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This is a W3C Working Draft for review by W3C members and other
interested parties. It is a draft document and may be updated,
replaced or obsoleted by other documents at any time. It is
inappropriate to use W3C Working Drafts as reference material or to
cite them as other than "work in progress". A list of current W3C
working drafts can be found at
http://www.w3.org/TR.
Note: Since working drafts are subject to frequent
change, you are advised to reference the above URL, rather than the
URLs for working drafts themselves.
This work is part of the W3C SGML
Activity (for current status, see
http://www.w3.org/MarkUp/SGML/Activity).
Extensible Markup Language (XML) is an extremely simple dialect of
SGML which is completely described in this document. The 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.
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 [ISO8879].
XML documents are made up of storage units called entities, which contain either text or binary data. Text is made up of characters, some of which form the character data in the document, 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, referred to as 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 very 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 RFCs 1738 and 1808 for Uniform Resource Locators, 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 (17 November 1997) is for public review and discussion. It may be distributed freely, as long as all text and legal notices remain intact.
Standards relevant to users and implementors of XML include:
The terminology used 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:
The formal grammar of XML is given 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 lowercase letter otherwise. Literal strings are quoted. The distinction between symbols which can and cannot be recognized using simple regular expressions may be used to set the boundary between an implementation's lexical scanner and its parser, but this specification neither constrains the placement of that boundary nor presupposes that all implementations will have one.
Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters:
#Xnn is a hexadecimal integer, the
expression matches the character in ISO/IEC 10646 whose canonical
(UCS-4) bit string, when interpreted as an unsigned binary number, has
the value indicated. The number of leading zeroes in the
#Xn form is insignificant; the number of leading
zeroes in the corresponding bit string 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 a simple expression:
expression)expression is treated as a unit,
and may be combined as described in this list.A?A or nothing; optional A.A BA followed by B.A | BA or B but not both.A - BA but does not match
B.
A+A.A*A./* ... */[ WFC: ... ][ VC: ... ]A textual object is an XML document if it is either valid or well-formed, as defined in this specification.
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.
The logical structure contains declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup. The two structures must be synchronous, as described below.
A textual object is said to be a well-formed XML document if it
matches the production labeled document and if it
meets all the well-formedness constraints given in this specification.
| 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. Then P is referred to as the
parent of C, and C as a
child of P.
The data stored in an XML entity is either text or binary. Binary data has an associated notation, identified by name; beyond a requirement that an XML processor make the notation's name and the associated system identifier available to the application, XML places no constraints on the contents of binary entities. So-called binary data might in fact be textual; its identification as binary means that an XML processor need not parse it in the fashion described by this specification. XML text is a sequence of characters.
A character is an atomic unit of text; valid characters are specified by ISO/IEC 10646 [ISO10646]. Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646.
| Character range | ||||||
|
The mechanism for encoding character values 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 are in use, or for bringing other encodings into play, are discussed later, in the discussion of character encodings.
Regardless of the specific encoding used, any character in the ISO/IEC 10646 character set may be referred to by the decimal or hexadecimal equivalent of its bit string.
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. Certain layout and format-control characters defined by
ISO/IEC 10646 should be ignored
when recognizing identifiers; these are defined by the
classes Ignorable and
Extender.
Full definitions of the specific characters in each class
are given in the appendix on character
classes.
A Name is a token
beginning with a letter or underscore character and continuing with
letters, digits, hyphens, underscores, 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') in the EBNF
notation of this spec) are
reserved for standardization in this or future versions of this
specification.
Note: the colon character is also allowed within XML names; it is reserved for experimentation with name spaces and schema scoping. Its meaning is expected to be standardized at some future point, at which point those documents using colon for experimental purposes will need to be updated. (Note: there is no guarantee that any name-space mechanism adopted for XML will in fact use colon as a name-space delimiter.) In practice, this means that authors should not use colon in XML names except as part of name-space experiments, but that XML processors should accept 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 replacement text of internal entities
(EntityValue),
the values of attributes (AttValue),
and external identifiers
(SystemLiteral); for some
purposes, the entire literal can be skipped without scanning for
markup within it (SkipLit):
| Literals | ||||||||||||||||||||||||||||||||||||||
|
XML text consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty elements, 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, processing instruction,
the replacement text of an internal
entity declaration,
or CDATA section.
If they are needed elsewhere,
they must be escaped
using either numeric character references or the strings
"&" and "<". The right angle
bracket (>) may be represented using the string
">", and must, for
compatibility, be
be escaped using
">" or a character reference when it appears in the string
"]]>", 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 except within other
markup.
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
Name is called the PI target; it is used
to identify the application to which the instruction is directed. XML
provides a mechanism, Notation, which
may be used for
formal declaration of such names.
CDATA sections
can 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, among other things, the version of XML being used.
The function of the markup in an XML document is to describe its storage and logical structures, and associate attribute-value pairs with the logical structure. XML provides a mechanism, the document type declaration, to define constraints on that logical structure and to support the use of predefined storage units. An XML document is said to be valid if there is 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 identification of the XML version as "1.0" 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.
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 XML document type declaration identifies 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.
| Document type definition | ||||||||||||||||
|
Validity Constraint -
Root Element Type
The Name in the document-type declaration must
match the element type of the root element.
Validity Constraint -
Declarations and PEs Synchronous
Parameter-entity replacement text must be synchronous 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.
The external subset must obey
the same grammatical constraints
as the internal subset; i.e. its contents must match the production for the
non-terminal symbol
markupdecls.
However, portions of the
external subset's contents may conditionally be ignored
by using
the conditional section
construct; this is not allowed in the internal subset.
| External subset | ||||
|
For example:
<?xml version="1.0"?> |
The system identifier "hello.dtd"
indicates
the location 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, an XML processor must read the internal subset first, then the external subset. This has the effect that entity and attribute 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 appears as a component of the XML declaration, signals the presence of such markup declarations.
| 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 such
external markup declarations.
Validity Constraint -
Standalone Document Declaration
The standalone document declaration must be accurate; that is to say,
its value must be "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 must be retained 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 distinguish white space in element content from other non-markup characters and signal to the application that white space in element content is not significant.
A special attribute
named "xml-space" may be inserted in
documents to signal an intention that the element to which this attribute
applies requires all white space to be treated as
significant 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".
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 text 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
CR (#Xd) and LF (#Xa).
To simplify the tasks of applications,
when an XML processor encounters any of
either a #Xd or #Xa standing alone, or the
two-character sequence "#Xd#Xa" it must pass to the application
the single character #Xa.
In document processing, it is often useful and sometimes essential to identify the language in which the document or some parts of it is written. Language is used here in the sense exemplified by both "Español" and "EBNF".
A special attribute, xml:lang, may be inserted in
documents to signal to specify the
language used in the contents and attribute values
of any element in an XML document.
The values of the attribute are language identifiers as defined
by RFC 1766, "Tags for the Identification of Languages":
| Language identification | ||||||||||||||||||||||||
|
The Langcode may be any of the following:
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 lowercase, the country code (if any) in upper case. Note that these values, unlike all 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 elements within the content of the element where it is specified,
unless overridden with another instance of xml:lang.
In valid documents, this attribute must be declared as described elsewhere in this document; a typical declaration will take the form
xml:lang NMTOKEN #IMPLIED |
but specific default values may also be given, if appropriate. In an 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 attributes. Each attribute 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 the string "XML"
are reserved for standardization in this or future versions of this
specification.
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
referred to as the attribute name and
the content of the
AttValue (the characters 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 the discussion of
attribute
declarations.)
Well-Formedness Constraint -
No External Entity References
Attribute values cannot contain entity references to
external entities.
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 | ||||
|
Well-Formedness Constraint -
GI Match
The Name in an element's end-tag must match
the element type in
the start-tag.
If an element is empty, it may 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 they are declared using the keyword
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 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.
Validity Constraint -
Element valid
An element is said to be 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
child elements or
content.children and
the sequence of
child elements
belongs to the language generated by the regular expression in
the content model.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.Examples of element type declarations:
<!ELEMENT br EMPTY> |
An element type may be declared to have
element content, which means that elements of that
type may only contain child
elements (no character data).
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 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 gives 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 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 reasons, it is an error if an element in the document can match more than one occurrence of an element type in the content model. 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 [Dragon]. 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. For more information, see the appendix on deterministic content models.
Validity Constraint -
Groups and PEs Synchronous
Parameter-entity replacement text must be synchronous 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 may be declared to contain mixed content, that is, text comprising character data optionally interspersed with child elements. In this case, the types of the child elements are constrained, but not their order nor 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; thus, the productions used to recognize them appear in the discussion of start-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, and at most one attribute definition
for a given attribute name.
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
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 this type 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
No element may have more than one ID attribute specified.
Validity Constraint -
IDREF
Values of this type must match
the Name
(for IDREFS, the Names) production;
each Name must match the value of an ID attribute on
some element in the XML document; i.e. IDREF values must
match some ID.
Validity Constraint -
Entity Name
Values of this type
must match the production for Name
(for ENTITIES, Names);
each Name must
match the
name of an external binary general entity declared in the DTD.
Validity Constraint -
Name Token
Values of this type
must consist of a string matching the Nmtoken nonterminal
(for NMTOKENS, the Nmtokens nonterminal) of the grammar defined
in this specification.
The XML processor must normalize attribute values before passing them to the application, as described in the section on attribute-value normalization.
Enumerated attributes can take one of a list of values provided in the declaration; there are two types:
| Enumerated attribute types' | ||||||||||||||||||||
|
Validity Constraint -
Notation Attributes
Values of this type must match
one of the notation names included in
the declaration.
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 | ||||||||||
|
#REQUIRED means that
the document is not valid
should the processor
encounter a
start-tag
for the element type in question which specifies no value for
this attribute.
#IMPLIED means that if the attribute is omitted
from an element of this type,
the XML processor must inform the application
that no value was specified; no constraint is placed on the behavior
of the application.
If the attribute
is neither #REQUIRED nor #IMPLIED, then the
AttValue value contains the declared
default value. If the #FIXED is present,
the document is not valid
if the attribute
is present with a different value from the default. 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 its value being
the declared default value.
Validity Constraint -
Attribute Default Legal
The declared
default value must meet the lexical constraints of the declared attribute type.
Examples of attribute-list declarations:
<!ATTLIST termdef |
Before the value of an attribute is passed to the application, the XML processor must normalize it as follows:
CDATA, all strings
of white space must be normalized to single space characters (#X20),
and leading and trailing white space must be removed.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 conditional section is read and
processed in the normal way. If the keyword is
"IGNORE", then the declarations within the conditional
section are ignored; the processor must read the conditional section 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 virtual storage units. These are called entities; they are (except for the document entity, see below) identified by name and have content. An entity may be stored in, but need not comprise the whole of, a single physical storage object such as a file or database field. 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 binary or text.
A text entity's
contents are referred to as its replacement text;
this text data is considered as an
integral part of the document.
A binary entity contains
binary data with an associated notation.
Text entities are invoked by name using entity references;
binary entities by name, given as the value of ENTITY
attributes.
The logical and physical structures (elements and entities) in an XML document must be synchronous. Tags and elements must each begin and end in the same entity, but may refer to other entities internally; comments, processing instructions, character references, and entity references must each be contained entirely within a single entity. Entities must each contain an integral number of elements, comments, processing instructions, and references, possibly together with character data not contained within any element in the entity, or else they must contain non-textual data, which by definition contains no elements.
A character reference refers to a specific character in the ISO/IEC 10646 character set, e.g. one not directly accessible from available input devices:
| Character reference | ||||||||
|
If the character begins with "&#X", the digits and
letters up to the terminating ";" provide a hexadecimal
representation of the character's value in ISO/IEC 10646.
If it begins just with "&#", the digits up to the terminating
";" provide a decimal representation of the character's value.
An entity
reference refers to the content of a named entity.
General entities are
text entities for use within the document itself; references to them
use ampersand (&) and semicolon (;) as
delimiters. In this specification,
general entities are sometimes referred to with the
unqualified term entity when this leads
to no ambiguity.
Parameter entities
are text entities for use within the DTD.
parameter-entity references use percent-sign (%) and
semicolon
(;) as delimiters.
| Entity reference | ||||||||||||||||||||||||||||||||||||||||||||||
|
Well-Formedness Constraint -
Entity Declared
The Name given in the entity reference must match the name given in the declaration
of the entity, except that well-formed documents need not declare
any of the following entities: amp,
lt,
gt,
apos,
quot. In valid
documents, these entities must be declared, in the form
specified in the section on
predefined entities.
In the case of parameter entities, the declaration
must precede the reference.
Note that an entity declaration which appears in the DTD's external subset
or in an external parameter entity may not be read by a non-validating
parser, which is not obligated
to process external entities.
Such processors are acknowledged to be incapable of detecting some cases of
this class of violations of well-formedness.
Well-Formedness Constraint -
Text Entity
An entity reference must not contain the name of a binary entity. Binary entities may be referred
to only in attribute values declared to
be of type ENTITY or ENTITIES.
Well-Formedness Constraint -
No Recursion
A text or parameter 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:
<!ENTITY % ISOLat2 |
Entities are declared thus:
| Entity declaration | ||||||||||||||||
|
The Name is that by which the entity is invoked by
match in an entity
reference.
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 replacement text of the entity is given in the
declaration.
Note that some processing of entity and character references in the
replacement text may be required to produce the correct replacement
text: see Construction of Internal
Entity Replacement Text.
An internal entity is a text 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 binary
data entity, otherwise a text entity.
Validity Constraint -
Notation Declared
The Name must match the declared name of a
notation.
The
SystemLiteral that follows the keyword
SYSTEM
is called the entity's system identifier. It is a URL,
which may be used to retrieve the entity.
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 URLs are relative to the location of the
entity or file within which the entity declaration occurs. Relative
URLs in entity declarations within the internal DTD subset are thus
relative to the location of the document; those in entity declarations
in the external subset are relative to the location of the files
containing the external subset.
In addition to a system literal, 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 URL. If the processor is unable to do so, it must use the URL specified in the system literal.
Examples of external entity declarations:
<!ENTITY open-hatch |
The table below summarizes the contexts in which character references, entity references, and invocations of binary 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.EntityValue.EntityValue.| References | Binary Entities |
||||
| Parameter | Internal General | External General | Character | ||
| in Content | Not recognized | Included | Included if validating | Included | Forbidden |
| in Attribute Value | Not recognized | Included | Forbidden | Included | Notify |
| in EntityValue | Included | Bypassed | Bypassed | Included | Forbidden |
| in DTD | Included as token | Forbidden | Forbidden | Forbidden | Forbidden |
Outside of the DTD, the % character has no
particular significance; thus what would be parameter entity references in the
DTD are not recognized as markup in content.
An entity is
included when its replacement text is retrieved and
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 text
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;" 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 general textual or parameter 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.
This rule is based on the recognition that the automatic inclusion provided by the SGML and XML text 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 text 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:
EntityValueWhen the name of an external entity appears as a token in the
value of an attribute of declared type ENTITY or ENTITIES,
the processor must inform the
application of the associated notation
name, and the notation's associated system
and public (if any)
identifiers.
When a general entity reference appears in the
EntityValue in an entity declaration,
it is ignored, and left as-is in the replacement text.
When a parameter entity reference is recognized in the DTD, it is included, but 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.
The replacement text 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
replacement text.
The actual replacement text that is
included as described above
must contain the replacement text for any included
parameter-entity and character references; however,
general-entity references must be left as-is, unexpanded,
in the replacement text that is to be included.
For example, given the following declarations
<!ENTITY % pub "Éditions Gallimard" > |
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 the appendix on expansion of entity 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. Valid XML documents must 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, as shown below.
<!ENTITY lt "<"> |
Notations identify by name the format of external binary entities, or the application to which processing instructions are 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 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.)
Each external text 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 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 only required to read entities in the UTF-8 and UTF-16 encodings, it is recognized that many other encodings are in daily use around the world, and it may be advantageous for XML processors to read entities that use these other encodings. For this purpose, XML provides an encoding declaration processing instruction, which, if it occurs, must appear at the beginning (preceded only by the Byte Order Mark, if present) of an external text entity. In the document entity, the encoding declaration is part of the XML declaration; in other entities, it is part of an encoding processing instruction:
| Encoding declaration | ||||||||||||||||||||||
|
Note that an XML declaration is only recognized, and can only affect the processing of an entity, when it appears at the beginning of the entity.
The values
UTF-8,
UTF-16,
ISO-10646-UTF-16, 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. 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.
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.
An entity which begins with neither a Byte Order Mark nor an encoding declaration must be in the UTF-8 encoding.
XML processors should make an effort to use all available information, internal and external, to aid in detecting an entity's correct encoding. Such information may include, but is not limited to:
Examples of encoding declarations:
<?xml encoding='UTF-8'?> |
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 might well appear on an input stream of the processor without any identification at all.
Conforming XML processors fall into two classes: validating and non-validating.
Validating and non-validating systems alike must report violations of the well-formedness constraints given in this specification.
Validating processors must report violations of the constraints expressed by the declarations in the DTD. They must also report all failures to fulfill the validity constraints given in this specification.
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 the accompanying note. This document also includes an SGML declaration which describes those constraints of XML applicable to an SGML parser.
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, extenders, and characters which should be ignored for purposes of recognizing identifiers are also distinguished.
| Characters | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion.
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.
Algorithms exist which allow many but not all non-deterministic content models to be reduced automatically to equivalent deterministic models; see Brüggemann-Klein 1991 [ABK].
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.
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 58 4D: 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 E7 D4: 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-line 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.
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 Steve DeRose, INSO Dave Hollander, HP Eliot Kimber, Highland Eve Maler, ArborText Tom Magliery, NCSA Murray Maloney, Grif Makoto Murata, Fuji Xerox Joel Nava, Adobe Peter Sharpe, SoftQuad John Tigue, DataChannel