement text.
The document entity is well-formed if it matches the production labeled document. An external general parsed entity is well-formed if it matches the production labeled extParsedEnt. All external parameter entities are well-formed by definition.
An internal general parsed entity is well-formed if its replacement text matches the production labeled content. All internal parameter entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and physical structures in an XML document are properly nested; no start-tag, end-tag, empty-element tag, element, comment, processing instruction, character reference, or entity reference can begin in one entity and end in another.
Each external parsed entity in an XML document may use a different encoding for its characters. All XML processors must be able to read entities in 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 name contains only Latin characters */|
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
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:
<?xml encoding='UTF-8'?> <?xml encoding='EUC-JP'?>
The table below summarizes the contexts in which character references, entity references, and invocations of unparsed entities might appear and the required behavior of an XML processor in each case. The labels in the leftmost column describe the recognition context:
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.
as a reference within either the internal or external subsets of the DTD, but outside of an EntityValue, AttValue, PI, Comment, SystemLiteral, PubidLiteral, or the contents of an ignored conditional section (see 3.4 Conditional Sections).
|Parameter||Internal General||External Parsed General||Unparsed|
|Reference in Content||Not recognized||Included||Included if validating||Forbidden||Included|
|Reference in Attribute Value||Not recognized||Included in literal||Forbidden||Forbidden||Included|
|Occurs as Attribute Value||Not recognized||Forbidden||Forbidden||Notify||Not recognized|
|Reference in EntityValue||Included in literal||Bypassed||Bypassed||Forbidden||Included|
|Reference in DTD||Included as PE||Forbidden||Forbidden||Forbidden||Forbidden|
Outside the DTD, the
% character has no special significance;
thus, what would be parameter entity references in the DTD are not recognized as
markup in content.
Similarly, the names of unparsed entities are not recognized except when they
appear in the value of an appropriately declared attribute.
[Definition: An entity is
included when its replacement text is retrieved and processed, in
place of the reference itself, as though it were part of the document at the
location the reference was recognized.] The replacement text may contain both character
data and (except for parameter entities) markup, which
must be recognized in the usual way. (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
When an XML processor recognizes a reference to a parsed entity, in order to validate the document, the processor must include its replacement text. If the entity is external, and the processor is not attempting to validate the XML document, the processor may, but need not, include the entity's replacement text. If a non-validating 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:
When an entity reference appears in an attribute value, or a parameter entity reference appears in a literal entity value, its replacement text is processed in place of the reference itself as though it were part of the document at the location the reference was recognized, except that a single or double quote character in the replacement text is always treated as a normal data character and will not terminate the literal. For example, this is well-formed:
<!-- --> <!ENTITY % YN '"Yes"' > <!ENTITY WhatHeSaid "He said %YN;" >
while this is not:
<!ENTITY EndAttr "27'" > <element attribute='a-&EndAttr;>
When the name of an unparsed entity appears as a token in the value of an attribute of declared type ENTITY or ENTITIES, a validating processor must inform the application of the system and public (if any) identifiers for both the entity and its associated notation.
When a general entity reference appears in the EntityValue in an entity declaration, it is bypassed and left as is.
Just as with external parsed entities, parameter entities need only be included if validating. When a parameter-entity reference is recognized in the DTD and included, its replacement text is enlarged by the attachment of one leading and one following space (#x20) character; the intent is to constrain the replacement text of parameter entities to contain an integral number of grammatical tokens in the DTD. This behavior does not apply to parameter entity references within entity values; these are described in 4.4.5 Included in Literal.
In discussing the treatment of internal entities, it is useful to distinguish two forms of the entity's value. [Definition: The literal entity value is the quoted string actually present in the entity declaration, corresponding to the non-terminal EntityValue.] [Definition: The replacement text is the content of the entity, after replacement of character references and parameter-entity references.]
The literal entity value as given in an internal entity declaration (EntityValue) may contain character, parameter-entity, and general-entity references. Such references must be contained entirely within the literal entity value. The actual replacement text that is included as described above must contain the replacement text of any parameter entities referred to, and must contain the character referred to, in place of any character references in the literal entity value; however, general-entity references must be left as-is, unexpanded. For example, given the following declarations:
<!ENTITY % pub "Éditions Gallimard" > <!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.
[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
not. For interoperability, valid XML documents
should declare these entities, like any others, before using them. If the
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 entities
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
<!ENTITY lt "&#60;"> <!ENTITY gt ">"> <!ENTITY amp "&#38;"> <!ENTITY apos "'"> <!ENTITY quot """>
[Definition: Notation declarations provide a name for the notation, for use in entity and attribute-list declarations and in attribute specifications, and an external identifier for the notation which may allow an XML processor or its client application to locate a helper application capable of processing data in the given notation.]
||[VC: Unique Notation Name]|
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.)
[Definition: The document entity serves as the root of the entity tree and a starting-point for an XML processor.] This specification does not specify how the document entity is to be located by an XML processor; unlike other entities, the document entity has no name and might well appear on a processor input stream without any identification at all.
Conforming XML processors fall into two classes: validating and non-validating.
Validating and non-validating processors alike must report violations of this specification's well-formedness constraints in the content of the document entity and any other parsed entities that they read.
[Definition: Validating processors must, 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
standalone="yes", they must not process entity
declarations or attribute-list declarations encountered
after a reference to a parameter entity that is not read, since the entity may
have contained overriding declarations.
The behavior of a validating XML processor is highly predictable; it must read every piece of a document and report all well-formedness and validity violations. Less is required of a non-validating processor; it need not read any part of the document other than the document entity. This has two effects that may be important to users of XML processors:
Certain well-formedness errors, specifically those that require reading external entities, may not be detected by a non-validating processor. Examples include the constraints entitled Entity Declared, Parsed Entity, and No Recursion, as well as some of the cases described as forbidden in 4.4 XML Processor Treatment of Entities and References.
The information passed from the processor to the application may vary, depending on whether the processor reads parameter and external entities. For example, a non-validating processor may not normalize attribute values, include the replacement text of internal entities, or supply default attribute values, where doing so depends on having read declarations in external or parameter entities.
For maximum reliability in interoperating between different XML processors, applications which use non-validating processors should not rely on any behaviors not required of such processors. Applications which require facilities such as the use of default attributes or internal entities which are declared in external entities should use validating XML processors.
The formal grammar of XML is given in this specification using a simple Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines one symbol, in the form
symbol ::= expression
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:
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.
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, Nl.
Name characters other than Name-start characters must have one of the categories Mc, Me, Mn, Lm, or Nd.
Characters in the compatibility area (i.e. with character code greater than #xF900 and less than #xFFFE) are not allowed in XML names.
Characters which have a font or compatibility decomposition (i.e. those with a "compatibility formatting tag" in field 5 of the database -- marked by field 5 beginning with a "<") are not allowed.
The following characters are treated as name-start characters rather than name characters, because the property file classifies them as Alphabetic: [#x02BB-#x02C1], #x0559, #x06E5, #x06E6.
Characters #x20DD-#x20E0 are excluded (in accordance with Unicode 2.0, section 5.14).
Character #x00B7 is classified as an extender, because the property list so identifies it.
Character #x0387 is added as a name character, because #x00B7 is its canonical equivalent.
Characters ':' and '_' are allowed as name-start characters.
Characters '-' and '.' are allowed as name characters.
XML is designed to be a subset of SGML, in that every 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].
This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion, as specified in 4.4 XML Processor Treatment of Entities and References.
If the DTD contains the declaration
<!ENTITY example "<p>An ampersand (&#38;) may be escaped numerically (&#38;#38;) or with a general entity (&amp;).</p>" >
then the XML processor will recognize the character references when it parses
the entity declaration, and resolve them before storing the following string as
the value of the entity "
<p>An ampersand (&) may be escaped numerically (&#38;) or with a general entity (&amp;).</p>
A reference in the document to "
&example;" will cause the
text to be reparsed, at which time the start- and end-tags of the
element will be recognized and the three references will be recognized and
expanded, resulting in a
p element with the following content (all
data, no delimiters or markup):
An ampersand (&) may be escaped numerically (&) or with a general entity (&).
A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference.
1 <?xml version='1.0'?> 2 <!DOCTYPE test [ 3 <!ELEMENT test (#PCDATA) > 4 <!ENTITY % xx '%zz;'> 5 <!ENTITY % zz '<!ENTITY tricky "error-prone" >' > 6 %xx; 7 ]> 8 <test>This sample shows a &tricky; method.</test>
This produces the following:
in line 4, the reference to character 37 is expanded immediately, and the
parameter entity "
xx" is stored in the symbol table with the
%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.
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].
The XML encoding declaration functions as an internal label on each entity, indicating which character encoding is in use. Before an XML processor can read the internal label, however, it apparently has to know what character encoding is in use--which is what the internal label is trying to indicate. In the general case, this is a hopeless situation. It is not entirely hopeless in XML, however, because XML limits the general case in two ways: each implementation is assumed to support only a finite set of character encodings, and the XML encoding declaration is restricted in position and content in order to make it feasible to autodetect the character encoding in use in each entity in normal cases. Also, in many cases other sources of information are available in addition to the XML data stream itself. Two cases may be distinguished, depending on whether the XML entity is presented to the processor without, or with, any accompanying (external) information. We consider the first case first.
Because each XML entity not 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:
||UCS-4, big-endian machine (1234 order)|
||UCS-4, little-endian machine (4321 order)|
||UCS-4, unusual octet order (2143)|
||UCS-4, unusual octet order (3412)|
Without a Byte Order Mark:
||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.|
||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)|
||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 which)|
||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 reliably|
||EBCDIC (in some flavor; the full encoding declaration must be read to tell which code page is in use)|
|Other||UTF-8 without an encoding declaration, or else the data stream is 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 present.
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.
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
If an XML entity is in a file, the Byte-Order Mark and encoding declaration are used (if present) to determine the character encoding.
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:
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 edition were:
This Second Edition was encoded in the XMLspec DTD (which has documentation available). The HTML versions were produced with a combination of the xmlspec.xsl, diffspec.xsl, and REC-xml-2e.xsl XSLT stylesheets. The PDF version was produced with the html2ps facility and a distiller program.