[Mirrored from: http://www.textuality.com/sgml-erb/WD-xml-lang.html, April 02, 1997]
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/pub/WWW/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/pub/WWW/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.
The 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].
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 SGML Editorial Review Board formed under the auspices of the World Wide Web Consortium (W3C) in 1996 and chaired by Jon Bosak of Sun Microsystems, with the very active participation of an SGML Working Group also organized by the W3C. The membership of the Editorial Review Board is given in an appendix. The ERB's contact with the W3C is Dan Connolly.
The design goals for XML are:
This specification, together with the associated standards, provides all the information necessary to understand XML version 1.0 and construct computer programs to process it.
This version of the XML specification (31 March 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:
Some terms used with special meaning in this specification are:
maymusterrorreportable errorvalidity constraintwell-formedness constraintat user optionmatchMixed and the content consists of character data and elements whose names match names in the content model, or if (b) the content model matches the rule for elements, and the sequence of child elements belongs to the language generated by the regular expression in the content model. case-foldingexact(ly) matchfor compatibilityfor interoperabilityThe 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 if a recursive grammar is required for recognition. Literal strings are quoted; unless otherwise noted they are case-insensitive. 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 meaning of symbols is as shown below.
#xNNNN[a-zA-Z], [#xNNNN-#xNNNN][^a-z], [^#xNNNN-#xNNNN][^abc], [^#xNNNN#xNNNN#xNNNN]"string"'string'a ba | ba - ba?a+a*%aS? a S?. Note that % has lower precedence than any of the suffix operators ?, *, or +; that is to say, %a* and %(a*) each mean that the result of including a parameter entity reference at the indicated location must match a*.(expression)% prefix operator, or a suffix operator: ?, *, or +./* ... */[ WFC: ... ][ VC: ... ]This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x0020) characters, carriage returns, line feeds, tabs, or ideographic space characters.
| White space | ||||
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Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO 10646.
| Character Range | ||||||
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Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic base character possibly followed by one or more combining characters, or of an ideographic character. Certain layout and format-control characters defined by ISO 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). The use of any name beginning with a string which matches "XML" in a fashion other than those described in this and related specifications is an error.
An Nmtoken (name token) is any mixture of name characters.
| Names and Tokens | ||||||||||||||||||||||||
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Literal data is any quoted string containing neither a left angle bracket nor the quotation mark used as a delimiter for that string. It may contain entity and character references. 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 | ||||||||||||||||||||||||||||||||||
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Within EntityValue, parameter-entity and character references are recognized and expanded immediately; general-entity references may occur, but they are not expanded until the entity containing them is referenced and expanded in the document itself. Within AttValue, general and character references are recognized and expanded; parameter-entity references are not recognized. Within SystemLiteral, no references of any kind are recognized.
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; it begins in a "root" or document entity, which may refer to other entities, and so on.
The logical structure contains declarations, elements, tags, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.
The two structures must be synchronous: see section 4.1.
A textual object is said to be a well-formed XML document if, first, it matches the production labeled document, and if for each entity reference which appears in the document, either the entity has been declared in the document type declaration or the entity name is one of: amp, lt, gt, apos, quot.
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 to make available the notation name and the system identifier, XML places no constraints on the contents or use 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 the specification. XML text data is a sequence of characters. A character is an atomic unit of text represented by a bit string; valid bit strings and their meanings are specified by ISO 10646. Users may extend the ISO 10646 character repertoire by exploiting the private use areas.
The mechanism for encoding character values into bit patterns may vary from entity to entity. All XML processors must accept the UTF-8 and UCS-2 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 10646 character set may be referred to by the decimal or hexadecimal equivalent of its bit string.
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 sections, 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 comments, processing instructions, or CDATA sections. 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 so represented 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 | ||||
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Comments may appear anywhere except in a CDATA section, i.e. within element content, in mixed content, or in a DTD. They must not occur within declarations or tags. 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 | ||||
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An example of a comment:
<!-- declarations for <head> & <body> --> |
Processing instructions (PIs) allow documents to contain instructions for applications.
| Processing Instructions | ||||
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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 an optional mechanism, NOTATION, for formal declaration of such names. The use of PI targets with names beginning "XML" in any way other than those described in this specification is an error.
CDATA sections can occur anywhere character data may occur; they are used to escape blocks of text which may contain characters which would otherwise be recognized as markup. CDATA sections begin with the string <![CDATA[ and end with the string ]]>:
| CDATA sections | ||||||||||||||||
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Within a CDATA section, only the CDEnd string is recognized, 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:
<![CDATA[<greeting>Hello, world!</greeting>]]> |
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 or source code.
An XML processor which does not read the DTD must always pass all characters in a document that are not markup through to the application. An XML processor which does read the DTD must always pass all characters in mixed content that are not markup through to the application. It may also choose to pass white space ocurring in element content to the application; if it does so, it must signal to the application that
A special attribute 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 must be declared as follows, if used:
XML-SPACE (DEFAULT|PRESERVE) #IMPLIED |
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.
The root element of any document, unless this attribute is provided or defaulted with a value of PRESERVE, is considered to have signaled no intentions as regards application space handling. Any space handling behavior specified for an element by the XML-SPACE attribute is inherited by that element's child elements as well.
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 start-tag in the document.
| XML document | ||||||||||||||||||||
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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 may include a pointer to an external entity containing a subset of the necessary markup declarations, and may also directly include another, internal, subset.
These two subsets make up the document type definition, abbreviated DTD. The DTD, in effect, provides a grammar which defines a class of documents. Properly speaking, the DTD consists of both subsets taken together, but it is a common practice for the bulk of the markup declarations to appear in the external subset, and for this subset, usually contained in a file, to be referred to as "the DTD" for a class of documents. The external subset must obey the same grammatical constraints as the internal subset; i.e. it must match the production for markupdecl.
| Document type definition | ||||||||||||||||
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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" ?> |
The version label version="1.0" indicates that the document conforms to version 1.0 of the XML specification. The character-set label encoding="UTF-8" indicates that the document entity is encoded using the UTF-8 transformation of ISO 10646.
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.
In some cases, an XML processor can read an XML document and accomplish useful tasks without having first processed the entire DTD. However, certain declarations can substantially affect the actions of an XML processor. A document author can communicate whether or not DTD processing is necessary using a required markup declaration (abbreviated RMD), which appears as a component of the XML declaration:
| Required markup declaration | ||||||||
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In an RMD, the value NONE indicates that an XML processor can parse the containing document correctly without first reading any part of the DTD. The value INTERNAL indicates that the XML processor must read and process the internal subset of the DTD, if provided, to parse the containing document correctly. The value ALL indicates that the XML processor must read and process the declarations in both the subsets of the DTD, if provided, to parse the containing document correctly.
The RMD must indicate that the entire DTD is required if the external subset contains any declarations of
If no RMD is provided, an XML processor must behave as though an RMD had been provided with the value ALL.
An example XML declaration with an RMD:
<?XML version="1.0" RMD='INTERNAL'?> |
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, are those of the start-tag. Each element has a type, identified by name (sometimes called its generic identifier or GI), and may have a set of attributes. Each attribute has a name and a value.
This specification does not constrain the semantics, use, or (beyond syntax) names of the elements and attributes.
The beginning of every XML element is marked by a start-tag.
| Start-tag | ||||||||||||||||||||||
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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.
Validity Constraint - Unique Att Spec:
No attribute may appear more than once in the same start-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.)
An example of a start tag:
<termdef id="dt-dog" term="dog"> |
The end of every element which is not empty is marked by an end-tag containing a name that echoes the element's type as given in the start-tag:
| End-tag | ||||
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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 | ||||||||||||||||
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If an element is empty, the start-tag constitutes the whole element. An empty element takes a special form:
| Tags for empty elements | ||||
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An example of an empty element:
<IMG align="left" |
The element structure of an XML document may, for validation purposes, be constrained using element and attribute declarations.
An element declaration constrains the element's type and its content.
Element 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 reference is made to an element type for which no declaration is provided, but this is not an error.
An element declaration takes the form:
| Element declaration | ||||||||||
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where the Name gives the type of the element.
Validity Constraint - Unique Element Declaration:
No element type may be declared more than once.
The content of an element can be categorized as element content or mixed content , as explained below. An element declared using the keyword EMPTY must be empty when it appears in the document.
If an element type is declared using the keyword ANY, then there are no validity constraints on its content: it may contain child elements of any type and number, interspersed with character data.
Examples of element declarations:
<!ELEMENT br EMPTY> |
An element type may be declared to have element content, which means that elements of that type may only contain other 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 | ||||||||||||||||||||
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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 appropriate location; content particles occurring in a sequence list must each appear in the element content in the order given. 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 respectively. 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 name 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 name 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. 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.
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 | ||||||||
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where the Names give the types of elements that may appear as children. 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. Attributes 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 | ||||||||
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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 entity 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. 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 | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Validity Constraint - Entity Name:
Values of this type must be match the production for Name (for ENTITIES, Names); each Name must exactly 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 (for NMTOKENS, 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 | ||||||||||||||||
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Validity Constraint - Notation Attributes:
The names in the declaration of NOTATION attributes must be names of declared notations (see the discussion of notations). 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 | ||||||
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#REQUIRED means that the document is invalid should the processor encounter a start-tag where this attribute is omitted, i.e. could occur but does not. #IMPLIED means that if an attribute is omitted, 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 invalid 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.
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:
Conditional sections are portions of the document type declaration internal or external subset which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.
| Conditional section | ||||||||||||
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Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, or nested conditional sections, intermingled with white space. Conditional sections must not occur within declarations, except the document-type declaration.
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 is replaced by its value (recursively if necessary) before the processor decides whether to include or ignore the conditional section.
An example:
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An XML document may consist of one or many virtual storage units. These are called entities; they are identified by name and have content. An entity may be stored in, but need not be coterminous with, a single physical storage object such as a file or stream. 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 contains text data which is to be considered as an integral part of the document. A binary entity contains binary data with an associated notation. References to text and binary entities cannot be distinguished by syntax; their types are established in their declarations.
The logical and physical structures 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 10646 character set, e.g. one not directly accessible from available input devices:
| Character reference | ||||||||
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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, or within conditional sections; references to them use percent-sign (%) and semicolon (;) as delimiters.
| Entity Reference | ||||||||||||||||||||||||||||||||||||||||||||||
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Well-Formedness Constraint - Entity Declared:
The Name given in the entity reference must exactly 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.
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.
Examples of character and entity references:
" " and "	" are white-space characters. |
Example of a parameter-entity reference:
<!ENTITY % ISOLat2 |
Entities are declared thus:
| Entity declaration | ||||||||||||||||
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The Name is that by which the entity is invoked by exact 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 just an EntityValue, this is called an internal entity. There is no separate physical storage object, and the replacement text of the entity is given in the declaration. Within the EntityValue, parameter-entity references and character references are recognized and expanded immediately. General-entity references are not recognized at the time the entity declaration is parsed, though they may be recognized when the entity itself is referred to.
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 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 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 |
Each 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 UCS-2. It is recognized that for some purposes, particularly work with Asian languages, the use of the UTF-16 transformation is required, and correct handling of this encoding is a desirable characteristic in XML processor implementations.
Entities encoded in UCS-2 must begin with the Byte Order Mark described by ISO 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, U+FEFF). 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 UCS-2 encoded documents.
Although an XML processor is only required to read entities in the UTF-8 and UCS-2, 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 encodings. For this purpose, XML provides an encoding declaration processing instruction, which, if it occurs, must appear at the beginning of a system entity, before any other character data or markup. 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 | ||||||||||||||||||||||
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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 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 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.
While XML provides mechanisms for distinguishing encodings, it is recognized that in a heterogeneous networked environment, there is often difficulty in reliably signaling the encoding of an entity. Errors in this area fall into two categories:
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.
XML allows character and general entity references in two places: the content of elements (content) and attribute values (AttValue). When an XML processor encounters such a reference, or the name of an external binary entity as the value of an ENTITY or ENTITIES attribute, then:
AT&T expands to AT&T; the remaining ampersand is not recognized as an entity-reference delimiter.) Since the entity may contain other entity references, an XML processor may have to repeat the inclusion process recursively.
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.
XML allows parameter entity references in a variety of places within the DTD. Parameter-entity references are always expanded immediately upon being recognized, and the DTD must match the relevant rules of the grammar after all parameter-entity references have been expanded. In addition, parameter entities referred to in specific contexts are required to satisfy certain constraints in their replacement text; for example, a parameter entity referred to within the internal DTD subset must match the rule for markupdecl.
Implementors of XML processors need to know the rules for expansion of references in more detail. These rules only come into play when the replacement text for an internal entity itself contains other references.
Since the replacement text of a general entity is parsed once when the general entity is declared and once when it is referred to, simple character references do not suffice to escape delimiters within the replacement text. Instead, delimiters must be escaped using a general entity reference (which is not expanded when the declaration is parsed) or a doubly-escaped character reference. See the appendix on expansion of entity references for detailed examples.
As mentioned in the discussion of Character Data and Markup, the characters used as markup delimiters by XML may all be escaped using entity references (for the entities amp, lt, gt, apos, quot).
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.
Notation declarations provide a name for the notation, for use in entity and attribute-list declarations and in attribute-value 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.
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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.)
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 systems must report locations in which the document does not comply with 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, using the same DTD, and that the parse trees produced by an SGML parser and an XML processor should be the same. To achieve this, XML was defined by removing features and options from the specification of SGML.
The following list describes syntactic characteristics which XML does not allow but which are legal in SGML. The list may not be complete.
<!-- comment text --> and can't have spaces within the markup of <!-- or -->. -- ... --) inside markup declarations. -- --. <![keyword[ or ]]>. The specific SGML declaration needed to enable SGML systems to process XML documents will vary from document to document, depending on the character set and the capacities and quantities required. Documents should be able to use the following SGML declaration:
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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, 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.
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This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion.
If the DTD contains the declaration
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then the XML processor will store the following string as the value of the entity example:
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and 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):
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A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference.
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This produces the following perhaps unexpected results:
%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.%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.For compatibility, it is required that content models in element 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.
The XML encoding declaration functions as an internal label on each entity, indicating which character set is in use. Before an XML processor can read the internal label, however, it apparently has to know what character set 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 UCS-2 format must begin with an XML encoding declaration, in which the first characters must be '<?XML', any conforming processor can detect, after four octets of input, which of the following cases apply (in reading this list, it may help to know that in Unicode, '<' is 0000 003C and '?' is 0000 003F, and the Byte Order Mark required of UCS-2 data streams is FEFF):
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 00 3C: UCS-2, big-endian FF FE 3C 00: UCS-2, little-endian 00 3C 00 3F: UCS-2, big-endian, no Byte Order Mark 3C 00 3F 00: UCS-2, little-endian, no Byte Order Mark 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-set routines should be careful to ensure the accuracy of the internal and external information used to label the entity.
The grammar given in the body of this specification is relatively simple, but for some purposes it is convenient to have an even simpler one. A very simple XML processor could parse a well-formed XML document, recognizing all element boundaries correctly (though not expanding entity references) using the following grammar:
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Most processors will require the more complex grammar given in the body of this specification.
This specification was prepared and approved for publication by the W3C SGML Editorial Review Board (ERB). ERB approval of this specification does not necessarily imply that all ERB members voted for its approval. At the time it approved this specification, the SGML ERB comprised the following members:
Jon Bosak, Sun (Chair); James Clark (Technical Lead); Tim Bray, Textuality (Co-editor); C. M. Sperberg-McQueen, U. of Ill. (Co-editor); Steve DeRose, INSO; Dave Hollander, HP; Eliot Kimber, Highland; Tom Magliery, NCSA; Eve Maler, ArborText; Jean Paoli, Microsoft; Peter Sharpe, SoftQuad;