REC-xml-19980210REC-xml-19980210The 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. A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.
This specification uses the term URI, which is defined by [Berners-Lee et al.], a work in progress expected to update [IETF RFC1738] and [IETF RFC1808].
The list of known errors in this specification is available at http://www.w3.org/XML/xml-19980210-errata.
Please report errors in this document to xml-editor@w3.org.
Extensible Markup Language, abbreviated XML, describes a class of data objects called XML documents and partially describes the behavior of computer programs which process them. XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.
XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.
A software module called an XML processor is used to read XML documents and provide access to their content and structure. It is assumed that an XML processor is doing its work on behalf of another module, called the application. This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.
XML was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the WG's contact with the W3C.
The design goals for XML are:
This specification, together with associated standards (Unicode and ISO/IEC 10646 for characters, Internet RFC 1766 for language identification tags, ISO 639 for language name codes, and ISO 3166 for country name codes), provides all the information necessary to understand XML Version 1.0 and construct computer programs to process it.
This version of the XML specification may be distributed freely, as long as all text and legal notices remain intact.
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:
A data object is an XML document if it is well-formed, as defined in this specification. A well-formed XML document may in addition be valid if it meets certain further constraints.
Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup. The logical and physical structures must nest properly, as described in "4.3.2 Well-Formed Parsed Entities".
A textual object is a well-formed XML document if:
document.
| Document | ||||
|
Matching the document production implies that:
As a
consequence of this, for each non-root element C in the document, there is one other element P in the document such that C is in the content of P, but is not in the content of any other element
that is in the content of P. P is referred to as the parent of C, and
C as a child of
P.
A parsed entity contains text, a sequence of characters, which may represent markup or character data. A character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646]. Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646. The use of "compatibility characters", as defined in section 6.8 of [Unicode], is discouraged.
| Character Range | ||||||
|
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.
| White Space | ||||
|
Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic or syllabic base character possibly followed by one or more combining characters, or of an ideographic character. Full definitions of the specific characters in each class are given in "B. Character Classes".
A Name is a token beginning with a letter or one of a few
punctuation characters, and continuing with letters, digits, hyphens,
underscores, colons, or full stops, together known as name characters. Names
beginning with the string "xml", or any string
which would match (('X'|'x') ('M'|'m')
('L'|'l')), are reserved for standardization in this or future versions
of this specification.
Note: The colon character within XML names is reserved for experimentation with name spaces. Its meaning is expected to be standardized at some future point, at which point those documents using the colon for experimental purposes may need to be updated. (There is no guarantee that any name-space mechanism adopted for XML will in fact use the colon as a name-space delimiter.) In practice, this means that authors should not use the colon in XML names except as part of name-space experiments, but that XML processors should accept the colon as a name character.
An Nmtoken (name token)
is any mixture of name characters.
| Names and Tokens | ||||||||||||||||||||
|
Literal data is any quoted string not containing the
quotation mark used as a delimiter for that string. Literals are used for
specifying the content of internal entities (EntityValue), the values of
attributes (AttValue), and
external identifiers (SystemLiteral). Note that a SystemLiteral can be parsed without
scanning for markup.
| Literals | ||||||||||||||||||||||||||||
|
Text consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character references, comments, CDATA section delimiters, document type declarations, and processing instructions.
All text that is not markup constitutes the character data of the document.
The ampersand character (&) and the left angle
bracket (<) may appear in their literal form only when used as markup delimiters, or within a comment,
a processing
instruction, or a CDATA section. They are also legal within the
literal entity value of an internal entity
declaration; see "4.3.2 Well-Formed Parsed Entities". If
they are needed elsewhere, they must be escaped using either numeric
character references or the strings "&" and "<" respectively. The right angle bracket
(>) may be represented using the string ">", and must, for compatibility, be
escaped using ">" or a character
reference when it appears in the string "]]>" in content, when that string is not marking
the end of a CDATA section.
In the content of elements, character data is any string
of characters which does not contain the start-delimiter of any markup. In a
CDATA section, character data is any string of characters not including the
CDATA-section-close delimiter, "]]>".
To allow attribute values to contain both single and
double quotes, the apostrophe or single-quote character (') may be represented
as "'", and the double-quote
character (") as """.
| Character Data | ||||
|
Comments may appear anywhere in a document outside
other markup; in addition, they may appear within the
document type declaration at places allowed by the grammar. They are not part of
the document's character data; an XML processor may, but need
not, make it possible for an application to retrieve the text of comments. For
compatibility, the string "--"
(double-hyphen) must not occur within comments.
| Comments | ||||
|
An example of a comment:
<!-- declarations for <head> & <body> --> |
Processing instructions (PIs) allow documents to contain instructions for applications.
| Processing Instructions | ||||||||
|
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.
CDATA sections may occur anywhere character data may
occur; they are used to escape blocks of text containing characters which would
otherwise be recognized as markup. CDATA sections begin with the string "<![CDATA[" and end with the string "]]>":
| CDATA Sections | ||||||||||||||||
|
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
data, not markup:
<![CDATA[<greeting>Hello, world!</greeting>]]> |
XML documents may, and should, begin with an XML declaration which specifies the version of XML being used. For example, the following is a complete XML document, well-formed but not valid:
<?xml version="1.0"?> |
and so is this:
<greeting>Hello, world!</greeting> |
The version number "1.0" should be used to indicate conformance to this
version of this specification; it is an error for a document to use the value
"1.0" if it does not conform to this version
of this specification. It is the intent of the XML working group to give later
versions of this specification numbers other than "1.0", but this intent does not indicate a commitment
to produce any future versions of XML, nor if any are produced, to use any
particular numbering scheme. Since future versions are not ruled out, this
construct is provided as a means to allow the possibility of automatic version
recognition, should it become necessary. Processors may signal an error if they
receive documents labeled with versions they do not support.
The function of the markup in an XML document is to describe its storage and logical structure and to associate attribute-value pairs with its logical structures. XML provides a mechanism, the document type declaration, to define constraints on the logical structure and to support the use of predefined storage units. An XML document is valid if it has an associated document type declaration and if the document complies with the constraints expressed in it.
The document type declaration must appear before the first element in the document.
| Prolog | ||||||||||||||||||||||||
|
The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.
A markup declaration is an element type declaration, an attribute-list declaration, an entity declaration, or a notation declaration. These declarations may be contained in whole or in part within parameter entities, as described in the well-formedness and validity constraints below. For fuller information, see "4. Physical Structures".
| Document Type Definition | ||||||||||||||||||
|
The markup declarations may be made up in whole or in
part of the replacement text of parameter entities. The
productions later in this specification for individual nonterminals (elementdecl, AttlistDecl, and so on) describe the
declarations after all the parameter entities have
been included.
Validity Constraint: Root
Element Type
The Name in
the document type declaration must match the element type of the root element.
Validity Constraint: Proper
Declaration/PE Nesting
Parameter-entity replacement text must be properly nested with
markup declarations. That is to say, if either the first character or the last
character of a markup declaration (markupdecl above) is contained in the
replacement text for a parameter-entity reference, both must be contained
in the same replacement text.
Well-Formedness Constraint: PEs
in Internal Subset
In the internal DTD subset, parameter-entity
references can occur only where markup declarations can occur, not within
markup declarations. (This does not apply to references that occur in external
parameter entities or to the external subset.)
Like the internal subset, the external subset and any
external parameter entities referred to in the DTD must consist of a series of
complete markup declarations of the types allowed by the non-terminal symbol
markupdecl, interspersed with white
space or parameter-entity references. However, portions of
the contents of the external subset or of external parameter entities may
conditionally be ignored by using the conditional section
construct; this is not allowed in the internal subset.
| External Subset | ||||||||
|
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"?> |
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" ?> |
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.
| 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:
amp, lt, gt, apos, quot), if references to those
entities appear in the document, or
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, denoted by the nonterminal S in this specification) to set apart the
markup for greater readability. Such white space is typically not intended for
inclusion in the delivered version of the document. On the other hand,
"significant" white space that should be preserved in the delivered version is
common, for example in poetry and source code.
An XML processor must always pass all characters in a document that are not markup through to the application. A validating XML processor must also inform the application which of these characters constitute white space appearing in element content.
A special attribute named xml:space may be attached to an element to signal an
intention that in that element, white space should be preserved by applications.
In valid documents, this attribute, like any other, must be declared
if it is used. When declared, it must be given as an enumerated type whose
only possible values are "default" and "preserve". For example:
<!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.
XML parsed entities are often stored in computer files which, for editing convenience, are organized into lines. These lines are typically separated by some combination of the characters carriage-return (#xD) and line-feed (#xA).
To simplify the tasks of applications, wherever an external parsed entity or the literal entity value of an internal parsed entity contains either the literal two-character sequence "#xD#xA" or a standalone literal #xD, an XML processor must pass to the application the single character #xA. (This behavior can conveniently be produced by normalizing all line breaks to #xA on input, before parsing.)
In document processing, it is often useful to identify
the natural or formal language in which the content is written. A special attribute
named xml:lang may be inserted in documents to
specify the language used in the contents and attribute values of any element in
an XML document. In valid documents, this attribute, like any other, must be declared
if it is used. The values of the attribute are language identifiers as defined
by [IETF RFC
1766], "Tags for the Identification of Languages":
| Language Identification | ||||||||||||||||||||||||
|
The Langcode may be any
of the following:
i-" (or "I-")
x-" or "X-" in
order to ensure that they do not conflict with names later standardized or
registered with IANA There may be any number of Subcode segments; if the first subcode
segment exists and the Subcode consists of two letters, then it must be a
country code from [ISO 3166], "Codes for the representation of names
of countries." If the first subcode consists of more than two letters, it must
be a subcode for the language in question registered with IANA, unless the Langcode begins with the prefix "x-" or "X-".
It is customary to give the language code in lower case, and the country code (if any) in upper case. Note that these values, unlike other names in XML documents, are case insensitive.
For example:
<p xml:lang="en">The quick brown fox jumps over the lazy dog.</p> |
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 'fr'> |
Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. Each element has a type, identified by name, sometimes called its "generic identifier" (GI), and may have a set of attribute specifications. Each attribute specification has a name and a value.
| Element | ||||||||||||||||
|
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:
EMPTY and the element has no content.
children and the sequence of child elements belongs to the language
generated by the regular expression in the content model, with optional white
space (characters matching the nonterminal S) between
each pair of child elements.
Mixed and the content consists of character data and child elements whose
types match names in the content model.
ANY, and the types of any child elements have been declared.
The beginning of every non-empty XML element is marked by a start-tag.
| Start-tag | ||||||||||||||||||||||||
|
The Name in the start- and
end-tags gives the element's type. The Name-AttValue pairs are referred to as the attribute specifications of the element, with the Name in
each pair referred to as the attribute name and the content of the AttValue (the text between the ' or " delimiters)
as the attribute value.
Well-Formedness Constraint:
Unique Att Spec
No attribute name may appear more
than once in the same start-tag or empty-element tag.
Validity Constraint: Attribute
Value Type
The attribute must have been declared;
the value must be of the type declared for it. (For attribute types, see "3.3 Attribute-List Declarations".)
Well-Formedness Constraint: No
External Entity References
Attribute values cannot
contain direct or indirect entity references to external entities.
Well-Formedness Constraint: No
< in Attribute Values
The replacement text of any entity referred to
directly or indirectly in an attribute value (other than "<") must not contain a <.
An example of a start-tag:
<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:
| End-tag | ||||
|
An example of an end-tag:
</termdef> |
The text between the start-tag and end-tag is called the element's content:
| 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 form:
| Tags for Empty Elements | ||||||
|
Empty-element tags may be used for any element which has
no content, whether or not it is declared using the keyword EMPTY. For interoperability, the empty-element tag must
be used, and can only be used, for elements which are declared EMPTY.
Examples of empty elements:
<IMG align="left" |
The element structure of an XML document may, for validation purposes, be constrained using element type and attribute-list declarations. An element type declaration constrains the element's content.
Element type declarations often constrain which element types can appear as children of the element. At user option, an XML processor may issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.
An element type declaration takes the form:
| Element Type Declaration | ||||||||||
|
where the Name gives the element
type being declared.
Validity Constraint: Unique
Element Type Declaration
No element type may be
declared more than once.
Examples of element type declarations:
<!ELEMENT br EMPTY> |
An
element type has element
content when elements of that type must contain only child elements (no character data),
optionally separated by white space (characters matching the nonterminal S). In this case, the constraint includes a
content model, a simple grammar governing the allowed types of the child
elements and the order in which they are allowed to appear. The grammar is built
on content particles (cps), which consist of
names, choice lists of content particles, or sequence lists of content
particles:
| Element-content Models | ||||||||||||||||||||
|
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?)> |
An element type has mixed content when elements of that type may contain character data, optionally interspersed with child elements. In this case, the types of the child elements may be constrained, but not their order or their number of occurrences:
| Mixed-content Declaration | ||||||||||||||||
|
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)*> |
Attributes are used to associate name-value pairs with elements. Attribute specifications may appear only within start-tags and empty-element tags; thus, the productions used to recognize them appear in "3.1 Start-Tags, End-Tags, and Empty-Element Tags". Attribute-list declarations may be used:
Attribute-list declarations specify the name, data type, and default value (if any) of each attribute associated with a given element type:
| Attribute-list Declaration | ||||||||
|
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.
XML attribute types are of three kinds: a string type, a set of tokenized types, and enumerated types. The string type may take any literal string as a value; the tokenized types have varying lexical and semantic constraints, as noted:
| Attribute Types | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Validity Constraint: ID
Values of type ID must
match the Name production. A name must not appear more
than once in an XML document as a value of this type; i.e., ID values must
uniquely identify the elements which bear them.
Validity Constraint: One ID per
Element Type
No element type may have more than one
ID attribute specified.
Validity Constraint: ID
Attribute Default
An ID attribute must have a
declared default of #IMPLIED or #REQUIRED.
Validity Constraint:
IDREF
Values of type IDREF must match the Name
production, and values of type IDREFS must
match Names; each Name
must match the value of an ID attribute on some element in the XML document;
i.e. IDREF values must match the value of some
ID attribute.
Validity Constraint: Entity
Name
Values of type ENTITY must match the Name
production, values of type ENTITIES must match
Names; each Name
must match the name of an unparsed entity declared in the DTD.
Validity Constraint: Name
Token
Values of type NMTOKEN must match the Nmtoken production; values of type NMTOKENS must match Nmtokens.
Enumerated attributes can take one of a list of values provided in the declaration. There are two kinds of enumerated types:
| Enumerated Attribute Types | ||||||||||||||||
|
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.
An attribute declaration provides information on whether the attribute's presence is required, and if not, how an XML processor should react if a declared attribute is absent in a document.
| Attribute Defaults | ||||||||||||||||||||||||||||
|
In an attribute declaration, #REQUIRED means that the attribute must always be
provided, #IMPLIED that no default value is
provided. If the declaration is
neither #REQUIRED nor #IMPLIED, then the AttValue value contains the declared default value; the #FIXED keyword states that the attribute must always
have the default value. If a default value is declared, when an XML processor
encounters an omitted attribute, it is to behave as though the attribute were
present with the declared default value.
Validity Constraint: Required
Attribute
If the default declaration is the keyword
#REQUIRED, then the attribute must be
specified for all elements of the type in the attribute-list declaration.
Validity Constraint: Attribute
Default Legal
The declared default value must meet
the lexical constraints of the declared attribute type.
Validity Constraint: Fixed
Attribute Default
If an attribute has a default
value declared with the #FIXED keyword,
instances of that attribute must match the default value.
Examples of attribute-list declarations:
<!ATTLIST termdef |
Before the value of an attribute is passed to the application or checked for validity, the XML processor must normalize it as follows:
If the declared value is not CDATA, then the XML processor must further process the normalized attribute value by discarding any leading and trailing space (#x20) characters, and by replacing sequences of space (#x20) characters by a single space (#x20) character.
All attributes for which no declaration has been read
should be treated by a non-validating parser as if declared CDATA.
Conditional sections are portions of the document type declaration external subset which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.
| Conditional Section | ||||||||||||||||||||
|
Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, processing instructions, or nested conditional sections, intermingled with white space.
If the keyword of the conditional section is INCLUDE, then the contents of the conditional
section are part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section
are not logically part of the DTD. Note that for reliable parsing, the contents
of even ignored conditional sections must be read in order to detect nested
conditional sections and ensure that the end of the outermost (ignored)
conditional section is properly detected. If a conditional section with a
keyword of INCLUDE occurs within a larger
conditional section with a keyword of IGNORE,
both the outer and the inner conditional sections are ignored.
If the keyword of the conditional section is a parameter-entity reference, the parameter entity must be replaced by its content before the processor decides whether to include or ignore the conditional section.
An example:
<!ENTITY % draft 'INCLUDE' > |
An XML document may consist of one or many storage units. These are called entities; they all have content and are all (except for the document entity, see below, and the external DTD subset) identified by name. Each XML document has one entity called the document entity, which serves as the starting point for the XML processor and may contain the whole document.
Entities may be either parsed or unparsed. A parsed entity's contents are referred to as its replacement text; this text is considered an integral part of the document.
An unparsed entity is a resource whose contents may or may not be text, and if text, may not be XML. Each unparsed entity has an associated notation, identified by name. Beyond a requirement that an XML processor make the identifiers for the entity and notation available to the application, XML places no constraints on the contents of unparsed entities.
Parsed entities are invoked by name using entity
references; unparsed entities by name, given in the value of ENTITY or ENTITIES
attributes.
General entities are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity. Parameter entities are parsed entities for use within the DTD. These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.
A character reference refers to a specific character in the ISO/IEC 10646 character set, for example one not directly accessible from available input devices.
| Character Reference | ||||||||||
|
Well-Formedness Constraint:
Legal Character
Characters referred to using
character references must match the production for Char.
&#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 (;) as delimiters.
| Entity Reference | ||||||||||||||||||||||||||||||||||||||||||||||
|
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. |
Example of a parameter-entity reference:
<!-- declare the parameter entity "ISOLat2"... --> |
| Entity Declaration | ||||||||||||||||||||
|
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.
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 |
If the entity is not internal, it is an external entity, declared as follows:
| External Entity Declaration | ||||||||||||||
|
If the NDataDecl is present, this is a general
unparsed entity; otherwise it is a parsed
entity.
Validity Constraint: Notation
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:
<!ENTITY open-hatch |
External parsed entities may each begin with a text declaration.
| 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 document entity is well-formed if it matches the
production labeled document. An
external general parsed entity is well-formed if it matches the production
labeled extParsedEnt.
An external parameter entity is well-formed if it matches the production labeled
extPE.
| Well-Formed External Parsed Entity | ||||||||
|
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 either UTF-8 or UTF-16.
Entities encoded in UTF-16 must begin with the Byte Order Mark described by ISO/IEC 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors must be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.
Although an XML processor is required to read only entities in the UTF-8 and UTF-16 encodings, it is recognized that other encodings are used around the world, and it may be desired for XML processors to read entities that use them. Parsed entities which are stored in an encoding other than UTF-8 or UTF-16 must begin with a text declaration containing an encoding declaration:
| Encoding Declaration | ||||||||||
|
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:
<?xml encoding='UTF-8'?> |
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:
content.
AttValue.
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.
EntityValue.
EntityValue or AttValue. | Entity Type | Character | ||||
| Parameter | Internal General |
External Parsed General |
Unparsed | ||
| Reference in Content |
Not recognized | Included | Included if validating | Forbidden | Included |
| Reference in Attribute Value |
Not recognized | Included in literal | Forbidden | Forbidden | Included |
| Occurs as Attribute Value |
Not recognized | Forbidden | Forbidden | Notify | Not recognized |
| Reference in EntityValue |
Included in literal | Bypassed | Bypassed | Forbidden | Included |
| Reference in DTD |
Included as PE | Forbidden | Forbidden | Forbidden | Forbidden |
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.
An entity is
included when its replacement text is
retrieved and processed, in place of the reference itself, as though it were
part of the document at the location the reference was recognized. The
replacement text may contain both character data and (except for parameter
entities) markup, which must be recognized in the usual
way, except that the replacement text of entities used to escape markup
delimiters (the entities amp, lt, gt, apos, quot) is
always treated as data. (The string "AT&T;" expands to "AT&T;" and the remaining ampersand is not
recognized as an entity-reference delimiter.) A character reference is included when the indicated character is processed in
place of the reference itself.
When an XML processor recognizes a reference to a parsed entity, in order to validate the document, the processor must include its replacement text. If the entity is external, and the processor is not attempting to validate the XML document, the processor may, but need not, include the entity's replacement text. If a non-validating parser does not include the replacement text, it must inform the application that it recognized, but did not read, the entity.
This rule is based on the recognition that the automatic inclusion provided by the SGML and XML entity mechanism, primarily designed to support modularity in authoring, is not necessarily appropriate for other applications, in particular document browsing. Browsers, for example, when encountering an external parsed entity reference, might choose to provide a visual indication of the entity's presence and retrieve it for display only on demand.
The following are forbidden, and constitute fatal errors:
EntityValue or AttValue.
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"' > |
while this is not:
<!ENTITY EndAttr "27'" > |
When the name of an unparsed entity appears as
a token in the value of an attribute of declared type ENTITY or ENTITIES, a validating processor must inform the
application of the system and public (if any) identifiers
for both the entity and its associated notation.
When a general entity reference appears in the EntityValue in an entity declaration,
it is bypassed and left as is.
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.
In discussing the treatment of internal entities, it is
useful to distinguish two forms of the entity's value. The literal entity value is
the quoted string actually present in the entity declaration, corresponding to
the non-terminal EntityValue. The replacement text is the content of the entity, after
replacement of character references and parameter-entity references.
The literal entity value as given in an internal entity
declaration (EntityValue) may
contain character, parameter-entity, and general-entity references. Such
references must be contained entirely within the literal entity value. The
actual replacement text that is included as described above must contain the replacement text of any parameter entities referred
to, and must contain the character referred to, in place of any character
references in the literal entity value; however, general-entity references must
be left as-is, unexpanded. For example, given the following declarations:
<!ENTITY % pub "Éditions Gallimard" > |
then the replacement text for the entity "book" is:
La Peste: Albert Camus, |
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".
Entity and
character references can both be used to escape the
left angle bracket, ampersand, and other delimiters. A set of general entities
(amp, lt,
gt, apos,
quot) is specified for this purpose. Numeric
character references may also be used; they are expanded immediately when
recognized and must be treated as character data, so the numeric character
references "<" and "&" may be used to escape < and &
when they occur in character data.
All XML processors must recognize these entities whether they are declared or not. For interoperability, valid XML documents should declare these entities, like any others, before using them. If the entities in question are declared, they must be declared as internal entities whose replacement text is the single character being escaped or a character reference to that character, as shown below.
<!ENTITY lt "&#60;"> |
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.
Notations identify by name the format of unparsed entities, the format of elements which bear a notation attribute, or the application to which a processing instruction is addressed.
Notation declarations provide a name for the notation, for use in entity and attribute-list declarations and in attribute specifications, and an external identifier for the notation which may allow an XML processor or its client application to locate a helper application capable of processing data in the given notation.
| Notation Declarations | ||||||||
|
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.)
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.
Validating processors must report violations of the constraints expressed by the declarations in the DTD, and failures to fulfill the validity constraints given in this specification. To accomplish this, validating XML processors must read and process the entire DTD and all external parsed entities referenced in the document.
Non-validating processors are required to check only the document entity, including the entire internal DTD subset, for well-formedness. While they are not required to check the document for validity, they are required to process all the declarations they read in the internal DTD subset and in any parameter entity that they read, up to the first reference to a parameter entity that they do not read; that is to say, they must use the information in those declarations to normalize attribute values, include the replacement text of internal entities, and supply default attribute values. They must not process entity declarations or attribute-list declarations encountered after a reference to a parameter entity that is not read, since the entity may have contained overriding declarations.
The behavior of a validating XML processor is highly predictable; it must read every piece of a document and report all well-formedness and validity violations. Less is required of a non-validating processor; it need not read any part of the document other than the document entity. This has two effects that may be important to users of XML processors:
For maximum reliability in interoperating between different XML processors, applications which use non-validating processors should not rely on any behaviors not required of such processors. Applications which require facilities such as the use of default attributes or internal entities which are declared in external entities should use validating XML processors.
The formal grammar of XML is given in this specification using a simple Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines one symbol, in the form
symbol ::= expression |
Symbols are written with an initial capital letter if they are defined by a regular expression, or with an initial lower case letter otherwise. Literal strings are quoted.
Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters:
#xN
N is a
hexadecimal integer, the expression matches the character in ISO/IEC 10646
whose canonical (UCS-4) code value, when interpreted as an unsigned binary
number, has the value indicated. The number of leading zeros in the #xN form is insignificant; the number of leading
zeros in the corresponding code value is governed by the character encoding in
use and is not significant for XML.
[a-zA-Z], [#xN-#xN]
[^a-z], [^#xN-#xN]
[^abc], [^#xN#xN#xN]
"string"
'string'
A and B represent simple expressions:
expression)
expression is
treated as a unit and may be combined as described in this list.
A?
A or
nothing; optional A.
A
B
A followed
by B.
A
| B
A or B but not both.
A
- B
A but does not match B.
A+
A.
A*
A. /*
... */
[
wfc: ... ]
[
vc: ... ]
Following the characteristics defined in the Unicode standard, characters are classed as base characters (among others, these contain the alphabetic characters of the Latin alphabet, without diacritics), ideographic characters, and combining characters (among others, this class contains most diacritics); these classes combine to form the class of letters. Digits and extenders are also distinguished.
| Characters | ||||||||||||||||||||||||
|
The character classes defined here can be derived from the Unicode character database as follows:
XML is designed to be a subset of SGML, in that every valid XML document should also be a conformant SGML document. For a detailed comparison of the additional restrictions that XML places on documents beyond those of SGML, see [Clark].
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 |
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 |
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 |
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'?> |
This produces the following:
xx"
is stored in the symbol table with the value "%zz;". Since the replacement text is not
rescanned, the reference to parameter entity "zz" is not recognized. (And it would be an error
if it were, since "zz" is not yet declared.)
<" 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.
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 "error-prone".
tricky" is recognized, and it is expanded, so the
full content of the "test" element is the
self-describing (and ungrammatical) string This
sample shows a error-prone method. For compatibility, it is required that content models in element type declarations be deterministic.
SGML requires deterministic content models (it calls them "unambiguous"); XML processors built using SGML systems may flag non-deterministic content models as errors.
For example, the content model ((b, c) | (b, d)) is non-deterministic, because
given an initial b the parser cannot know
which b in the model is being matched without
looking ahead to see which element follows the b. In this case, the two references to b can be collapsed into a single reference, making
the model read (b, (c | d)). An initial b now clearly matches only a single name in the
content model. The parser doesn't need to look ahead to see what follows; either
c or d would
be accepted.
More formally: a finite state automaton may be constructed from the content model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi, and Ullman [Aho/Ullman]. In many such algorithms, a follow set is constructed for each position in the regular expression (i.e., each leaf node in the syntax tree for the regular expression); if any position has a follow set in which more than one following position is labeled with the same element type name, then the content model is in error and may be reported as an error.
Algorithms exist which allow many but not all non-deterministic content models to be reduced automatically to equivalent deterministic models; see Brüggemann-Klein 1991 [Brüggemann-Klein].
The XML encoding declaration functions as an internal label on each entity, indicating which character encoding is in use. Before an XML processor can read the internal label, however, it apparently has to know what character encoding is in use--which is what the internal label is trying to indicate. In the general case, this is a hopeless situation. It is not entirely hopeless in XML, however, because XML limits the general case in two ways: each implementation is assumed to support only a finite set of character encodings, and the XML encoding declaration is restricted in position and content in order to make it feasible to autodetect the character encoding in use in each entity in normal cases. Also, in many cases other sources of information are available in addition to the XML data stream itself. Two cases may be distinguished, depending on whether the XML entity is presented to the processor without, or with, any accompanying (external) information. We consider the first case first.
Because each XML entity not in UTF-8 or UTF-16 format
must begin with an XML encoding declaration, in
which the first characters must be '<?xml',
any conforming processor can detect, after two to four octets of input, which of
the following cases apply. In reading this list, it may help to know that in
UCS-4, '<' is "#x0000003C" and '?' is
"#x0000003F", and the Byte Order Mark required
of UTF-16 data streams is "#xFEFF".
00 00 00 3C: UCS-4,
big-endian machine (1234 order)
3C 00 00 00: UCS-4,
little-endian machine (4321 order)
00 00 3C 00: UCS-4,
unusual octet order (2143)
00 3C 00 00: UCS-4,
unusual octet order (3412)
FE FF: UTF-16,
big-endian
FF FE: UTF-16,
little-endian
00 3C 00 3F:
UTF-16, big-endian, no Byte Order Mark (and thus, strictly speaking, in error)
3C 00 3F 00:
UTF-16, little-endian, no Byte Order Mark (and thus, strictly speaking, in
error)
3C 3F 78 6D: UTF-8,
ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any other 7-bit,
8-bit, or mixed-width encoding which ensures that the characters of ASCII have
their normal positions, width, and values; the actual encoding declaration
must be read to detect which of these applies, but since all of these
encodings use the same bit patterns for the ASCII characters, the encoding
declaration itself may be read reliably
4C 6F A7 94: EBCDIC
(in some flavor; the full encoding declaration must be read to tell which code
page is in use)
This level of autodetection is enough to read the XML encoding declaration and parse the character-encoding identifier, which is still necessary to distinguish the individual members of each family of encodings (e.g. to tell UTF-8 from 8859, and the parts of 8859 from each other, or to distinguish the specific EBCDIC code page in use, and so on).
Because the contents of the encoding declaration are restricted to ASCII characters, a processor can reliably read the entire encoding declaration as soon as it has detected which family of encodings is in use. Since in practice, all widely used character encodings fall into one of the categories above, the XML encoding declaration allows reasonably reliable in-band labeling of character encodings, even when external sources of information at the operating-system or transport-protocol level are unreliable.
Once the processor has detected the character encoding in use, it can act appropriately, whether by invoking a separate input routine for each case, or by calling the proper conversion function on each character of input.
Like any self-labeling system, the XML encoding declaration will not work if any software changes the entity's character set or encoding without updating the encoding declaration. Implementors of character-encoding routines should be careful to ensure the accuracy of the internal and external information used to label the entity.
The second possible case occurs when the XML entity is accompanied by encoding information, as in some file systems and some network protocols. When multiple sources of information are available, their relative priority and the preferred method of handling conflict should be specified as part of the higher-level protocol used to deliver XML. Rules for the relative priority of the internal label and the MIME-type label in an external header, for example, should be part of the RFC document defining the text/xml and application/xml MIME types. In the interests of interoperability, however, the following rules are recommended.
charset parameter on the
MIME type determines the character encoding method; all other heuristics and
sources of information are solely for error recovery.
This specification was prepared and approved for publication by the W3C XML Working Group (WG). WG approval of this specification does not necessarily imply that all WG members voted for its approval. The current and former members of the XML WG are:
Jon Bosak, Sun (Chair); James Clark (Technical Lead); Tim Bray, Textuality and Netscape (XML Co-editor); Jean Paoli, Microsoft (XML Co-editor); C. M. Sperberg-McQueen, U. of Ill. (XML Co-editor); Dan Connolly, W3C (W3C Liaison); Paula Angerstein, Texcel; Steve DeRose, INSO; Dave Hollander, HP; Eliot Kimber, ISOGEN; Eve Maler, ArborText; Tom Magliery, NCSA; Murray Maloney, Muzmo and Grif; Makoto Murata, Fuji Xerox Information Systems; Joel Nava, Adobe; Conleth O'Connell, Vignette; Peter Sharpe, SoftQuad; John Tigue, DataChannel