Extensible Markup Language (XML)

W3C Working Draft 17-November-1997

This version:
Previous versions:
Tim Bray (Textuality and Netscape) <tbray@textuality.com>
Jean Paoli (Microsoft) <jeanpa@microsoft.com>
C. M. Sperberg-McQueen (University of Illinois at Chicago) <cmsmcq@uic.edu>

Status of this document

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 (XML)

Version 1.0

Table of Contents

1. Introduction
    1.1 Origin and Goals
    1.2 Relationship to Existing Standards
    1.3 Terminology
    1.4 Notation
2. Documents
    2.1 Well-Formed XML Documents
    2.2 Characters
    2.3 Common Syntactic Constructs
    2.4 Character Data and Markup
    2.6 Processing Instructions
    2.7 CDATA Sections
    2.8 Prolog and Document Type Declaration
    2.9 Standalone Document Declaration
    2.10 White Space Handling
    2.11 End-of-Line Handling
    2.12 Language Identification
3. Logical Structures
    3.1 Start-Tags, End-Tags, and Empty-Element Tags
    3.2 Element Type Declarations
        3.2.1 Element Content
        3.2.2 Mixed Content
    3.3 Attribute-List Declarations
        3.3.1 Attribute Types
        3.3.2 Attribute Defaults
        3.3.3 Attribute-Value Normalization
    3.4 Conditional Sections
4. Physical Structures
    4.1 Logical and Physical Structures
    4.2 Character and Entity References
    4.3 Entity Declarations
        4.3.1 Internal Entities
        4.3.2 External Entities
    4.4 XML Processor Treatment of Entities and References
        4.4.1 Not Recognized
        4.4.2 Included
        4.4.3 Included If Validating
        4.4.4 Forbidden
        4.4.5 Notify
        4.4.6 Bypassed
        4.4.7 Included as Token
    4.5 Construction of Internal Entity Replacement Text
    4.6 Predefined Entities
    4.7 Notation Declarations
    4.8 Character Encoding in Entities
    4.9 Document Entity
5. Conformance


B. Character Classes
C. Expansion of Entity and Character References
D. Deterministic Content Models
E. Autodetection of Character Encodings
F. References
G. W3C XML Working Group

1. Introduction

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.

1.1 Origin and Goals

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:

  1. XML shall be straightforwardly usable over the Internet.
  2. XML shall support a wide variety of applications.
  3. XML shall be compatible with SGML.
  4. It shall be easy to write programs which process XML documents.
  5. The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
  6. XML documents should be human-legible and reasonably clear.
  7. The XML design should be prepared quickly.
  8. The design of XML shall be formal and concise.
  9. XML documents shall be easy to create.
  10. Terseness in XML markup is of minimal importance.

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.

1.2 Relationship to Existing Standards

Standards relevant to users and implementors of XML include:

1.3 Terminology

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:

Conforming documents and XML processors are permitted to but need not behave as described.
Conforming documents and XML processors are required to behave as described; otherwise they are in error.
A violation of the rules of this specification; results are undefined. Conforming software may detect and report an error and may recover from it.
fatal error
An error which conforming software must detect and report to the application. After encountering a fatal error, an XML processor may continue processing the data to search for further errors and may report such errors to the application. In order to support correction of errors, the processor may make unprocessed text from the document (with intermingled character data and markup) available to the application. Once a fatal error is detected, however, the processor must not continue normal processing (i.e. it must not continue to pass character data and information about the document's logical structure to the application in the normal way).
validity constraint
A rule which applies to all valid XML documents. Violations of validity constraints are errors; they must, at user option, be reported by validating XML processors.
well-formedness constraint
A rule which applies to all well-formed XML documents. Violations of well-formedness constraints are fatal errors.
at user option
Conforming software may or must (depending on the modal verb in the sentence) behave as described; if it does, it must provide users a means to enable or disable the behavior described.
(Of strings or names:) Two strings or names being compared must be identical. Characters with multiple possible representations in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic forms) match only if they have the same representation in both strings. At user option, processors may normalize such characters to their canonical form. Note that no processig of characters with respect to case is part of the matching process. (Of strings and rules in the grammar:) A string matches a grammatical production if it belongs to the language generated by that production. (Of content and content models:) An element is said to match its declaration when it conforms in the fashion described in the element validity constraint.
for compatibility
A feature of XML included solely to ensure that XML remains compatible with SGML.
for interoperability
A non-binding recommendation included to increase the chances that XML documents can be processed by the existing installed base of SGML processors which predate the WebSGML Adaptations Annex to ISO 8879 now being prepared by ISO/IEC JTC1.

1.4 Notation

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:

where n 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]
matches any character with a value in the range(s) indicated (inclusive).
[^a-z], [^#Xn-#Xn]
matches any character with a value outside the range indicated.
[^abc], [^#Xn#Xn#Xn]
matches any character with a value not among the characters given.
matches a literal string matching that given inside the double quotes.
matches a literal string matching that given inside the single quotes.
These symbols may be combined to match more complex patterns as follows, where A and B represent a simple expression:
expression is treated as a unit, and may be combined as described in this list.
matches A or nothing; optional A.
matches A followed by B.
A | B
matches A or B but not both.
A - B
matches any string that matches A but does not match B.
matches one or more occurrences of A.
matches zero or more occurrences of A.
Other notations used in the productions are:
/* ... */
[ WFC: ... ]
Well-formedness check; this identifies by name a check for well-formedness associated with a production.
[ VC: ... ]
Validity check; this identifies by name a check for validity associated with a production.

2. Documents

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.

2.1 Well-Formed XML Documents

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.

[1] document ::= prolog element Misc*

Matching the document production implies that:

  1. It contains one or more elements.
  2. There is exactly one element, called the root, or document element, no part of which appears in the content of any other element. For all other elements, if the start-tag is in the content of another element, the end-tag is in the content of the same element. More simply stated, the elements, delimited by start- and end-tags, nest within each other.

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.

2.2 Characters

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
[2] Char ::= #X9 | #Xa | #Xd | [#X20-#Xfffd] | [#X10000-#x10FFFF] /* any Unicode character, FFFE and FFFF excluded */

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.

2.3 Common Syntactic Constructs

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
[3] S ::= (#X20 | #X9 | #Xd | #Xa)+

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
[4] MiscName ::= '.' | '-' | '_' | ':' | CombiningChar | Ignorable | Extender
[5] NameChar ::= Letter | Digit | MiscName
[6] Name ::= (Letter | '_' | ':') (NameChar)*
[7] Names ::= Name (S Name)*
[8] Nmtoken ::= (NameChar)+
[9] Nmtokens ::= Nmtoken (S Nmtoken)*

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):

[10] EntityValue ::= '"' ([^%&"] | PEReference | Reference)* '"'
|  "'" ([^%&'] | PEReference | Reference)* "'"
[11] AttValue ::= '"' ([^<&"] | Reference)* '"'
|  "'" ([^<&'] | Reference)* "'"
[12] SystemLiteral ::= '"' URLchar* '"' | "'" (URLchar - "'")* "'"
[13] URLchar ::= /* See RFC 1738 and 1808 */
[14] PubidLiteral ::= '"' PubidChar* '"' | "'" (PubidChar - "'")* "'"
[15] PubidChar ::= #X20 | #X9 | #Xd | #Xa | [a-zA-Z0-9] | [-'()+,./:=?]
[16] SkipLit ::= ('"' [^"]* '"') | ("'" [^']* "'")

2.4 Character Data and Markup

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 "&amp;" and "&lt;". The right angle bracket (>) may be represented using the string "&gt;", and must, for compatibility, be be escaped using "&gt;" 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 "&apos;", and the double-quote character (") as "&quot;".

Character data
[17] PCData ::= [^<&]*


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.

[18] Comment ::= '<--' ((Char - '-') | ('-' (Char - '-'))* "-->"

An example of a comment:

<!-- declarations for <head> & <body> -->

2.6 Processing Instructions

Processing instructions (PIs) allow documents to contain instructions for applications.

Processing instructions
[19] PI ::= '<?' Name (S (Char* - (Char* '?>' Char*)))? '?>'

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.

2.7 CDATA Sections

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
[20] CDSect ::= CDStart CData CDEnd
[21] CDStart ::= '<![CDATA['
[22] CData ::= (Char* - (Char* ']]>' Char*))
[23] CDEnd ::= ']]>'

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 "&lt;" and "&amp;". 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>]]>

2.8 Prolog and Document Type Declaration

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.

[24] prolog ::= XMLDecl? Misc* (doctypedecl Misc*)?
[25] XMLDecl ::= '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
[26] VersionInfo ::= S 'version' Eq ('"1.0"' | "'1.0'")
[27] Eq ::= S? '=' S?
[28] Misc ::= Comment | PI | S

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"?>
<greeting>Hello, world!</greeting>

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
[29] doctypedecl ::= '<!DOCTYPE' S Name (S ExternalID)? S? ('[' (markupdecls | S)? ']' S?)? '>' [ vc: Root Element Type ]
[30] markupdecls ::= (S? markupdecl S?)*
[31] markupdecl ::= elementdecl | AttlistDecl | EntityDecl | NotationDecl | PI | Comment [ vc: Declarations and PEs Synchronous ]

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
[32] extSubset ::= ( markupdecl | conditionalSect | S )*

For example:

<?xml version="1.0"?>
<!DOCTYPE greeting SYSTEM "hello.dtd">
<greeting>Hello, world!</greeting>

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" ?>
<!DOCTYPE greeting [
  <!ELEMENT greeting (#PCDATA)>
<greeting>Hello, world!</greeting>

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.

2.9 Standalone Document Declaration

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
[33] SDDecl ::= S 'standalone' Eq "'" ('yes' | 'no') "'"
| S 'standalone' Eq '"' ('yes' | 'no') '"' [ vc: 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

An example XML declaration with a Standalone Document Declaration:

<?xml version="1.0" standalone='yes'?>

2.10 White Space Handling

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.

2.11 End-of-Line Handling

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.

2.12 Language Identification

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
[34] LanguageID ::= Langcode ('-' Subcode)*
[35] Langcode ::= ISO639Code | IanaCode | UserCode
[36] ISO639Code ::= ([a-z] | [A-Z]) ([a-z] | [A-Z])
[37] IanaCode ::= ('i' | 'I') '-' ([a-z] | [A-Z])+
[38] UserCode ::= ('x' | 'X') '-' ([a-z] | [A-Z])+
[39] Subcode ::= ([a-z] | [A-Z])+

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>
<p xml:lang="en-GB">What colour is it?</p>
<p xml:lang="en-US">What color is it?</p>
<sp who="Faust" xml:lang="de">
  <l>Habe nun, ach! Philosophie,</l>
  <l>Juristerei, und Medizin</l>
  <l>durchaus studiert mit heißem Bemüh'n.</l>

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


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'>
    <!ATTLIST gloss  xml:lang NMTOKEN 'en'>
    <!ATTLIST note   xml:lang NMTOKEN 'en'>

3. Logical Structures

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.

[40] element ::= EmptyElemTag
| STag content ETag [ wfc: GI Match ]

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.

3.1 Start-Tags, End-Tags, and Empty-Element Tags

The beginning of every non-empty XML element is marked by a start-tag.

[41] STag ::= '<' Name (S Attribute)* S? '>' [ wfc: Unique Att Spec ]
[42] Attribute ::= Name Eq AttValue [ vc: Attribute Value Type ]
[ wfc: No External Entity References ]

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:

[43] ETag ::= '</' Name S? '>'

An example of an end-tag:


The text between the start-tag and end-tag is called the element's content:

Content of elements
[44] content ::= (element | PCData | Reference | CDSect | PI | Comment)*

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
[45] EmptyElemTag ::= '<' Name (S Attribute)* S? '/>' [ wfc: Unique Att Spec ]

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"
 src="http://www.w3.org/Icons/WWW/w3c_home" />

3.2 Element Type Declarations

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
[46] elementdecl ::= '<!ELEMENT' S Name S contentspec S? '>' [ vc: Unique Element Type Declaration ]
[47] contentspec ::= 'EMPTY' | 'ANY' | Mixed | children [ vc: Element valid ]

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:

  1. The declaration matches EMPTY and the element has no child elements or content.
  2. The declaration matches children and the sequence of child elements belongs to the language generated by the regular expression in the content model.
  3. The declaration matches mixed and the content consists of character data and child elements whose types match names in the content model.
  4. The declaration matches ANY, and the types of any child elements have been declared.

Examples of element type declarations:

<!ELEMENT p (#PCDATA|emph)* >
<!ELEMENT %name.para; %content.para; >
<!ELEMENT container ANY>

3.2.1 Element Content

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
[48] children ::= (choice | seq) ('?' | '*' | '+')?
[49] cp ::= (Name | choice | seq) ('?' | '*' | '+')?
[50] choice ::= '(' S? cp ( S? '|' S? cp )* S? ')' [ vc: Groups and PEs Synchronous ]
[51] seq ::= '(' S? cp ( S? ',' S? cp )* S? ')' [ vc: Groups and PEs Synchronous ]

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?)>
<!ELEMENT div1 (head, (p | list | note)*, div2*)>
<!ELEMENT head (%head.content; | %head.misc;)*>

3.2.2 Mixed Content

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
[52] Mixed ::= '(' S? '#PCDATA' (S? '|' S? Name)* S? ')*'
| '(' S? '#PCDATA' S? ')' [ vc: Groups and PEs Synchronous ]
[ vc: No Duplicate Types ]

where the Names give the types of elements that may appear as children.

Validity Constraint - No Duplicate Types
The same name must not appear more than once in a single mixed-content declaration.

Examples of mixed content declarations:

<!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
<!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* >

3.3 Attribute-List Declarations

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
[53] AttlistDecl ::= '<!ATTLIST' S Name S AttDef+ S? '>'
[54] AttDef ::= S Name S AttType S Default

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.

3.3.1 Attribute Types

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
[55] AttType ::= StringType | TokenizedType | EnumeratedType
[56] StringType ::= 'CDATA'
[57] TokenizedType ::= 'ID' [ vc: ID ]
[ vc: One ID per Element ]
| 'IDREF' [ vc: IDREF ]
| 'IDREFS' [ vc: IDREF ]
| 'ENTITY' [ vc: Entity Name ]
| 'ENTITIES' [ vc: Entity Name ]
| 'NMTOKEN' [ vc: Name Token ]
| 'NMTOKENS' [ vc: Name Token ]

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'
[58] EnumeratedType ::= NotationType | Enumeration
[59] NotationType ::= 'NOTATION' S '(' S? Ntoks (S? '|' S? Ntoks)* S? ')' [ vc: Notation Attributes ]
[60] Ntoks ::= Name (S? '|' S? Name)*
[61] Enumeration ::= '(' S? Nmtoken (S? '|' S? Nmtoken)* S? ')' [ vc: Enumeration ]

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.

3.3.2 Attribute Defaults

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
[62] Default ::= '#REQUIRED' | '#IMPLIED' [ vc: Attribute Default Legal ]
| (('#FIXED' S)? AttValue)

#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
          id      ID      #REQUIRED
          name    CDATA   #IMPLIED>
<!ATTLIST list
          type    (bullets|ordered|glossary)  "ordered">
<!ATTLIST form
          method  CDATA   #FIXED "POST">

3.3.3 Attribute-Value Normalization

Before the value of an attribute is passed to the application, the XML processor must normalize it as follows:

  1. Line-end characters (or, on some systems, record boundaries) must be replaced by single space (#X20) characters.
  2. Character references and references to internal text entities must be expanded. References to external entities are an error.
  3. If the attribute is not of type CDATA, all strings of white space must be normalized to single space characters (#X20), and leading and trailing white space must be removed.
All attributes for which no declaration have been read should be treated as CDATA.

3.4 Conditional Sections

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
[63] conditionalSect ::= includeSect | ignoreSect
[64] includeSect ::= '<![' S? 'INCLUDE' S? '[' markupdecls ']]>'
[65] ignoreSect ::= '<![' S? 'IGNORE' S? '[' ignoreSectContents* ']]>'
[66] ignoreSectContents ::= Ignore ('<![' ignoreSectContents ']]>' Ignore)*
[67] Ignore ::= Char* - (Char* ('<![' | ']]>') Char*)

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' >
<!ENTITY % final 'IGNORE' >
<!ELEMENT book (comments*, title, body, supplements?)>
<!ELEMENT book (title, body, supplements?)>

4. Physical Structures

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.

4.1 Logical and Physical Structures

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.

4.2 Character and Entity References

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
[68] CharRef ::= '&#' [0-9]+ ';'
| '&#X' [0-9a-fA-F]+ ';'

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
[69] Reference ::= EntityRef | CharRef
[70] EntityRef ::= '&' Name ';' [ wfc: Entity Declared ]
[ wfc: Text Entity ]
[ wfc: No Recursion ]
[71] PEReference ::= '%' Name ';' [ wfc: Entity Declared ]
[ wfc: Text Entity ]
[ wfc: No Recursion ]
[ wfc: In DTD ]

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> (&#X3C;) to save options.
This document was prepared on &docdate; and
is classified &security-level;.

Example of a parameter-entity reference:

         SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >

4.3 Entity Declarations

Entities are declared thus:

Entity declaration
[72] EntityDecl ::= '<!ENTITY' S Name S EntityDef S? '>' /* General entities */
| '<!ENTITY' S '%' S Name S EntityDef S? '>' /* Parameter entities */
[73] EntityDef ::= EntityValue | ExternalDef

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.

4.3.1 Internal Entities

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

4.3.2 External Entities

If the entity is not internal, it is an external entity, declared as follows:

External entity declaration
[74] ExternalDef ::= ExternalID NDataDecl?
[75] ExternalID ::= 'SYSTEM' S SystemLiteral
|  'PUBLIC' S PubidLiteral S SystemLiteral
[76] NDataDecl ::= S 'NDATA' S Name [ vc: Notation Declared ]

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
         SYSTEM "http://www.textuality.com/boilerplate/OpenHatch.xml">
<!ENTITY open-hatch
         PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN"
<!ENTITY hatch-pic
         SYSTEM "../grafix/OpenHatch.gif"
         NDATA gif >

4.4 XML Processor Treatment of Entities and References

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:

in Content
anywhere after the start-tag and before the end-tag of an element; corresponds to the nonterminal content.
in Attribute Value
within the value of an attribute in a start-tag; corresponds to the nonterminal AttValue.
in Entity Value
within a parameter or internal entity's replacement text within the entity's declaration; corresponds to the nonterminal EntityValue.
in DTD
within either the internal or external subsets of the DTD, but outside of an EntityValue.
References Binary
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

4.4.1 Not Recognized

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.

4.4.2 Included

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&amp;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.

4.4.3 Included If Validating

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.

4.4.4 Forbidden

The following are forbidden, and constitute fatal errors:

4.4.5 Notify

When 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.

4.4.6 Bypassed

When a general entity reference appears in the EntityValue in an entity declaration, it is ignored, and left as-is in the replacement text.

4.4.7 Included as Token

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.

4.5 Construction of Internal Entity Replacement Text

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    "&#Xc9;ditions Gallimard" >
<!ENTITY   rights "All rights reserved" >
<!ENTITY   book   "La Peste: Albert Camus, &#Xa9; 1947 %pub;. &rights;" >

then the replacement text for the entity "book" is

La Peste: Albert Camus, © 1947 Éditions Gallimard. &rights;

The general-entity reference "&rights;" would be expanded should the reference "&book;" appear in the document's content or an attribute value.

These simple rules may have complex interactions; for a detailed discussion of a difficult example, see the appendix on expansion of entity references.

4.6 Predefined Entities

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 "&#60;" and "&#38;" 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     "&#60;"> 
<!ENTITY gt     "&#62;"> 
<!ENTITY amp    "&#38;"> 
<!ENTITY apos   "&#39;"> 
<!ENTITY quot   "&#34;"> 

4.7 Notation Declarations

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
[77] NotationDecl ::= '<!NOTATION' S Name S ExternalID S? '>'

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.)

4.8 Character Encoding in Entities

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
[78] EncodingDecl ::= S 'encoding' Eq QEncoding
[79] EncodingPI ::= '<?xml' S 'encoding' Eq QEncoding S? '?>'
[80] QEncoding ::= '"' Encoding '"' | "'" Encoding "'"
[81] Encoding ::= LatinName
[82] LatinName ::= [A-Za-z] ([A-Za-z0-9._] | '-')* /* Name containing only Latin characters */

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:

If an XML processor encounters an entity with an encoding that it is unable to process, it may inform the application of this fact and may allow the application to request either that the entity should be treated as an binary entity, or that processing should cease.

Examples of encoding declarations:

<?xml encoding='UTF-8'?>
<?xml encoding='EUC-JP'?>

4.9 Document Entity

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.

5. Conformance

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.

B. Character Classes

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.

[83] BaseChar ::= [#X41-#X5a] | [#X61-#X7a] /* Latin 1 upper and lowercase */
| #Xaa | #Xb5 | #Xba | [#Xc0-#Xd6] | [#Xd8-#Xf6] | [#Xf8-#Xff] /* Latin 1 supplementary */
| [#X0100-#X017f] | [#X0180-#X01f5] | [#X01fa-#X0217] /* Extended Latin-A and B */
| [#X0250-#X02a8] /* IPA Extensions */
| [#X02b0-#X02b8] | [#X02bb-#X02c1] | [#X02e0-#X02e4] /* Spacing Modifiers */
| #X037a | #X0386 | [#X0388-#X038a] | #X038c | [#X038e-#X03a1] | [#X03a3-#X03ce] | [#X03d0-#X03d6] | #X03da | #X03dc | #X03de | #X03e0 | [#X03e2-#X03f3] /* Greek and Coptic */
| [#X0401-#X040c] | [#X040e-#X044f] | [#X0451-#X045c] | [#X045e-#X0481] | [#X0490-#X04c4] | [#X04c7-#X04c8] | [#X04cb-#X04cc] | [#X04d0-#X04eb] | [#X04ee-#X04f5] | [#X04f8-#X04f9] /* Cyrillic */
| [#X0531-#X0556] | #X0559 | [#X0561-#X0587] /* Armenian */
| [#X05d0-#X05ea] | [#X05f0-#X05f2] /* Hebrew */
| [#X0621-#X063a] | [#X0641-#X064a] | [#X0671-#X06b7] | [#X06ba-#X06be] | [#X06c0-#X06ce] | [#X06d0-#X06d3] | #X06d5 | [#X06e5-#X06e6] /* Arabic */
| [#X0905-#X0939] | #X093d | [#X0958-#X0961] /* Devanagari */
| [#X0985-#X098c] | [#X098f-#X0990] | [#X0993-#X09a8] | [#X09aa-#X09b0] | #X09b2 | [#X09b6-#X09b9] | [#X09dc-#X09dd] | [#X09df-#X09e1] | [#X09f0-#X09f1] /* Bengali */
| [#X0a05-#X0a0a] | [#X0a0f-#X0a10] | [#X0a13-#X0a28] | [#X0a2a-#X0a30] | [#X0a32-#X0a33] | [#X0a35-#X0a36] | [#X0a38-#X0a39] | [#X0a59-#X0a5c] | #X0a5e | [#X0a72-#X0a74] | [#X0a85-#X0a8b] | #X0a8d /* Gurmukhi */
| [#X0a8f-#X0a91] | [#X0a93-#X0aa8] | [#X0aaa-#X0ab0] | [#X0ab2-#X0ab3] | [#X0ab5-#X0ab9] | #X0abd | #X0ae0 /* Gujarati */
| [#X0b05-#X0b0c] | [#X0b0f-#X0b10] | [#X0b13-#X0b28] | [#X0b2a-#X0b30] | [#X0b32-#X0b33] | [#X0b36-#X0b39] | #X0b3d | [#X0b5c-#X0b5d] | [#X0b5f-#X0b61] /* Oriya */
| [#X0b85-#X0b8a] | [#X0b8e-#X0b90] | [#X0b92-#X0b95] | [#X0b99-#X0b9a] | #X0b9c | [#X0b9e-#X0b9f] | [#X0ba3-#X0ba4] | [#X0ba8-#X0baa] | [#X0bae-#X0bb5] | [#X0bb7-#X0bb9] /* Tamil */
| [#X0c05-#X0c0c] | [#X0c0e-#X0c10] | [#X0c12-#X0c28] | [#X0c2a-#X0c33] | [#X0c35-#X0c39] | [#X0c60-#X0c61] /* Telugu */
| [#X0c85-#X0c8c] | [#X0c8e-#X0c90] | [#X0c92-#X0ca8] | [#X0caa-#X0cb3] | [#X0cb5-#X0cb9] | #X0cde | [#X0ce0-#X0ce1] /* Kannada */
| [#X0d05-#X0d0c] | [#X0d0e-#X0d10] | [#X0d12-#X0d28] | [#X0d2a-#X0d39] | [#X0d60-#X0d61] /* Malayalam */
| [#X0e01-#X0e2e] | #X0e30 | [#X0e32-#X0e33] | [#X0e40-#X0e45] /* Thai */
| [#X0e81-#X0e82] | #X0e84 | [#X0e87-#X0e88] | #X0e8a | #X0e8d | [#X0e94-#X0e97] | [#X0e99-#X0e9f] | [#X0ea1-#X0ea3] | #X0ea5 | #X0ea7 | [#X0eaa-#X0eab] | [#X0ead-#X0eae] | #X0eb0 | [#X0eb2-#X0eb3] | #X0ebd | [#X0ec0-#X0ec4] | [#X0edc-#X0edd] /* Lao */
| [#X0f40-#X0f47] | [#X0f49-#X0f69] /* Tibetan */
| [#X10a0-#X10c5] | [#X10d0-#X10f6] /* Georgian */
| [#X1100-#X1159] | [#X115f-#X11a2] | [#X11a8-#X11f9] /* Hangul Jamo */
| [#X1e00-#X1e9b] | [#X1ea0-#X1ef9] /* Add'l Extended Latin */
| [#X1f00-#X1f15] | [#X1f18-#X1f1d] | [#X1f20-#X1f45] | [#X1f48-#X1f4d] | [#X1f50-#X1f57] | #X1f59 | #X1f5b | #X1f5d | [#X1f5f-#X1f7d] | [#X1f80-#X1fb4] | [#X1fb6-#X1fbc] | #X1fbe | [#X1fc2-#X1fc4] | [#X1fc6-#X1fcc] | [#X1fd0-#X1fd3] | [#X1fd6-#X1fdb] | [#X1fe0-#X1fec] | [#X1ff2-#X1ff4] | [#X1ff6-#X1ffc] /* Greek Extensions */
| #X207f /* Super-, subscripts */
| #X2102 | #X2107 | [#X210a-#X2113] | #X2115 | [#X2118-#X211d] | #X2124 | #X2126 | #X2128 | [#X212a-#X2131] | [#X2133-#X2138] /* Letterlike Symbols */
| [#X2160-#X2182] /* Number forms */
| [#X3041-#X3094] /* Hiragana */
| [#X30a1-#X30fa] /* Katakana */
| [#X3105-#X312c] /* Bopomofo */
| [#X3131-#X318e] /* Hangul Jamo */
| [#Xac00-#Xd7a3] /* Hangul syllables */
[84] Ideographic ::= [#X4e00-#X9fa5] | [#Xf900-#Xfa2d] | #X3007 | [#X3021-#X3029]
[85] CombiningChar ::= [#X0300-#X0345] | [#X0360-#X0361] | [#X0483-#X0486] | [#X0591-#X05a1] | [#X05a3-#X05b9] | [#X05bb-#X05bd] | #X05bf | [#X05c1-#X05c2] | #X05c4 | [#X064b-#X0652] | #X0670 | [#X06d6-#X06dc] | [#X06dd-#X06df] | [#X06e0-#X06e4] | [#X06e7-#X06e8] | [#X06ea-#X06ed] | [#X0901-#X0903] | #X093c | [#X093e-#X094c] | #X094d | [#X0951-#X0954] | [#X0962-#X0963] | [#X0981-#X0983] | #X09bc | #X09be | #X09bf | [#X09c0-#X09c4] | [#X09c7-#X09c8] | [#X09cb-#X09cd] | #X09d7 | [#X09e2-#X09e3] | #X0a02 | #X0a3c | #X0a3e | #X0a3f | [#X0a40-#X0a42] | [#X0a47-#X0a48] | [#X0a4b-#X0a4d] | [#X0a70-#X0a71] | [#X0a81-#X0a83] | #X0abc | [#X0abe-#X0ac5] | [#X0ac7-#X0ac9] | [#X0acb-#X0acd] | [#X0b01-#X0b03] | #X0b3c | [#X0b3e-#X0b43] | [#X0b47-#X0b48] | [#X0b4b-#X0b4d] | [#X0b56-#X0b57] | [#X0b82-#X0b83] | [#X0bbe-#X0bc2] | [#X0bc6-#X0bc8] | [#X0bca-#X0bcd] | #X0bd7 | [#X0c01-#X0c03] | [#X0c3e-#X0c44] | [#X0c46-#X0c48] | [#X0c4a-#X0c4d] | [#X0c55-#X0c56] | [#X0c82-#X0c83] | [#X0cbe-#X0cc4] | [#X0cc6-#X0cc8] | [#X0cca-#X0ccd] | [#X0cd5-#X0cd6] | [#X0d02-#X0d03] | [#X0d3e-#X0d43] | [#X0d46-#X0d48] | [#X0d4a-#X0d4d] | #X0d57 | #X0e31 | [#X0e34-#X0e3a] | [#X0e47-#X0e4e] | #X0eb1 | [#X0eb4-#X0eb9] | [#X0ebb-#X0ebc] | [#X0ec8-#X0ecd] | [#X0f18-#X0f19] | #X0f35 | #X0f37 | #X0f39 | #X0f3e | #X0f3f | [#X0f71-#X0f84] | [#X0f86-#X0f8b] | [#X0f90-#X0f95] | #X0f97 | [#X0f99-#X0fad] | [#X0fb1-#X0fb7] | #X0fb9 | [#X20d0-#X20dc] | #X20e1 | [#X302a-#X302f] | #X3099 | #X309a
[86] Letter ::= BaseCharIdeographic
[87] Digit ::= [#X30-#X39] /* ISO 646 digits */
| [#X0660-#X0669] /* Arabic-Indic digits */
| [#X06f0-#X06f9] /* Eastern Arabic-Indic digits */
| [#X0966-#X096f] /* Devanagari digits */
| [#X09e6-#X09ef] /* Bengali digits */
| [#X0a66-#X0a6f] /* Gurmukhi digits */
| [#X0ae6-#X0aef] /* Gujarati digits */
| [#X0b66-#X0b6f] /* Oriya digits */
| [#X0be7-#X0bef] /* Tamil digits (no zero) */
| [#X0c66-#X0c6f] /* Telugu digits */
| [#X0ce6-#X0cef] /* Kannada digits */
| [#X0d66-#X0d6f] /* Malayalam digits */
| [#X0e50-#X0e59] /* Thai digits */
| [#X0ed0-#X0ed9] /* Lao digits */
| [#X0f20-#X0f29] /* Tibetan digits */
[88] Ignorable ::= [#X200c-#X200f] /* zw layout */
| [#X202a-#X202e] /* bidi formatting */
| [#X206a-#X206f] /* alt formatting */
[89] Extender ::= #Xb7 | #X02d0 | #X02d1 | #X0387 | #X0640 | #X0e46 | #X0ec6 | #X3005 | [#X3031-#X3035] | [#X309b-#X309e] | [#X30fc-#X30fe]

C. Expansion of Entity and Character References

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;#38;) may be escaped
numerically (&#38;#38;#38;) or with a general entity
(&amp;amp;).</p>" >

then the XML processor will recognize the character references when it parses the entity declaration, and resolve them before storing the following string as the value of the entity "example":

<p>An ampersand (&#38;) may be escaped
numerically (&#38;#38;) or with a general entity

A reference in the document to "&example;" will cause the text to be reparsed, at which time the start- and end-tags of the "p" element will be recognized and the three references will be recognized and expanded, resulting in a "p" element with the following content (all data, no delimiters or markup):

An ampersand (&) may be escaped
numerically (&#38;) or with a general entity

A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference.

1 <?xml version='1.0'?>
2 <!DOCTYPE test [
3 <!ELEMENT test (#PCDATA) >
4 <!ENTITY % xx '&#37;zz;'>
5 <!ENTITY % zz '&#60;!ENTITY tricky "error-prone" >' >
6 %xx;
7 ]>
8 <test>This sample shows a &tricky; method.</test>

This produces the following:

D. Deterministic Content Models

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].

E. Autodetection of Character Encodings

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"):

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.

F. References

Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. Compilers: Principles, Techniques, and Tools. Reading: Addison-Wesley, 1986, rpt. corr. 1988.
Brüggemann-Klein, Anne. Regular Expressions into Finite Automata. Extended abstract in I. Simon, Hrsg., LATIN 1992, S. 97-98. Springer-Verlag, Berlin 1992. Full Version in Theoretical Computer Science 120: 197-213, 1993.
Brüggemann-Klein, Anne, and Derick Wood. Deterministic Regular Languages. Universität Freiburg, Institut für Informatik, Bericht 38, Oktober 1991.
ISO 639
(International Organization for Standardization). ISO 8879:1988 (E). Code for the representation of names of languages. [Geneva]: International Organization for Standardization, 1988.
ISO 3166
(International Organization for Standardization). ISO 3166-1:1997 (E). Codes for the representation of names of countries and their subdivisions -- Part 1: Country codes [Geneva]: International Organization for Standardization, 1997.
ISO/IEC 8879
ISO (International Organization for Standardization). ISO/IEC 8879-1986 (E). Information processing -- Text and Office Systems -- Standard Generalized Markup Language (SGML). First edition -- 1986-10-15. [Geneva]: International Organization for Standardization, 1986.
ISO/IEC 10646
ISO (International Organization for Standardization). ISO/IEC 10646-1993 (E). Information technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane. [Geneva]: International Organization for Standardization, 1993 (plus amendments AM 1 through AM 5).
ISO/IEC 10744
ISO (International Organization for Standardization). ISO/IEC 10744-1992 (E). Information technology -- Hypermedia/Time-based Structuring Language (HyTime). [Geneva]: International Organization for Standardization, 1992. Extended Facilities Annexe. [Geneva]: International Organization for Standardization, 1996.
IETF (Internet Engineering Task Force). RFC 1738: Uniform Resource Locators. 1991.
IETF (Internet Engineering Task Force). RFC 1766: Tags for the Identification of Languages. 1995.
The Unicode Consortium. The Unicode Standard, Version 2.0. Reading, Mass.: Addison-Wesley Developers Press, 1996.

G. W3C XML Working Group

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