This document specifies a syntax created by subsetting an existing, widely used international text processing standard (Standard Generalized Markup Language, ISO 8879:1986(E) as amended and corrected) for use on the World Wide Web. It is a product of the W3C XML Activity, details of which can be found at http://www.w3.org/XML. A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.
This specification uses the term URI, which is defined by [Berners-Lee et al.], a work in progress expected to update [IETF RFC1738] and [IETF RFC1808].
The list of known errors in this specification is available at http://www.w3.org/XML/xml-19980210-errata.
Please report errors in this document to email@example.com.
XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.
A software module called an XML processor is used to read XML documents and provide access to their content and structure. It is assumed that an XML processor is doing its work on behalf of another module, called the application. This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.
The design goals for XML are:
This version of the XML specification may be distributed freely, as long as all text and legal notices remain intact.
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".
Matching the document production implies that:
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".
S (white space) consists of one or more
space (#x20) characters, carriage returns, line feeds, or tabs.
Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic or syllabic base character possibly followed by one or more combining characters, or of an ideographic character. Full definitions of the specific characters in each class are given in "B. Character Classes".
A Name is a token beginning with a letter or one of a few punctuation characters, and continuing with letters, digits, hyphens, underscores, colons, or full stops, together known as name characters. Names beginning with the string "xml", or any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.
Note: The colon character within XML names is reserved for experimentation with name spaces. Its meaning is expected to be standardized at some future point, at which point those documents using the colon for experimental purposes may need to be updated. (There is no guarantee that any name-space mechanism adopted for XML will in fact use the colon as a name-space delimiter.) In practice, this means that authors should not use the colon in XML names except as part of name-space experiments, but that XML processors should accept the colon as a name character.
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.
All text that is not markup constitutes the character data of the document.
The ampersand character (&) and the left angle bracket (<) may appear in their literal form only when used as markup delimiters, or within a comment, a processing instruction, or a CDATA section. They are also legal within the literal entity value of an internal entity declaration; see "4.3.2 Well-Formed Parsed Entities". If they are needed elsewhere, they must be escaped using either numeric character references or the strings "&" and "<" respectively. The right angle bracket (>) may be represented using the string ">", and must, for compatibility, be escaped using ">" or a character reference when it appears in the string "]]>" in content, when that string is not marking the end of a CDATA section.
In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup. In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, "]]>".
To allow attribute values to contain both single and double quotes,
the apostrophe or single-quote character (') may be represented as "'",
and the double-quote character (") as """.
An example of a comment:
|<!-- declarations for <head> & <body> -->|
PIs are not part of the document's character data, but must be passed through to the application. The PI begins with a target (PITarget) used to identify the application to which the instruction is directed. The target names "XML", "xml", and so on are reserved for standardization in this or future versions of this specification. The XML Notation mechanism may be used for formal declaration of PI targets.
Within a CDATA section, only the CDEnd string is recognized as markup, so that left angle brackets and ampersands may occur in their literal form; they need not (and cannot) be escaped using "<" and "&". CDATA sections cannot nest.
An example of a CDATA section, in which "<greeting>" and
"</greeting>" are recognized as character
and so is this:
The version number "1.0" should be used to indicate conformance to this version of this specification; it is an error for a document to use the value "1.0" if it does not conform to this version of this specification. It is the intent of the XML working group to give later versions of this specification numbers other than "1.0", but this intent does not indicate a commitment to produce any future versions of XML, nor if any are produced, to use any particular numbering scheme. Since future versions are not ruled out, this construct is provided as a means to allow the possibility of automatic version recognition, should it become necessary. Processors may signal an error if they receive documents labeled with versions they do not support.
The function of the markup in an XML document is to describe its storage and logical structure and to associate attribute-value pairs with its logical structures. XML provides a mechanism, the document type declaration, to define constraints on the logical structure and to support the use of predefined storage units. An XML document is valid if it has an associated document type declaration and if the document complies with the constraints expressed in it.
The document type declaration must appear before the first element
in the document.
The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.
A markup declaration is an element
type declaration, an attribute-list declaration,
an entity declaration, or a notation
declaration. These declarations may be contained in whole or in part
within parameter entities, as described in the well-formedness
and validity constraints below. For fuller information, see "4.
|Document Type Definition|
The markup declarations may be made up in whole or in part of the replacement
text of parameter entities. The productions later
in this specification for individual nonterminals (elementdecl,
and so on) describe the declarations after all the parameter entities
have been included.
Validity Constraint: Root Element Type
The Name in the document type declaration must match the element type of the root element.
Validity Constraint: Proper Declaration/PE Nesting
Parameter-entity replacement text must be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration (markupdecl above) is contained in the replacement text for a parameter-entity reference, both must be contained in the same replacement text.
Well-Formedness Constraint: PEs in Internal Subset
In the internal DTD subset, parameter-entity references can occur only where markup declarations can occur, not within markup declarations. (This does not apply to references that occur in external parameter entities or to the external subset.)
Like the internal subset, the external subset and any external parameter
entities referred to in the DTD must consist of a series of complete markup
declarations of the types allowed by the non-terminal symbol
interspersed with white space or parameter-entity references.
However, portions of the contents of the external subset or of external
parameter entities may conditionally be ignored by using the conditional
section construct; this is not allowed in the internal subset.
The external subset and external parameter entities also differ from the internal subset in that in them, parameter-entity references are permitted within markup declarations, not only between markup declarations.
An example of an XML document with a document type declaration:
<!DOCTYPE greeting SYSTEM "hello.dtd">
The system identifier "hello.dtd" gives the URI of a DTD for the document.
The declarations can also be given locally, as in this example:
|<?xml version="1.0" encoding="UTF-8" ?>
<!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.
|Standalone Document Declaration|
In a standalone document declaration, the value "yes" indicates that there are no markup declarations external to the document entity (either in the DTD external subset, or in an external parameter entity referenced from the internal subset) which affect the information passed from the XML processor to the application. The value "no" indicates that there are or may be such external markup declarations. Note that the standalone document declaration only denotes the presence of external declarations; the presence, in a document, of references to external entities, when those entities are internally declared, does not change its standalone status.
If there are no external markup declarations, the standalone document declaration has no meaning. If there are external markup declarations but there is no standalone document declaration, the value "no" is assumed.
Any XML document for which standalone="no" holds can be converted
algorithmically to a standalone document, which may be desirable for some
network delivery applications.
Validity Constraint: Standalone Document Declaration
The standalone document declaration must have the value "no" if any external markup declarations contain declarations of:
|<?xml version="1.0" standalone='yes'?>|
An XML processor must always pass all characters in a document that are not markup through to the application. A validating XML processor must also inform the application which of these characters constitute white space appearing in element content.
A special attribute named xml:space
may be attached to an element to signal an intention that in that element,
white space should be preserved by applications. In valid documents, this
attribute, like any other, must be declared if
it is used. When declared, it must be given as an enumerated
type whose only possible values are "default" and "preserve".
|<!ATTLIST poem xml:space (default|preserve) 'preserve'>|
The value "default" signals that applications' default white-space processing modes are acceptable for this element; the value "preserve" indicates the intent that applications preserve all the white space. This declared intent is considered to apply to all elements within the content of the element where it is specified, unless overriden with another instance of the xml:space attribute.
The root element of any document is considered to have signaled no intentions as regards application space handling, unless it provides a value for this attribute or the attribute is declared with a default value.
To simplify the tasks of applications, wherever an external parsed entity or the literal entity value of an internal parsed entity contains either the literal two-character sequence "#xD#xA" or a standalone literal #xD, an XML processor must pass to the application the single character #xA. (This behavior can conveniently be produced by normalizing all line breaks to #xA on input, before parsing.)
The Langcode may be any of the following:
It is customary to give the language code in lower case, and the country code (if any) in upper case. Note that these values, unlike other names in XML documents, are case insensitive.
|<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 content.
A simple declaration for xml:lang might take the form
|xml:lang NMTOKEN #IMPLIED|
but specific default values may also be given, if appropriate. In a
collection of French poems for English students, with glosses and notes
in English, the xml:lang attribute might be declared this way:
| <!ATTLIST poem xml:lang NMTOKEN
<!ATTLIST gloss xml:lang NMTOKEN 'en'>
<!ATTLIST note xml:lang NMTOKEN 'en'>
This specification does not constrain the semantics, use, or (beyond
syntax) names of the element types and attributes, except that names beginning
with a match to (('X'|'x')('M'|'m')('L'|'l')) are reserved for
standardization in this or future versions of this specification.
Well-Formedness Constraint: Element Type Match
The Name in an element's end-tag must match the element type in the start-tag.
Validity Constraint: Element Valid
An element is valid if there is a declaration matching elementdecl where the Name matches the element type, and one of the following holds:
The Name in the start- and end-tags
gives the element's type.The Name-AttValue
pairs are referred to as the attribute specifications of the element,with
the Name in each pair referred to as the
attribute name andthe content of the
(the text between the
' or " delimiters) as the attribute
Well-Formedness Constraint: Unique Att Spec
No attribute name may appear more than once in the same start-tag or empty-element tag.
Validity Constraint: Attribute Value Type
The attribute must have been declared; the value must be of the type declared for it. (For attribute types, see "3.3 Attribute-List Declarations".)
Well-Formedness Constraint: No External Entity References
Attribute values cannot contain direct or indirect entity references to external entities.
Well-Formedness Constraint: No < in Attribute Values
The replacement text of any entity referred to directly or indirectly in an attribute value (other than "<") must not contain a <.
An example of a start-tag:
|<termdef id="dt-dog" term="dog">|
The end of every element that begins with a start-tag
must be marked by an end-tag containing a name that echoes the element's
type as given in the start-tag:
An example of an end-tag:
The text between the
start-tag and end-tag is called the element's
|Content of Elements|
If an element is empty, it must be represented
either by a start-tag immediately followed by an end-tag or by an empty-element
tag.An empty-element tag takes a special
|Tags for Empty Elements|
Empty-element tags may be used for any element which has no content, whether or not it is declared using the keyword EMPTY. For interoperability, the empty-element tag must be used, and can only be used, for elements which are declared EMPTY.
Examples of empty elements:
Element type declarations often constrain which element types can appear as children of the element. At user option, an XML processor may issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.
An element type declaration takes the
|Element Type Declaration|
where the Name gives the element type
Validity Constraint: Unique Element Type Declaration
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>
where each Name is the type of an element which may appear as a child. Any content particle in a choice list may appear in the element content at the location where the choice list appears in the grammar; content particles occurring in a sequence list must each appear in the element content in the order given in the list. The optional character following a name or list governs whether the element or the content particles in the list may occur one or more (+), zero or more (*), or zero or one times (?). The absence of such an operator means that the element or content particle must appear exactly once. This syntax and meaning are identical to those used in the productions in this specification.
The content of an element matches a content model if and only if it
is possible to trace out a path through the content model, obeying the
sequence, choice, and repetition operators and matching each element in
the content against an element type in the content model. For
compatibility, it is an error if an element in the document can match
more than one occurrence of an element type in the content model. For more
information, see "E. Deterministic Content Models".
Validity Constraint: Proper Group/PE Nesting
Parameter-entity replacement text must be properly nested with parenthetized groups. That is to say, if either of the opening or closing parentheses in a choice, seq, or Mixed construct is contained in the replacement text for a parameter entity, both must be contained in the same replacement text. For interoperability, if a parameter-entity reference appears in a choice, seq, or Mixed construct, its replacement text should not be empty, and neither the first nor last non-blank character of the replacement text should be a connector (| or ,).
Examples of element-content models:
|<!ELEMENT spec (front, body, back?)>
<!ELEMENT div1 (head, (p | list | note)*, div2*)>
<!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*>
where the Names give the types of elements
that may appear as children.
Validity Constraint: No Duplicate Types
The same name must not appear more than once in a single mixed-content declaration.
Examples of mixed content declarations:
|<!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
<!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* >
<!ELEMENT b (#PCDATA)>
The Name in the AttlistDecl rule is the type of an element. At user option, an XML processor may issue a warning if attributes are declared for an element type not itself declared, but this is not an error. The Name in the AttDef rule is the name of the attribute.
When more than one AttlistDecl is provided for a given element type, the contents of all those provided are merged. When more than one definition is provided for the same attribute of a given element type, the first declaration is binding and later declarations are ignored. For interoperability, writers of DTDs may choose to provide at most one attribute-list declaration for a given element type, at most one attribute definition for a given attribute name, and at least one attribute definition in each attribute-list declaration. For interoperability, an XML processor may at user option issue a warning when more than one attribute-list declaration is provided for a given element type, or more than one attribute definition is provided for a given attribute, but this is not an error.
Validity Constraint: ID
Values of type ID must match the Name production. A name must not appear more than once in an XML document as a value of this type; i.e., ID values must uniquely identify the elements which bear them.
Validity Constraint: One ID per Element Type
No element type may have more than one ID attribute specified.
Validity Constraint: ID Attribute Default
An ID attribute must have a declared default of #IMPLIED or #REQUIRED.
Validity Constraint: IDREF
Values of type IDREF must match the Name production, and values of type IDREFS must match Names; each Name must match the value of an ID attribute on some element in the XML document; i.e. IDREF values must match the value of some ID attribute.
Validity Constraint: Entity Name
Values of type ENTITY must match the Name production, values of type ENTITIES must match Names; each Name must match the name of an unparsed entity declared in the DTD.
Validity Constraint: Name Token
Values of type NMTOKEN must match the Nmtoken production; values of type NMTOKENS must match Nmtokens.
Enumerated attributes can take one
of a list of values provided in the declaration. There are two kinds of
|Enumerated Attribute Types|
A NOTATION attribute identifies a notation,
declared in the DTD with associated system and/or public identifiers, to
be used in interpreting the element to which the attribute is attached.
Validity Constraint: Notation Attributes
Values of this type must match one of the notation names included in the declaration; all notation names in the declaration must be declared.
Validity Constraint: Enumeration
Values of this type must match one of the Nmtoken tokens in the declaration.
For interoperability, the same Nmtoken should not occur more than once in the enumerated attribute types of a single element type.
In an attribute declaration, #REQUIRED means that the attribute
must always be provided, #IMPLIED that no default value is provided. If
the declaration is neither #REQUIRED nor #IMPLIED, then
AttValue value contains the declared
value; the #FIXED keyword states that the attribute must always
have the default value. If a default value is declared, when an XML processor
encounters an omitted attribute, it is to behave as though the attribute
were present with the declared default value.
Validity Constraint: Required Attribute
If the default declaration is the keyword #REQUIRED, then the attribute must be specified for all elements of the type in the attribute-list declaration.
Validity Constraint: Attribute Default Legal
The declared default value must meet the lexical constraints of the declared attribute type.
Validity Constraint: Fixed Attribute Default
If an attribute has a default value declared with the #FIXED keyword, instances of that attribute must match the default value.
Examples of attribute-list declarations:
id ID #REQUIRED
name CDATA #IMPLIED>
type (bullets|ordered|glossary) "ordered">
method CDATA #FIXED "POST">
All attributes for which no declaration has been read should be treated by a non-validating parser as if declared CDATA.
Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, processing instructions, or nested conditional sections, intermingled with white space.
If the keyword of the conditional section is INCLUDE, then the contents of the conditional section are part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section are not logically part of the DTD. Note that for reliable parsing, the contents of even ignored conditional sections must be read in order to detect nested conditional sections and ensure that the end of the outermost (ignored) conditional section is properly detected. If a conditional section with a keyword of INCLUDE occurs within a larger conditional section with a keyword of IGNORE, both the outer and the inner conditional sections are ignored.
If the keyword of the conditional section is a parameter-entity reference, the parameter entity must be replaced by its content before the processor decides whether to include or ignore the conditional section.
|<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >
Entities may be either parsed or unparsed.A parsed entity's contents are referred to as its replacement text; this text is considered an integral part of the document.
An unparsed entity is a resource whose contents may or may not be text, and if text, may not be XML. Each unparsed entity has an associated notation, identified by name. Beyond a requirement that an XML processor make the identifiers for the entity and notation available to the application, XML places no constraints on the contents of unparsed entities.
Parsed entities are invoked by name using entity references; unparsed entities by name, given in the value of ENTITY or ENTITIES attributes.
General entities are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity. Parameter entities are parsed entities for use within the DTD. These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.
Well-Formedness Constraint: 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 with "&#", the digits up to the terminating ; provide a decimal representation of the character's code point.
An entity reference refers to the content
of a named entity.References to parsed general entities
use ampersand (&) and semicolon (;) as delimiters.Parameter-entity
references use percent-sign (%) and semicolon (;)
Well-Formedness Constraint: Entity Declared
In a document without any DTD, a document with only an internal DTD subset which contains no parameter entity references, or a document with "standalone='yes'", the Name given in the entity reference must match that in an entity declaration, except that well-formed documents need not declare any of the following entities: amp, lt, gt, apos, quot. The declaration of a parameter entity must precede any reference to it. Similarly, the declaration of a general entity must precede any reference to it which appears in a default value in an attribute-list declaration. Note that if entities are declared in the external subset or in external parameter entities, a non-validating processor is not obligated to read and process their declarations; for such documents, the rule that an entity must be declared is a well-formedness constraint only if standalone='yes'.
Validity Constraint: Entity Declared
In a document with an external subset or external parameter entities with "standalone='no'", the Name given in the entity reference must match that in an entity declaration. For interoperability, valid documents should declare the entities amp, lt, gt, apos, quot, in the form specified in "4.6 Predefined Entities". The declaration of a parameter entity must precede any reference to it. Similarly, the declaration of a general entity must precede any reference to it which appears in a default value in an attribute-list declaration.
Well-Formedness Constraint: Parsed Entity
An entity reference must not contain the name of an unparsed entity. Unparsed entities may be referred to only in attribute values declared to be of type ENTITY or ENTITIES.
Well-Formedness Constraint: No Recursion
A parsed entity must not contain a recursive reference to itself, either directly or indirectly.
Well-Formedness Constraint: In DTD
Parameter-entity references may only appear in the DTD.
Examples of character and entity references:
|Type <key>less-than</key> (<) to save options.
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. -->
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.
An internal entity is a parsed entity.
Example of an internal entity declaration:
|<!ENTITY Pub-Status "This is a pre-release of the
|External Entity Declaration|
If the NDataDecl is present, this
is a general unparsed entity; otherwise it is
a parsed entity.
Validity Constraint: Notation Declared
The Name must match the declared name of a notation.
The SystemLiteral is called the entity's system identifier. It is a URI, which may be used to retrieve the entity. Note that the hash mark (#) and fragment identifier frequently used with URIs are not, formally, part of the URI itself; an XML processor may signal an error if a fragment identifier is given as part of a system identifier. Unless otherwise provided by information outside the scope of this specification (e.g. a special XML element type defined by a particular DTD, or a processing instruction defined by a particular application specification), relative URIs are relative to the location of the resource within which the entity declaration occurs. A URI might thus be relative to the document entity, to the entity containing the external DTD subset, or to some other external parameter entity.
An XML processor should handle a non-ASCII character in a URI by representing the character in UTF-8 as one or more bytes, and then escaping these bytes with the URI escaping mechanism (i.e., by converting each byte to %HH, where HH is the hexadecimal notation of the byte value).
In addition to a system identifier, an external identifier may include a public identifier. An XML processor attempting to retrieve the entity's content may use the public identifier to try to generate an alternative URI. If the processor is unable to do so, it must use the URI specified in the system literal. Before a match is attempted, all strings of white space in the public identifier must be normalized to single space characters (#x20), and leading and trailing white space must be removed.
Examples of external entity declarations:
PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
NDATA gif >
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.
|Well-Formed External Parsed Entity|
An internal general parsed entity is well-formed if its replacement text matches the production labeled content. All internal parameter entities are well-formed by definition.
A consequence of well-formedness in entities is that the logical and physical structures in an XML document are properly nested; no start-tag, end-tag, empty-element tag, element, comment, processing instruction, character reference, or entity reference can begin in one entity and end in another.
Entities encoded in UTF-16 must begin with the Byte Order Mark described by ISO/IEC 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors must be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.
Although an XML processor is required to read only entities in the UTF-8
and UTF-16 encodings, it is recognized that other encodings are used around
the world, and it may be desired for XML processors to read entities that
use them. Parsed entities which are stored in an encoding other than UTF-8
or UTF-16 must begin with a text declaration containing
an encoding declaration:
In the document entity, the encoding declaration is part of the XML declaration. The EncName is the name of the encoding used.
In an encoding declaration, the values "UTF-8", "UTF-16", "ISO-10646-UCS-2", and "ISO-10646-UCS-4" should be used for the various encodings and transformations of Unicode / ISO/IEC 10646, the values "ISO-8859-1", "ISO-8859-2", ... "ISO-8859-9" should be used for the parts of ISO 8859, and the values "ISO-2022-JP", "Shift_JIS", and "EUC-JP" should be used for the various encoded forms of JIS X-0208-1997. XML processors may recognize other encodings; it is recommended that character encodings registered (as charsets) with the Internet Assigned Numbers Authority [IANA], other than those just listed, should be referred to using their registered names. Note that these registered names are defined to be case-insensitive, so processors wishing to match against them should do so in a case-insensitive way.
In the absence of information provided by an external transport protocol (e.g. HTTP or MIME), it is an error for an entity including an encoding declaration to be presented to the XML processor in an encoding other than that named in the declaration, for an encoding declaration to occur other than at the beginning of an external entity, or for an entity which begins with neither a Byte Order Mark nor an encoding declaration to use an encoding other than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary ASCII entities do not strictly need an encoding declaration.
It is a fatal error when an XML processor encounters an entity with an encoding that it is unable to process.
Examples of encoding declarations:
|Not recognized||Included||Included if validating||Forbidden||Included|
in Attribute Value
|Not recognized||Included in literal||Forbidden||Forbidden||Included|
|Not recognized||Forbidden||Forbidden||Notify||Not recognized|
|Included in literal||Bypassed||Bypassed||Forbidden||Included|
|Included as PE||Forbidden||Forbidden||Forbidden||Forbidden|
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.
|<!ENTITY % YN '"Yes"' >
<!ENTITY WhatHeSaid "He said &YN;" >
while this is not:
|<!ENTITY EndAttr "27'" >
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 "book" is:
|La Peste: Albert Camus,
© 1947 Éditions Gallimard. &rights;
The general-entity reference "&rights;" would be expanded should the reference "&book;" appear in the document's content or an attribute value.
These simple rules may have complex interactions; for a detailed discussion of a difficult example, see "D. Expansion of Entity and Character References".
All XML processors must recognize these entities whether they are declared
or not. For interoperability, valid XML documents
should declare these entities, like any others, before using them. If the
entities in question are declared, they must be declared as internal entities
whose replacement text is the single character being escaped or a character
reference to that character, as shown below.
|<!ENTITY lt "&#60;">
<!ENTITY gt ">">
<!ENTITY amp "&#38;">
<!ENTITY apos "'">
<!ENTITY quot """>
Note that the < and & characters in the declarations of "lt" and "amp" are doubly escaped to meet the requirement that entity replacement be well-formed.
Notation declarations provide a name
for the notation, for use in entity and attribute-list declarations and
in attribute specifications, and an external identifier for the notation
which may allow an XML processor or its client application to locate a
helper application capable of processing data in the given notation.
XML processors must provide applications with the name and external identifier(s) of any notation declared and referred to in an attribute value, attribute definition, or entity declaration. They may additionally resolve the external identifier into the system identifier, file name, or other information needed to allow the application to call a processor for data in the notation described. (It is not an error, however, for XML documents to declare and refer to notations for which notation-specific applications are not available on the system where the XML processor or application is running.)
Validating and non-validating processors alike must report violations of this specification's well-formedness constraints in the content of the document entity and any other parsed entities that they read.
Validating processors must report violations of the constraints expressed by the declarations in the DTD, and failures to fulfill the validity constraints given in this specification. To accomplish this, validating XML processors must read and process the entire DTD and all external parsed entities referenced in the document.
Non-validating processors are required to check only the document entity, including the entire internal DTD subset, for well-formedness.While they are not required to check the document for validity, they are required to process all the declarations they read in the internal DTD subset and in any parameter entity that they read, up to the first reference to a parameter entity that they do not read; that is to say, they must use the information in those declarations to normalize attribute values, include the replacement text of internal entities, and supply default attribute values. They must not processentity 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.
|symbol ::= expression|
Symbols are written with an initial capital letter if they are defined by a regular expression, or with an initial lower case letter otherwise. Literal strings are quoted.
Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters:
The character classes defined here can be derived from the Unicode character database as follows:
If the DTD contains the declaration
|<!ENTITY example "<p>An ampersand (&#38;) may be
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 "example":
|<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 "p"
element will be recognized and the three references will be recognized
and expanded, resulting in a "p" element with the following content
(all data, no delimiters or markup):
|An ampersand (&) may be escaped
numerically (&) or with a general entity
A more complex example will illustrate the rules and their effects fully.
In the following example, the line numbers are solely for reference.
|1 <?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:
SGML requires deterministic content models (it calls them "unambiguous"); XML processors built using SGML systems may flag non-deterministic content models as errors.
For example, the content model ((b, c) | (b, d)) is non-deterministic, because given an initial b the parser cannot know which b in the model is being matched without looking ahead to see which element follows the b. In this case, the two references to b can be collapsed into a single reference, making the model read (b, (c | d)). An initial b now clearly matches only a single name in the content model. The parser doesn't need to look ahead to see what follows; either c or d would be accepted.
More formally: a finite state automaton may be constructed from the content model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi, and Ullman [Aho/Ullman]. In many such algorithms, a follow set is constructed for each position in the regular expression (i.e., each leaf node in the syntax tree for the regular expression); if any position has a follow set in which more than one following position is labeled with the same element type name, then the content model is in error and may be reported as an error.
Algorithms exist which allow many but not all non-deterministic content models to be reduced automatically to equivalent deterministic models; see Brüggemann-Klein 1991 [Brüggemann-Klein].
Because each XML entity not in UTF-8 or UTF-16 format must begin with an XML encoding declaration, in which the first characters must be '<?xml', any conforming processor can detect, after two to four octets of input, which of the following cases apply. In reading this list, it may help to know that in UCS-4, '<' is "#x0000003C" and '?' is "#x0000003F", and the Byte Order Mark required of UTF-16 data streams is "#xFEFF".
Because the contents of the encoding declaration are restricted to ASCII characters, a processor can reliably read the entire encoding declaration as soon as it has detected which family of encodings is in use. Since in practice, all widely used character encodings fall into one of the categories above, the XML encoding declaration allows reasonably reliable in-band labeling of character encodings, even when external sources of information at the operating-system or transport-protocol level are unreliable.
Once the processor has detected the character encoding in use, it can act appropriately, whether by invoking a separate input routine for each case, or by calling the proper conversion function on each character of input.
Like any self-labeling system, the XML encoding declaration will not work if any software changes the entity's character set or encoding without updating the encoding declaration. Implementors of character-encoding routines should be careful to ensure the accuracy of the internal and external information used to label the entity.
The second possible case occurs when the XML entity is accompanied by encoding information, as in some file systems and some network protocols. When multiple sources of information are available, their relative priority and the preferred method of handling conflict should be specified as part of the higher-level protocol used to deliver XML. Rules for the relative priority of the internal label and the MIME-type label in an external header, for example, should be part of the RFC document defining the text/xml and application/xml MIME types. In the interests of interoperability, however, the following rules are recommended.
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