Extensible Markup Language (XML) 1.0 (Fifth Edition)

W3C Recommendation 26 November 2008

Note: On 7 February 2013, this specification was modified in place to replace broken links to RFC4646 and RFC4647.

This version:
Latest version:
Previous versions:
Tim Bray, Textuality and Netscape <tbray@textuality.com>
Jean Paoli, Microsoft <jeanpa@microsoft.com>
C. M. Sperberg-McQueen, W3C <cmsmcq@w3.org>
Eve Maler, Sun Microsystems, Inc. <eve.maler@east.sun.com>
François Yergeau

Please refer to the errata for this document, which may include some normative corrections.

The previous errata for this document, are also available.

See also translations.

This document is also available in these non-normative formats: XML and XHTML with color-coded revision indicators.


The Extensible Markup Language (XML) is a subset of SGML that is completely described in this document. Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document specifies a syntax created by subsetting an existing, widely used international text processing standard (Standard Generalized Markup Language, ISO 8879:1986(E) as amended and corrected) for use on the World Wide Web. It is a product of the XML Core Working Group as part of the XML Activity. The English version of this specification is the only normative version. However, for translations of this document, see http://www.w3.org/2003/03/Translations/byTechnology?technology=xml.

This document is a W3C Recommendation. This fifth edition is not a new version of XML. As a convenience to readers, it incorporates the changes dictated by the accumulated errata (available at http://www.w3.org/XML/xml-V10-4e-errata) to the Fourth Edition of XML 1.0, dated 16 August 2006. In particular, erratum [E09] relaxes the restrictions on element and attribute names, thereby providing in XML 1.0 the major end user benefit currently achievable only by using XML 1.1. As a consequence, many possible documents which were not well-formed according to previous editions of this specification are now well-formed, and previously invalid documents using the newly-allowed name characters in, for example, ID attributes, are now valid.

This edition supersedes the previous W3C Recommendation of 16 August 2006.

Please report errors in this document to the public xml-editor@w3.org mail list; public archives are available. For the convenience of readers, an XHTML version with color-coded revision indicators is also provided; this version highlights each change due to an erratum published in the errata list for the previous edition, together with a link to the particular erratum in that list. Most of the errata in the list provide a rationale for the change. The errata list for this fifth edition is available at http://www.w3.org/XML/xml-V10-5e-errata.

An implementation report is available at http://www.w3.org/XML/2008/01/xml10-5e-implementation.html. A Test Suite is maintained to help assessing conformance to this specification.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1 Introduction
    1.1 Origin and Goals
    1.2 Terminology
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 Character and Entity References
    4.2 Entity Declarations
        4.2.1 Internal Entities
        4.2.2 External Entities
    4.3 Parsed Entities
        4.3.1 The Text Declaration
        4.3.2 Well-Formed Parsed Entities
        4.3.3 Character Encoding in 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 Included in Literal
        4.4.6 Notify
        4.4.7 Bypassed
        4.4.8 Included as PE
        4.4.9 Error
    4.5 Construction of Entity Replacement Text
    4.6 Predefined Entities
    4.7 Notation Declarations
    4.8 Document Entity
5 Conformance
    5.1 Validating and Non-Validating Processors
    5.2 Using XML Processors
6 Notation


A References
    A.1 Normative References
    A.2 Other References
B Character Classes
C XML and SGML (Non-Normative)
D Expansion of Entity and Character References (Non-Normative)
E Deterministic Content Models (Non-Normative)
F Autodetection of Character Encodings (Non-Normative)
    F.1 Detection Without External Encoding Information
    F.2 Priorities in the Presence of External Encoding Information
G W3C XML Working Group (Non-Normative)
H W3C XML Core Working Group (Non-Normative)
I Production Notes (Non-Normative)
J Suggestions for XML Names (Non-Normative)

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 [ISO 8879]. By construction, XML documents are conforming SGML documents.

XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form markup. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.

[Definition: A software module called an XML processor is used to read XML documents and provide access to their content and structure.] [Definition: It is assumed that an XML processor is doing its work on behalf of another module, called the application.] This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.

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 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 Working Group'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 [Unicode] and ISO/IEC 10646 [ISO/IEC 10646] for characters, Internet BCP 47 [IETF BCP 47] and the Language Subtag Registry [IANA-LANGCODES] for language identification tags), provides all the information necessary to understand XML Version 1.0 and construct computer programs to process it.

This version of the XML specification may be distributed freely, as long as all text and legal notices remain intact.

1.2 Terminology

The terminology used to describe XML documents is defined in the body of this specification. The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when EMPHASIZED, are to be interpreted as described in [IETF RFC 2119]. In addition, the terms defined in the following list are used in building those definitions and in describing the actions of an XML processor:


[Definition: A violation of the rules of this specification; results are undefined. Unless otherwise specified, failure to observe a prescription of this specification indicated by one of the keywords MUST, REQUIRED, MUST NOT, SHALL and SHALL NOT is an error. Conforming software MAY detect and report an error and MAY recover from it.]

fatal error

[Definition: An error which a conforming XML processor MUST detect and report to the application. After encountering a fatal error, the 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 data 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).]

at user option

[Definition: 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.]

validity constraint

[Definition: 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

[Definition: A rule which applies to all well-formed XML documents. Violations of well-formedness constraints are fatal errors.]


[Definition: (Of strings or names:) Two strings or names being compared are 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. No case folding is performed. (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 matches its declaration when it conforms in the fashion described in the constraint [VC: Element Valid].]

for compatibility

[Definition: Marks a sentence describing a feature of XML included solely to ensure that XML remains compatible with SGML.]

for interoperability

[Definition: Marks a sentence describing 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.]

2 Documents

[Definition: A data object is an XML document if it is well-formed, as defined in this specification. In addition, the XML document is valid if it meets certain further constraints.]

Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup. The logical and physical structures MUST nest properly, as described in 4.3.2 Well-Formed Parsed Entities.

2.1 Well-Formed XML Documents

[Definition: A textual object is a well-formed XML document if:]

  1. Taken as a whole, it matches the production labeled document.

  2. It meets all the well-formedness constraints given in this specification.

  3. Each of the parsed entities which is referenced directly or indirectly within the document is well-formed.

[1]   document   ::=    prolog element Misc*

Matching the document production implies that:

  1. It contains one or more elements.

  2. [Definition: 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 properly within each other.

[Definition: As a consequence of this, for each non-root element C in the document, there is one other element P in the document such that C is in the content of P, but is not in the content of any other element that is in the content of P. P is referred to as the parent of C, and C as a child of P.]

2.2 Characters

[Definition: A parsed entity contains text, a sequence of characters, which may represent markup or character data.] [Definition: A character is an atomic unit of text as specified by ISO/IEC 10646:2000 [ISO/IEC 10646]. Legal characters are tab, carriage return, line feed, and the legal characters of Unicode and ISO/IEC 10646. The versions of these standards cited in A.1 Normative References were current at the time this document was prepared. New characters may be added to these standards by amendments or new editions. Consequently, XML processors MUST accept any character in the range specified for Char. ]

Character Range
[2]   Char   ::=   #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]/* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */

The mechanism for encoding character code points into bit patterns may vary from entity to entity. All XML processors MUST accept the UTF-8 and UTF-16 encodings of Unicode [Unicode]; the mechanisms for signaling which of the two is in use, or for bringing other encodings into play, are discussed later, in 4.3.3 Character Encoding in Entities.


Document authors are encouraged to avoid "compatibility characters", as defined in section 2.3 of [Unicode]. The characters defined in the following ranges are also discouraged. They are either control characters or permanently undefined Unicode characters:

[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDEF],
[#x1FFFE-#x1FFFF], [#x2FFFE-#x2FFFF], [#x3FFFE-#x3FFFF],
[#x4FFFE-#x4FFFF], [#x5FFFE-#x5FFFF], [#x6FFFE-#x6FFFF],
[#x7FFFE-#x7FFFF], [#x8FFFE-#x8FFFF], [#x9FFFE-#x9FFFF],

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)+


The presence of #xD in the above production is maintained purely for backward compatibility with the First Edition. As explained in 2.11 End-of-Line Handling, all #xD characters literally present in an XML document are either removed or replaced by #xA characters before any other processing is done. The only way to get a #xD character to match this production is to use a character reference in an entity value literal.

An Nmtoken (name token) is any mixture of name characters.

[Definition: A Name is an Nmtoken with a restricted set of initial characters.] Disallowed initial characters for Names include digits, diacritics, the full stop and the hyphen.

Names beginning with the string "xml", or with any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.


The Namespaces in XML Recommendation [XML Names] assigns a meaning to names containing colon characters. Therefore, authors should not use the colon in XML names except for namespace purposes, but XML processors must accept the colon as a name character.

The first character of a Name MUST be a NameStartChar, and any other characters MUST be NameChars; this mechanism is used to prevent names from beginning with European (ASCII) digits or with basic combining characters. Almost all characters are permitted in names, except those which either are or reasonably could be used as delimiters. The intention is to be inclusive rather than exclusive, so that writing systems not yet encoded in Unicode can be used in XML names. See J Suggestions for XML Names for suggestions on the creation of names.

Document authors are encouraged to use names which are meaningful words or combinations of words in natural languages, and to avoid symbolic or white space characters in names. Note that COLON, HYPHEN-MINUS, FULL STOP (period), LOW LINE (underscore), and MIDDLE DOT are explicitly permitted.

The ASCII symbols and punctuation marks, along with a fairly large group of Unicode symbol characters, are excluded from names because they are more useful as delimiters in contexts where XML names are used outside XML documents; providing this group gives those contexts hard guarantees about what cannot be part of an XML name. The character #x037E, GREEK QUESTION MARK, is excluded because when normalized it becomes a semicolon, which could change the meaning of entity references.

Names and Tokens
[4]   NameStartChar   ::=   ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a]   NameChar   ::=   NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]   Name   ::=   NameStartChar (NameChar)*
[6]   Names   ::=   Name (#x20 Name)*
[7]   Nmtoken   ::=   (NameChar)+
[8]   Nmtokens   ::=   Nmtoken (#x20 Nmtoken)*


The Names and Nmtokens productions are used to define the validity of tokenized attribute values after normalization (see 3.3.1 Attribute Types).

Literal data is any quoted string not containing the quotation mark used as a delimiter for that string. Literals are used for specifying the content of internal entities (EntityValue), the values of attributes (AttValue), and external identifiers (SystemLiteral). Note that a SystemLiteral can be parsed without scanning for markup.

[9]   EntityValue   ::=   '"' ([^%&"] | PEReference | Reference)* '"'
|  "'" ([^%&'] | PEReference | Reference)* "'"
[10]   AttValue   ::=   '"' ([^<&"] | Reference)* '"'
|  "'" ([^<&'] | Reference)* "'"
[11]   SystemLiteral   ::=   ('"' [^"]* '"') | ("'" [^']* "'")
[12]   PubidLiteral   ::=   '"' PubidChar* '"' | "'" (PubidChar - "'")* "'"
[13]   PubidChar   ::=   #x20 | #xD | #xA | [a-zA-Z0-9] | [-'()+,./:=?;!*#@$_%]


Although the EntityValue production allows the definition of a general entity consisting of a single explicit < in the literal (e.g., <!ENTITY mylt "<">), it is strongly advised to avoid this practice since any reference to that entity will cause a well-formedness error.

2.4 Character Data and Markup

Text consists of intermingled character data and markup. [Definition: Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character references, comments, CDATA section delimiters, document type declarations, processing instructions, XML declarations, text declarations, and any white space that is at the top level of the document entity (that is, outside the document element and not inside any other markup).]

[Definition: All text that is not markup constitutes the character data of the document.]

The ampersand character (&) and the left angle bracket (<) MUST NOT appear in their literal form, except when used as markup delimiters, or within a comment, a processing instruction, or a CDATA section. If they are needed elsewhere, they MUST be escaped using either numeric character references or the strings " &amp; " and " &lt; " respectively. The right angle bracket (>) may be represented using the string " &gt; ", and MUST, for compatibility, be escaped using either " &gt; " or a character reference when it appears in the string " ]]> " in content, when that string is not marking the end of a CDATA section.

In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup and does not include the CDATA-section-close delimiter, " ]]> ". 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
[14]   CharData   ::=   [^<&]* - ([^<&]* ']]>' [^<&]*)


[Definition: Comments may appear anywhere in a document outside other markup; in addition, they may appear within the document type declaration at places allowed by the grammar. They are not part of the document's character data; an XML processor MAY, but need not, make it possible for an application to retrieve the text of comments. For compatibility, the string " -- " (double-hyphen) MUST NOT occur within comments.] Parameter entity references MUST NOT be recognized within comments.

[15]   Comment   ::=   '<!--' ((Char - '-') | ('-' (Char - '-')))* '-->'

An example of a comment:

Note that the grammar does not allow a comment ending in --->. The following example is not well-formed.

2.6 Processing Instructions

[Definition: Processing instructions (PIs) allow documents to contain instructions for applications.]

Processing Instructions
[16]   PI   ::=   '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
[17]   PITarget   ::=    Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

PIs are not part of the document's character data, but MUST be passed through to the application. The PI begins with a target (PITarget) used to identify the application to which the instruction is directed. The target names " XML ", " xml ", and so on are reserved for standardization in this or future versions of this specification. The XML Notation mechanism may be used for formal declaration of PI targets. Parameter entity references MUST NOT be recognized within processing instructions.

2.7 CDATA Sections

[Definition: CDATA sections may occur anywhere character data may occur; they are used to escape blocks of text containing characters which would otherwise be recognized as markup. CDATA sections begin with the string " <![CDATA[ " and end with the string " ]]> ":]

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

[Definition: XML documents SHOULD begin with an XML declaration which specifies the version of XML being used.] For example, the following is a complete XML document, well-formed but not valid:

<?xml version="1.0"?>
<greeting>Hello, world!</greeting> 

and so is this:

<greeting>Hello, world!</greeting>

The function of the markup in an XML document is to describe its storage and logical structure and to associate attribute name-value pairs with its logical structures. XML provides a mechanism, the document type declaration, to define constraints on the logical structure and to support the use of predefined storage units. [Definition: An XML document is valid if it has an associated document type declaration and if the document complies with the constraints expressed in it.]

The document type declaration MUST appear before the first element in the document.

[22]   prolog   ::=    XMLDecl? Misc* (doctypedecl Misc*)?
[23]   XMLDecl   ::=   '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
[24]   VersionInfo   ::=    S 'version' Eq ("'" VersionNum "'" | '"' VersionNum '"')
[25]   Eq   ::=    S? '=' S?
[26]   VersionNum   ::=   '1.' [0-9]+
[27]   Misc   ::=    Comment | PI | S

Even though the VersionNum production matches any version number of the form '1.x', XML 1.0 documents SHOULD NOT specify a version number other than '1.0'.


When an XML 1.0 processor encounters a document that specifies a 1.x version number other than '1.0', it will process it as a 1.0 document. This means that an XML 1.0 processor will accept 1.x documents provided they do not use any non-1.0 features.

[Definition: The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.]

[Definition: A markup declaration is an element type declaration, an attribute-list declaration, an entity declaration, or a notation declaration.] These declarations may be contained in whole or in part within parameter entities, as described in the well-formedness and validity constraints below. For further information, see 4 Physical Structures.

Document Type Definition
[28]   doctypedecl   ::=   '<!DOCTYPE' S Name (S ExternalID)? S? ('[' intSubset ']' S?)? '>'[VC: Root Element Type]
[WFC: External Subset]
[28a]   DeclSep   ::=    PEReference | S [WFC: PE Between Declarations]
[28b]   intSubset   ::=   (markupdecl | DeclSep)*
[29]   markupdecl   ::=    elementdecl | AttlistDecl | EntityDecl | NotationDecl | PI | Comment [VC: Proper Declaration/PE Nesting]
[WFC: PEs in Internal Subset]

Note that it is possible to construct a well-formed document containing a doctypedecl that neither points to an external subset nor contains an internal subset.

The markup declarations may be made up in whole or in part of the replacement text of parameter entities. The productions later in this specification for individual nonterminals (elementdecl, AttlistDecl, and so on) describe the declarations after all the parameter entities have been included.

Parameter entity references are recognized anywhere in the DTD (internal and external subsets and external parameter entities), except in literals, processing instructions, comments, and the contents of ignored conditional sections (see 3.4 Conditional Sections). They are also recognized in entity value literals. The use of parameter entities in the internal subset is restricted as described below.

Validity constraint: Root Element Type

The Name in the document type declaration MUST match the element type of the root element.

Validity constraint: Proper Declaration/PE Nesting

Parameter-entity replacement text MUST be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration (markupdecl above) is contained in the replacement text for a parameter-entity reference, both MUST be contained in the same replacement text.

Well-formedness constraint: PEs in Internal Subset

In the internal DTD subset, parameter-entity references MUST NOT occur within markup declarations; they may occur where markup declarations can occur. (This does not apply to references that occur in external parameter entities or to the external subset.)

Like the internal subset, the external subset and any external parameter entities referenced in a DeclSep MUST consist of a series of complete markup declarations of the types allowed by the non-terminal symbol markupdecl, interspersed with white space or parameter-entity references. However, portions of the contents of the external subset or of these external parameter entities may conditionally be ignored by using the conditional section construct; this is not allowed in the internal subset but is allowed in external parameter entities referenced in the internal subset.

External Subset
[30]   extSubset   ::=    TextDecl? extSubsetDecl
[31]   extSubsetDecl   ::=   ( markupdecl | conditionalSect | DeclSep)*

The external subset and external parameter entities also differ from the internal subset in that in them, parameter-entity references are permitted within markup declarations, not only between markup declarations.

An example of an XML document with a document type declaration:

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

The system identifier " hello.dtd " gives the address (a URI reference) 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, the internal subset MUST be considered to occur before the external subset. This has the effect that entity and attribute-list declarations in the internal subset take precedence over those in the external subset.

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 may appear as a component of the XML declaration, signals whether or not there are such declarations which appear external to the document entity or in parameter entities. [Definition: An external markup declaration is defined as a markup declaration occurring in the external subset or in a parameter entity (external or internal, the latter being included because non-validating processors are not required to read them).]

Standalone Document Declaration
[32]   SDDecl   ::=    S 'standalone' Eq (("'" ('yes' | 'no') "'") | ('"' ('yes' | 'no') '"')) [VC: Standalone Document Declaration]

In a standalone document declaration, the value "yes" indicates that there are no external markup declarations which affect the information passed from the XML processor to the application. The value "no" indicates that there are or may be such external markup declarations. Note that the standalone document declaration only denotes the presence of external declarations; the presence, in a document, of references to external entities, when those entities are internally declared, does not change its standalone status.

If there are no external markup declarations, the standalone document declaration has no meaning. If there are external markup declarations but there is no standalone document declaration, the value "no" is assumed.

Any XML document for which standalone="no" holds can be converted algorithmically to a standalone document, which may be desirable for some network delivery applications.

Validity constraint: Standalone Document Declaration

The standalone document declaration MUST have the value "no" if any external markup declarations contain declarations of:

  • attributes with default values, if elements to which these attributes apply appear in the document without specifications of values for these attributes, or

  • entities (other than amp, lt, gt, apos, quot), if references to those entities appear in the document, or

  • attributes with tokenized types, where the attribute appears in the document with a value such that normalization will produce a different value from that which would be produced in the absence of the declaration, or

  • element types with element content, if white space occurs directly within any instance of those types.

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) to set apart the markup for greater readability. Such white space is typically not intended for inclusion in the delivered version of the document. On the other hand, "significant" white space that should be preserved in the delivered version is common, for example in poetry and source code.

An XML processor MUST always pass all characters in a document that are not markup through to the application. A validating XML processor MUST also inform the application which of these characters constitute white space appearing in element content.

A special attribute named xml:space may be attached to an element to signal an intention that in that element, white space should be preserved by applications. In valid documents, this attribute, like any other, MUST be declared if it is used. When declared, it MUST be given as an enumerated type whose values are one or both of "default" and "preserve". For example:

<!ATTLIST poem  xml:space (default|preserve) 'preserve'>

<!ATTLIST pre xml:space (preserve) #FIXED '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 overridden with another instance of the xml:space attribute. This specification does not give meaning to any value of xml:space other than "default" and "preserve". It is an error for other values to be specified; the XML processor MAY report the error or MAY recover by ignoring the attribute specification or by reporting the (erroneous) value to the application. Applications may ignore or reject erroneous values.

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 parsed entities are often stored in computer files which, for editing convenience, are organized into lines. These lines are typically separated by some combination of the characters CARRIAGE RETURN (#xD) and LINE FEED (#xA).

To simplify the tasks of applications, the XML processor MUST behave as if it normalized all line breaks in external parsed entities (including the document entity) on input, before parsing, by translating both the two-character sequence #xD #xA and any #xD that is not followed by #xA to a single #xA character.

2.12 Language Identification

In document processing, it is often useful to identify the natural or formal language in which the content is written. A special attribute named xml:lang may be inserted in documents to specify the language used in the contents and attribute values of any element in an XML document. In valid documents, this attribute, like any other, MUST be declared if it is used. The values of the attribute are language identifiers as defined by [IETF BCP 47], Tags for the Identification of Languages; in addition, the empty string may be specified.

(Productions 33 through 38 have been removed.)

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" desc='leise' xml:lang="de">
  <l>Habe nun, ach! Philosophie,</l>
  <l>Juristerei, und Medizin</l>
  <l>und leider auch Theologie</l>
  <l>durchaus studiert mit heißem Bemüh'n.</l>

The language specified by xml:lang applies to the element where it is specified (including the values of its attributes), and to all elements in its content unless overridden with another instance of xml:lang. In particular, the empty value of xml:lang is used on an element B to override a specification of xml:lang on an enclosing element A, without specifying another language. Within B, it is considered that there is no language information available, just as if xml:lang had not been specified on B or any of its ancestors. Applications determine which of an element's attribute values and which parts of its character content, if any, are treated as language-dependent values described by xml:lang.


Language information may also be provided by external transport protocols (e.g. HTTP or MIME). When available, this information may be used by XML applications, but the more local information provided by xml:lang should be considered to override it.

A simple declaration for xml:lang might take the form


but specific default values may also be given, if appropriate. In a collection of French poems for English students, with glosses and notes in English, the xml:lang attribute might be declared this way:

<!ATTLIST poem   xml:lang CDATA 'fr'>
<!ATTLIST gloss  xml:lang CDATA 'en'>
<!ATTLIST note   xml:lang CDATA 'en'>

3 Logical Structures

[Definition: Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. Each element has a type, identified by name, sometimes called its "generic identifier" (GI), and may have a set of attribute specifications.] Each attribute specification has a name and a value.

[39]   element   ::=    EmptyElemTag
| STag content ETag [WFC: Element Type Match]
[VC: Element Valid]

This specification does not constrain the application semantics, use, or (beyond syntax) names of the element types and attributes, except that names beginning with a match to (('X'|'x')('M'|'m')('L'|'l')) are reserved for standardization in this or future versions of this specification.

Well-formedness constraint: Element Type Match

The Name in an element's end-tag MUST match the element type in the start-tag.

Validity constraint: Element Valid

An element is valid if there is a declaration matching elementdecl where the Name matches the element type, and one of the following holds:

  1. The declaration matches EMPTY and the element has no content (not even entity references, comments, PIs or white space).

  2. The declaration matches children and the sequence of child elements belongs to the language generated by the regular expression in the content model, with optional white space, comments and PIs (i.e. markup matching production [27] Misc) between the start-tag and the first child element, between child elements, or between the last child element and the end-tag. Note that a CDATA section containing only white space or a reference to an entity whose replacement text is character references expanding to white space do not match the nonterminal S, and hence cannot appear in these positions; however, a reference to an internal entity with a literal value consisting of character references expanding to white space does match S, since its replacement text is the white space resulting from expansion of the character references.

  3. The declaration matches Mixed, and the content (after replacing any entity references with their replacement text) consists of character data (including CDATA sections), comments, PIs and child elements whose types match names in the content model.

  4. The declaration matches ANY, and the content (after replacing any entity references with their replacement text) consists of character data, CDATA sections, comments, PIs and child elements whose types have been declared.

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

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

[40]   STag   ::=   '<' Name (S Attribute)* S? '>'[WFC: Unique Att Spec]
[41]   Attribute   ::=    Name Eq AttValue [VC: Attribute Value Type]
[WFC: No External Entity References]
[WFC: No < in Attribute Values]

The Name in the start- and end-tags gives the element's type. [Definition: The Name-AttValue pairs are referred to as the attribute specifications of the element], [Definition: with the Name in each pair referred to as the attribute name ] and [Definition: the content of the AttValue (the text between the ' or " delimiters) as the attribute value.] Note that the order of attribute specifications in a start-tag or empty-element tag is not significant.

Well-formedness constraint: No < in Attribute Values

The replacement text of any entity referred to directly or indirectly in an attribute value MUST NOT contain a <.

An example of a start-tag:

<termdef id="dt-dog" term="dog">

[Definition: 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:]

[42]   ETag   ::=   '</' Name S? '>'

An example of an end-tag:

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

Content of Elements
[43]   content   ::=    CharData? ((element | Reference | CDSect | PI | Comment) CharData?)*

[Definition: An element with no content is said to be empty.] The representation of an empty element is either a start-tag immediately followed by an end-tag, or an empty-element tag. [Definition: An empty-element tag takes a special form:]

Tags for Empty Elements
[44]   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 it is declared using the keyword EMPTY. For interoperability, the empty-element tag SHOULD be used, and SHOULD only be used, for elements which are declared 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-list declarations. An element type declaration constrains the element's content.

Element type declarations often constrain which element types can appear as children of the element. At user option, an XML processor MAY issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.

[Definition: An element type declaration takes the form:]

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

where the Name gives the element type being declared.

Examples of element type declarations:

3.2.1 Element Content

[Definition: An element type has element content when elements of that type MUST contain only child elements (no character data), optionally separated by white space (characters matching the nonterminal S).] [Definition: In this case, the constraint includes a content model, a simple grammar governing the allowed types of the child elements and the order in which they are allowed to appear.] The grammar is built on content particles (cps), which consist of names, choice lists of content particles, or sequence lists of content particles:

Element-content Models
[47]   children   ::=   (choice | seq) ('?' | '*' | '+')?
[48]   cp   ::=   (Name | choice | seq) ('?' | '*' | '+')?
[49]   choice   ::=   '(' S? cp ( S? '|' S? cp )+ S? ')'[VC: Proper Group/PE Nesting]
[50]   seq   ::=   '(' S? cp ( S? ',' S? cp )* S? ')'[VC: Proper Group/PE Nesting]

where each Name is the type of an element which may appear as a child. Any content particle in a choice list may appear in the element content at the location where the choice list appears in the grammar; content particles occurring in a sequence list MUST each appear in the element content in the order given in the list. The optional character following a name or list governs whether the element or the content particles in the list may occur one or more (+), zero or more (*), or zero or one times (?). The absence of such an operator means that the element or content particle MUST appear exactly once. This syntax and meaning are identical to those used in the productions in this specification.

The content of an element matches a content model if and only if it is possible to trace out a path through the content model, obeying the sequence, choice, and repetition operators and matching each element in the content against an element type in the content model. For compatibility, it is an error if the content model allows an element to match more than one occurrence of an element type in the content model. For more information, see E Deterministic Content Models.

Validity constraint: Proper Group/PE Nesting

Parameter-entity replacement text MUST be properly nested with parenthesized 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 contain at least one non-blank character, and neither the first nor last non-blank character of the replacement text SHOULD be a connector (| or ,).

Examples of element-content models:

<!ELEMENT spec (front, body, back?)>
<!ELEMENT div1 (head, (p | list | note)*, div2*)>
<!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*>

3.2.2 Mixed Content

[Definition: An element type has mixed content when elements of that type may contain character data, optionally interspersed with child elements.] In this case, the types of the child elements may be constrained, but not their order or their number of occurrences:

Mixed-content Declaration
[51]   Mixed   ::=   '(' S? '#PCDATA' (S? '|' S? Name)* S? ')*'
| '(' S? '#PCDATA' S? ')' [VC: Proper Group/PE Nesting]
[VC: No Duplicate Types]

where the Names give the types of elements that may appear as children. The keyword #PCDATA derives historically from the term "parsed character data."

Examples of mixed content declarations:

3.3 Attribute-List Declarations

Attributes are used to associate name-value pairs with elements. Attribute specifications MUST NOT appear outside of start-tags and empty-element tags; thus, the productions used to recognize them appear in 3.1 Start-Tags, End-Tags, and Empty-Element Tags. Attribute-list declarations may be used:

  • To define the set of attributes pertaining to a given element type.

  • To establish type constraints for these attributes.

  • To provide default values for attributes.

[Definition: 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
[52]   AttlistDecl   ::=   '<!ATTLIST' S Name AttDef* S? '>'
[53]   AttDef   ::=    S Name S AttType S DefaultDecl

The Name in the AttlistDecl rule is the type of an element. At user option, an XML processor MAY issue a warning if attributes are declared for an element type not itself declared, but this is not an error. The Name in the AttDef rule is the name of the attribute.

When more than one AttlistDecl is provided for a given element type, the contents of all those provided are merged. When more than one definition is provided for the same attribute of a given element type, the first declaration is binding and later declarations are ignored. For interoperability, writers of DTDs may choose to provide at most one attribute-list declaration for a given element type, at most one attribute definition for a given attribute name in an attribute-list declaration, and at least one attribute definition in each attribute-list declaration. For interoperability, an XML processor MAY at user option issue a warning when more than one attribute-list declaration is provided for a given element type, or more than one attribute definition is provided for a given attribute, but this is not an error.

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 are more constrained. The validity constraints noted in the grammar are applied after the attribute value has been normalized as described in 3.3.3 Attribute-Value Normalization.

Attribute Types
[54]   AttType   ::=    StringType | TokenizedType | EnumeratedType
[55]   StringType   ::=   'CDATA'
[56]   TokenizedType   ::=   'ID'[VC: ID]
[VC: One ID per Element Type]
[VC: ID Attribute Default]
| 'ENTITY'[VC: Entity Name]
| 'ENTITIES'[VC: Entity Name]
| 'NMTOKEN'[VC: Name Token]
| 'NMTOKENS'[VC: Name Token]

Validity constraint: ID

Values of type ID MUST match the Name production. A name MUST NOT appear more than once in an XML document as a value of this type; i.e., ID values MUST uniquely identify the elements which bear them.

Validity constraint: IDREF

Values of type IDREF MUST match the Name production, and values of type IDREFS MUST match Names; each Name MUST match the value of an ID attribute on some element in the XML document; i.e. IDREF values MUST match the value of some ID attribute.

Validity constraint: Entity Name

Values of type ENTITY MUST match the Name production, values of type ENTITIES MUST match Names; each Name MUST match the name of an unparsed entity declared in the DTD.

[Definition: Enumerated attributes have a list of allowed values in their declaration ]. They MUST take one of those values. There are two kinds of enumerated attribute types:

Enumerated Attribute Types
[57]   EnumeratedType   ::=    NotationType | Enumeration
[58]   NotationType   ::=   'NOTATION' S '(' S? Name (S? '|' S? Name)* S? ')' [VC: Notation Attributes]
[VC: One Notation Per Element Type]
[VC: No Notation on Empty Element]
[VC: No Duplicate Tokens]
[59]   Enumeration   ::=   '(' S? Nmtoken (S? '|' S? Nmtoken)* S? ')'[VC: Enumeration]
[VC: No Duplicate Tokens]

A NOTATION attribute identifies a notation, declared in the DTD with associated system and/or public identifiers, to be used in interpreting the element to which the attribute is attached.

Validity constraint: Notation Attributes

Values of this type MUST match one of the notation names included in the declaration; all notation names in the declaration MUST be declared.

Validity constraint: No Notation on Empty Element

For compatibility, an attribute of type NOTATION MUST NOT be declared on an element declared EMPTY.

Validity constraint: No Duplicate Tokens

The notation names in a single NotationType attribute declaration, as well as the NmTokens in a single Enumeration attribute declaration, MUST all be distinct.

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 is to react if a declared attribute is absent in a document.

Attribute Defaults
[60]   DefaultDecl   ::=   '#REQUIRED' | '#IMPLIED'
| (('#FIXED' S)? AttValue)[VC: Required Attribute]
[VC: Attribute Default Value Syntactically Correct]
[WFC: No < in Attribute Values]
[VC: Fixed Attribute Default]
[WFC: No External Entity References]

In an attribute declaration, #REQUIRED means that the attribute MUST always be provided, #IMPLIED that no default value is provided. [Definition: If the declaration is neither #REQUIRED nor #IMPLIED, then the AttValue value contains the declared default value; the #FIXED keyword states that the attribute MUST always have the default value. When an XML processor encounters an element without a specification for an attribute for which it has read a default value declaration, it MUST report the attribute with the declared default value to the application.]

Validity constraint: Attribute Default Value Syntactically Correct

The declared default value MUST meet the syntactic constraints of the declared attribute type. That is, the default value of an attribute:

Note that only the syntactic constraints of the type are required here; other constraints (e.g. that the value be the name of a declared unparsed entity, for an attribute of type ENTITY) will be reported by a validating parser only if an element without a specification for this attribute actually occurs.

Examples of attribute-list declarations:

3.3.3 Attribute-Value Normalization

Before the value of an attribute is passed to the application or checked for validity, the XML processor MUST normalize the attribute value by applying the algorithm below, or by using some other method such that the value passed to the application is the same as that produced by the algorithm.

  1. All line breaks MUST have been normalized on input to #xA as described in 2.11 End-of-Line Handling, so the rest of this algorithm operates on text normalized in this way.

  2. Begin with a normalized value consisting of the empty string.

  3. For each character, entity reference, or character reference in the unnormalized attribute value, beginning with the first and continuing to the last, do the following:

    • For a character reference, append the referenced character to the normalized value.

    • For an entity reference, recursively apply step 3 of this algorithm to the replacement text of the entity.

    • For a white space character (#x20, #xD, #xA, #x9), append a space character (#x20) to the normalized value.

    • For another character, append the character to the normalized value.

If the attribute type is not CDATA, then the XML processor MUST further process the normalized attribute value by discarding any leading and trailing space (#x20) characters, and by replacing sequences of space (#x20) characters by a single space (#x20) character.

Note that if the unnormalized attribute value contains a character reference to a white space character other than space (#x20), the normalized value contains the referenced character itself (#xD, #xA or #x9). This contrasts with the case where the unnormalized value contains a white space character (not a reference), which is replaced with a space character (#x20) in the normalized value and also contrasts with the case where the unnormalized value contains an entity reference whose replacement text contains a white space character; being recursively processed, the white space character is replaced with a space character (#x20) in the normalized value.

All attributes for which no declaration has been read SHOULD be treated by a non-validating processor as if declared CDATA.

It is an error if an attribute value contains a reference to an entity for which no declaration has been read.

Following are examples of attribute normalization. Given the following declarations:

<!ENTITY d "&#xD;">
<!ENTITY a "&#xA;">
<!ENTITY da "&#xD;&#xA;">

the attribute specifications in the left column below would be normalized to the character sequences of the middle column if the attribute a is declared NMTOKENS and to those of the right columns if a is declared CDATA.

Attribute specificationa is NMTOKENSa is CDATA

x y z
#x20 #x20 x y z
A #x20 B
#x20 #x20 A #x20 #x20 #x20 B #x20 #x20
#xD #xD A #xA #xA B #xD #xA
#xD #xD A #xA #xA B #xD #xA

Note that the last example is invalid (but well-formed) if a is declared to be of type NMTOKENS.

3.4 Conditional Sections

[Definition: Conditional sections are portions of the document type declaration external subset or of external parameter entities which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.]

Conditional Section
[61]   conditionalSect   ::=    includeSect | ignoreSect
[62]   includeSect   ::=   '<![' S? 'INCLUDE' S? '[' extSubsetDecl ']]>' [VC: Proper Conditional Section/PE Nesting]
[63]   ignoreSect   ::=   '<![' S? 'IGNORE' S? '[' ignoreSectContents* ']]>'[VC: Proper Conditional Section/PE Nesting]
[64]   ignoreSectContents   ::=    Ignore ('<![' ignoreSectContents ']]>' Ignore)*
[65]   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 contents of the conditional section MUST be processed as part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section MUST NOT be processed as part of the DTD. 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 MUST be ignored. The contents of an ignored conditional section MUST be parsed by ignoring all characters after the "[" following the keyword, except conditional section starts "<![" and ends "]]>", until the matching conditional section end is found. Parameter entity references MUST NOT be recognized in this process.

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:

4 Physical Structures

[Definition: An XML document may consist of one or many storage units. These are called entities; they all have content and are all (except for the document entity and the external DTD subset) identified by entity name.] Each XML document has one entity called the document entity, which serves as the starting point for the XML processor and may contain the whole document.

Entities may be either parsed or unparsed. [Definition: The contents of a parsed entity are referred to as its replacement text; this text is considered an integral part of the document.]

[Definition: An unparsed entity is a resource whose contents may or may not be text, and if text, may be other than XML. Each unparsed entity has an associated notation, identified by name. Beyond a requirement that an XML processor make the identifiers for the entity and notation available to the application, XML places no constraints on the contents of unparsed entities.]

Parsed entities are invoked by name using entity references; unparsed entities by name, given in the value of ENTITY or ENTITIES attributes.

[Definition: General entities are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity.] [Definition: Parameter entities are parsed entities for use within the DTD.] These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.

4.1 Character and Entity References

[Definition: A character reference refers to a specific character in the ISO/IEC 10646 character set, for example one not directly accessible from available input devices.]

Character Reference
[66]   CharRef   ::=   '&#' [0-9]+ ';'
| '&#x' [0-9a-fA-F]+ ';'[WFC: Legal Character]

If the character reference begins with " &#x ", the digits and letters up to the terminating ; provide a hexadecimal representation of the character's code point in ISO/IEC 10646. If it begins just with " &# ", the digits up to the terminating ; provide a decimal representation of the character's code point.

[Definition: An entity reference refers to the content of a named entity.] [Definition: References to parsed general entities use ampersand (&) and semicolon (;) as delimiters.] [Definition: Parameter-entity references use percent-sign (%) and semicolon (;) as delimiters.]

Entity Reference
[67]   Reference   ::=    EntityRef | CharRef
[68]   EntityRef   ::=   '&' Name ';'[WFC: Entity Declared]
[VC: Entity Declared]
[WFC: Parsed Entity]
[WFC: No Recursion]
[69]   PEReference   ::=   '%' Name ';'[VC: Entity Declared]
[WFC: No Recursion]

Well-formedness constraint: Entity Declared

In a document without any DTD, a document with only an internal DTD subset which contains no parameter entity references, or a document with " standalone='yes' ", for an entity reference that does not occur within the external subset or a parameter entity, the Name given in the entity reference MUST match that in an entity declaration that does not occur within the external subset or a parameter entity, except that well-formed documents need not declare any of the following entities: amp, lt, gt, apos, quot. The declaration of a general entity MUST precede any reference to it which appears in a default value in an attribute-list declaration.

Note that non-validating processors are not obligated to read and process entity declarations occurring in parameter entities or in the external subset; for such documents, the rule that an entity must be declared is a well-formedness constraint only if standalone='yes'.

Validity constraint: Entity Declared

In a document with an external subset or parameter entity references, if the document is not standalone (either "standalone='no'" is specified or there is no standalone declaration), then the Name given in the entity reference MUST match that in an entity declaration. For interoperability, valid documents SHOULD declare the entities amp, lt, gt, apos, quot, in the form specified in 4.6 Predefined Entities. The declaration of a parameter entity MUST precede any reference to it. Similarly, the declaration of a general entity MUST precede any attribute-list declaration containing a default value with a direct or indirect reference to that general entity.

Well-formedness constraint: Parsed Entity

An entity reference MUST NOT contain the name of an unparsed entity. Unparsed entities may be referred to only in attribute values declared to be of type ENTITY or ENTITIES.

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:

<!-- declare the parameter entity "ISOLat2"... -->
         SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >
<!-- ... now reference it. -->

4.2 Entity Declarations

[Definition: Entities are declared thus:]

Entity Declaration
[70]   EntityDecl   ::=    GEDecl | PEDecl
[71]   GEDecl   ::=   '<!ENTITY' S Name S EntityDef S? '>'
[72]   PEDecl   ::=   '<!ENTITY' S '%' S Name S PEDef S? '>'
[73]   EntityDef   ::=    EntityValue | (ExternalID NDataDecl?)
[74]   PEDef   ::=    EntityValue | ExternalID

The Name identifies the entity in an entity reference or, in the case of an unparsed entity, in the value of an ENTITY or ENTITIES attribute. If the same entity is declared more than once, the first declaration encountered is binding; at user option, an XML processor MAY issue a warning if entities are declared multiple times.

4.2.1 Internal Entities

[Definition: If the entity definition is an EntityValue, the defined entity is called an internal entity. There is no separate physical storage object, and the content of the entity is given in the declaration.] Note that some processing of entity and character references in the literal entity value may be required to produce the correct replacement text: see 4.5 Construction of Entity Replacement Text.

An internal entity is a parsed entity.

Example of an internal entity declaration:

<!ENTITY Pub-Status "This is a pre-release of the

4.2.2 External Entities

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

External Entity Declaration
[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 general unparsed entity; otherwise it is a parsed entity.

Validity constraint: Notation Declared

The Name MUST match the declared name of a notation.

[Definition: The SystemLiteral is called the entity's system identifier. It is meant to be converted to a URI reference (as defined in [IETF RFC 3986]), as part of the process of dereferencing it to obtain input for the XML processor to construct the entity's replacement text.] It is an error for a fragment identifier (beginning with a # character) to be part of a system identifier. Unless otherwise provided by information outside the scope of this specification (e.g. a special XML element type defined by a particular DTD, or a processing instruction defined by a particular application specification), relative URIs are relative to the location of the resource within which the entity declaration occurs. This is defined to be the external entity containing the '<' which starts the declaration, at the point when it is parsed as a declaration. A URI might thus be relative to the document entity, to the entity containing the external DTD subset, or to some other external parameter entity. Attempts to retrieve the resource identified by a URI may be redirected at the parser level (for example, in an entity resolver) or below (at the protocol level, for example, via an HTTP Location: header). In the absence of additional information outside the scope of this specification within the resource, the base URI of a resource is always the URI of the actual resource returned. In other words, it is the URI of the resource retrieved after all redirection has occurred.

System identifiers (and other XML strings meant to be used as URI references) may contain characters that, according to [IETF RFC 3986], must be escaped before a URI can be used to retrieve the referenced resource. The characters to be escaped are the control characters #x0 to #x1F and #x7F (most of which cannot appear in XML), space #x20, the delimiters '<' #x3C, '>' #x3E and '"' #x22, the unwise characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and '`' #x60, as well as all characters above #x7F. Since escaping is not always a fully reversible process, it MUST be performed only when absolutely necessary and as late as possible in a processing chain. In particular, neither the process of converting a relative URI to an absolute one nor the process of passing a URI reference to a process or software component responsible for dereferencing it SHOULD trigger escaping. When escaping does occur, it MUST be performed as follows:

  1. Each character to be escaped is represented in UTF-8 [Unicode] as one or more bytes.

  2. The resulting bytes are escaped with the URI escaping mechanism (that is, converted to % HH, where HH is the hexadecimal notation of the byte value).

  3. The original character is replaced by the resulting character sequence.


In a future edition of this specification, the XML Core Working Group intends to replace the preceding paragraph and list of steps with a normative reference to an upcoming revision of IETF RFC 3987, which will define "Legacy Extended IRIs (LEIRIs)". When this revision is available, it is the intent of the XML Core WG to use it to replace language similar to the above in any future revisions of XML-related specifications under its purview.

[Definition: In addition to a system identifier, an external identifier may include a public identifier.] An XML processor attempting to retrieve the entity's content may use any combination of the public and system identifiers as well as additional information outside the scope of this specification to try to generate an alternative URI reference. If the processor is unable to do so, it MUST use the URI reference specified in the system literal. Before a match is attempted, all strings of white space in the public identifier MUST be normalized to single space characters (#x20), and leading and trailing white space MUST be removed.

Examples of external entity declarations:

<!ENTITY open-hatch
         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 >

4.3 Parsed Entities

4.3.2 Well-Formed Parsed Entities

The document entity is well-formed if it matches the production labeled document. An external general parsed entity is well-formed if it matches the production labeled extParsedEnt. All external parameter entities are well-formed by definition.


Only parsed entities that are referenced directly or indirectly within the document are required to be well-formed.

Well-Formed External Parsed Entity
[78]   extParsedEnt   ::=    TextDecl? content

An internal general parsed entity is well-formed if its replacement text matches the production labeled content. All internal parameter entities are well-formed by definition.

A consequence of well-formedness in general entities is that the logical and physical structures in an XML document are properly nested; no start-tag, end-tag, empty-element tag, element, comment, processing instruction, character reference, or entity reference can begin in one entity and end in another.

4.3.3 Character Encoding in Entities

Each external parsed entity in an XML document may use a different encoding for its characters. All XML processors MUST be able to read entities in both the UTF-8 and UTF-16 encodings. The terms "UTF-8" and "UTF-16" in this specification do not apply to related character encodings, including but not limited to UTF-16BE, UTF-16LE, or CESU-8.

Entities encoded in UTF-16 MUST and entities encoded in UTF-8 MAY begin with the Byte Order Mark described by Annex H of [ISO/IEC 10646:2000], section 16.8 of [Unicode] (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors MUST be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.

If the replacement text of an external entity is to begin with the character U+FEFF, and no text declaration is present, then a Byte Order Mark MUST be present, whether the entity is encoded in UTF-8 or UTF-16.

Although an XML processor is required to read only entities in the UTF-8 and UTF-16 encodings, it is recognized that other encodings are used around the world, and it may be desired for XML processors to read entities that use them. In the absence of external character encoding information (such as MIME headers), parsed entities which are stored in an encoding other than UTF-8 or UTF-16 MUST begin with a text declaration (see 4.3.1 The Text Declaration) containing an encoding declaration:

Encoding Declaration
[80]   EncodingDecl   ::=    S 'encoding' Eq ('"' EncName '"' | "'" EncName "'" )
[81]   EncName   ::=   [A-Za-z] ([A-Za-z0-9._] | '-')*/* Encoding name contains only Latin characters */

In the document entity, the encoding declaration is part of the XML declaration. The EncName is the name of the encoding used.

In an encoding declaration, the values " UTF-8 ", " UTF-16 ", " ISO-10646-UCS-2 ", and " ISO-10646-UCS-4 " SHOULD be used for the various encodings and transformations of Unicode / ISO/IEC 10646, the values " ISO-8859-1 ", " ISO-8859-2 ", ... " ISO-8859- n " (where n is the part number) SHOULD be used for the parts of ISO 8859, and the values " ISO-2022-JP ", " Shift_JIS ", and " EUC-JP " SHOULD be used for the various encoded forms of JIS X-0208-1997. It is RECOMMENDED that character encodings registered (as charsets) with the Internet Assigned Numbers Authority [IANA-CHARSETS], other than those just listed, be referred to using their registered names; other encodings SHOULD use names starting with an "x-" prefix. XML processors SHOULD match character encoding names in a case-insensitive way and SHOULD either interpret an IANA-registered name as the encoding registered at IANA for that name or treat it as unknown (processors are, of course, not required to support all IANA-registered encodings).

In the absence of information provided by an external transport protocol (e.g. HTTP or MIME), it is a fatal 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, or for an entity which begins with neither a Byte Order Mark nor an encoding declaration to use an encoding other than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary ASCII entities do not strictly need an encoding declaration.

It is a fatal error for a TextDecl to occur other than at the beginning of an external entity.

It is a fatal error when an XML processor encounters an entity with an encoding that it is unable to process. It is a fatal error if an XML entity is determined (via default, encoding declaration, or higher-level protocol) to be in a certain encoding but contains byte sequences that are not legal in that encoding. Specifically, it is a fatal error if an entity encoded in UTF-8 contains any ill-formed code unit sequences, as defined in section 3.9 of Unicode [Unicode]. Unless an encoding is determined by a higher-level protocol, it is also a fatal error if an XML entity contains no encoding declaration and its content is not legal UTF-8 or UTF-16.

Examples of text declarations containing encoding declarations:

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

4.4 XML Processor Treatment of Entities and References

The table below summarizes the contexts in which character references, entity references, and invocations of unparsed entities might appear and the REQUIRED behavior of an XML processor in each case. The labels in the leftmost column describe the recognition context:

Reference in Content

as a reference anywhere after the start-tag and before the end-tag of an element; corresponds to the nonterminal content.

Reference in Attribute Value

as a reference within either the value of an attribute in a start-tag, or a default value in an attribute declaration; corresponds to the nonterminal AttValue.

Occurs as Attribute Value

as a Name, not a reference, appearing either as the value of an attribute which has been declared as type ENTITY, or as one of the space-separated tokens in the value of an attribute which has been declared as type ENTITIES.

Reference in Entity Value

as a reference within a parameter or internal entity's literal entity value in the entity's declaration; corresponds to the nonterminal EntityValue.

Reference in DTD

as a reference within either the internal or external subsets of the DTD, but outside of an EntityValue, AttValue, PI, Comment, SystemLiteral, PubidLiteral, or the contents of an ignored conditional section (see 3.4 Conditional Sections).


Entity TypeCharacter
ParameterInternal GeneralExternal Parsed GeneralUnparsed
Reference in Content Not recognized Included Included if validating Forbidden Included
Reference in Attribute Value Not recognized Included in literal Forbidden Forbidden Included
Occurs as Attribute Value Not recognized Forbidden Forbidden Notify Not recognized
Reference in EntityValue Included in literal Bypassed Bypassed Error Included
Reference in DTD Included as PE Forbidden Forbidden Forbidden Forbidden

4.4.1 Not Recognized

Outside the DTD, the % character has no special significance; thus, what would be parameter entity references in the DTD are not recognized as markup in content. Similarly, the names of unparsed entities are not recognized except when they appear in the value of an appropriately declared attribute.

4.4.2 Included

[Definition: An entity is included when its replacement text is retrieved and processed, in place of the reference itself, as though it were part of the document at the location the reference was recognized.] The replacement text may contain both character data and (except for parameter entities) markup, which MUST be recognized in the usual way. (The string " AT&amp;T; " expands to " AT&T; " and the remaining ampersand is not recognized as an entity-reference delimiter.) A character reference is included when the indicated character is processed in place of the reference itself.

4.4.3 Included If Validating

When an XML processor recognizes a reference to a parsed entity, in order to validate the document, the processor MUST include its replacement text. If the entity is external, and the processor is not attempting to validate the XML document, the processor MAY, but need not, include the entity's replacement text. If a non-validating processor does not include the replacement text, it MUST inform the application that it recognized, but did not read, the entity.

This rule is based on the recognition that the automatic inclusion provided by the SGML and XML entity mechanism, primarily designed to support modularity in authoring, is not necessarily appropriate for other applications, in particular document browsing. Browsers, for example, when encountering an external parsed entity reference, might choose to provide a visual indication of the entity's presence and retrieve it for display only on demand.

4.4.4 Forbidden

The following are forbidden, and constitute fatal errors:

  • the appearance of a reference to an unparsed entity, except in the EntityValue in an entity declaration.

  • the appearance of any character or general-entity reference in the DTD except within an EntityValue or AttValue.

  • a reference to an external entity in an attribute value.

4.4.5 Included in Literal

When an entity reference appears in an attribute value, or a parameter entity reference appears in a literal entity value, its replacement text MUST be processed in place of the reference itself as though it were part of the document at the location the reference was recognized, except that a single or double quote character in the replacement text MUST always be treated as a normal data character and MUST NOT terminate the literal. For example, this is well-formed:

<!ENTITY % YN '"Yes"' >
<!ENTITY WhatHeSaid "He said %YN;" >

while this is not:

<!ENTITY EndAttr "27'" >
<element attribute='a-&EndAttr;>

4.4.6 Notify

When the name of an unparsed entity appears as a token in the value of an attribute of declared type ENTITY or ENTITIES, a validating processor MUST inform the application of the system and public (if any) identifiers for both the entity and its associated notation.

4.4.7 Bypassed

When a general entity reference appears in the EntityValue in an entity declaration, it MUST be bypassed and left as is.

4.4.8 Included as PE

Just as with external parsed entities, parameter entities need only be included if validating. When a parameter-entity reference is recognized in the DTD and included, its replacement text MUST be 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. This behavior MUST NOT apply to parameter entity references within entity values; these are described in 4.4.5 Included in Literal.

4.4.9 Error

It is an error for a reference to an unparsed entity to appear in the EntityValue in an entity declaration.

4.5 Construction of Entity Replacement Text

In discussing the treatment of entities, it is useful to distinguish two forms of the entity's value. [Definition: For an internal entity, the literal entity value is the quoted string actually present in the entity declaration, corresponding to the non-terminal EntityValue.] [Definition: For an external entity, the literal entity value is the exact text contained in the entity.] [Definition: For an internal entity, the replacement text is the content of the entity, after replacement of character references and parameter-entity references.] [Definition: For an external entity, the replacement text is the content of the entity, after stripping the text declaration (leaving any surrounding whitespace) if there is one but without any replacement of character references or parameter-entity references.]

The literal entity value as given in an internal entity declaration (EntityValue) may contain character, parameter-entity, and general-entity references. Such references MUST be contained entirely within the literal entity value. The actual replacement text that is included (or included in literal) as described above MUST contain the replacement text of any parameter entities referred to, and MUST contain the character referred to, in place of any character references in the literal entity value; however, general-entity references MUST be left as-is, unexpanded. For example, given the following declarations:

<!ENTITY % pub    "&#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 D Expansion of Entity and Character References.

4.6 Predefined Entities

[Definition: Entity and character references may 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. For interoperability, valid XML documents SHOULD declare these entities, like any others, before using them. If the entities lt or amp are declared, they MUST be declared as internal entities whose replacement text is a character reference to the respective character (less-than sign or ampersand) being escaped; the double escaping is REQUIRED for these entities so that references to them produce a well-formed result. If the entities gt, apos, or quot are declared, they MUST be declared as internal entities whose replacement text is the single character being escaped (or a character reference to that character; the double escaping here is OPTIONAL but harmless). For example:

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

4.7 Notation Declarations

[Definition: Notations identify by name the format of unparsed entities, the format of elements which bear a notation attribute, or the application to which a processing instruction is addressed.]

[Definition: 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
[82]   NotationDecl   ::=   '<!NOTATION' S Name S (ExternalID | PublicID) S? '>'[VC: Unique Notation Name]
[83]   PublicID   ::=   'PUBLIC' S PubidLiteral

Validity constraint: Unique Notation Name

A given Name MUST NOT be declared in more than one notation declaration.

XML processors MUST provide applications with the name and external identifier(s) of any notation declared and referred to in an attribute value, attribute definition, or entity declaration. They MAY additionally resolve the external identifier into the system identifier, file name, or other information needed to allow the application to call a processor for data in the notation described. (It is not an error, however, for XML documents to declare and refer to notations for which notation-specific applications are not available on the system where the XML processor or application is running.)

4.8 Document Entity

[Definition: The document entity serves as the root of the entity tree and a starting-point for an XML processor.] This specification does not specify how the document entity is to be located by an XML processor; unlike other entities, the document entity has no name and might well appear on a processor input stream without any identification at all.

5 Conformance

5.1 Validating and Non-Validating Processors

Conforming XML processors fall into two classes: validating and non-validating.

Validating and non-validating processors alike MUST report violations of this specification's well-formedness constraints in the content of the document entity and any other parsed entities that they read.

[Definition: Validating processors MUST, at user option, report violations of the constraints expressed by the declarations in the DTD, and failures to fulfill the validity constraints given in this specification.] To accomplish this, validating XML processors MUST read and process the entire DTD and all external parsed entities referenced in the document.

Non-validating processors are REQUIRED to check only the document entity, including the entire internal DTD subset, for well-formedness. [Definition: While they are not required to check the document for validity, they are REQUIRED to process all the declarations they read in the internal DTD subset and in any parameter entity that they read, up to the first reference to a parameter entity that they do not read; that is to say, they MUST use the information in those declarations to normalize attribute values, include the replacement text of internal entities, and supply default attribute values.] Except when standalone="yes", they MUST NOT process entity declarations or attribute-list declarations encountered after a reference to a parameter entity that is not read, since the entity may have contained overriding declarations; when standalone="yes", processors MUST process these declarations.

Note that when processing invalid documents with a non-validating processor the application may not be presented with consistent information. For example, several requirements for uniqueness within the document may not be met, including more than one element with the same id, duplicate declarations of elements or notations with the same name, etc. In these cases the behavior of the parser with respect to reporting such information to the application is undefined.

5.2 Using XML Processors

The behavior of a validating XML processor is highly predictable; it must read every piece of a document and report all well-formedness and validity violations. Less is required of a non-validating processor; it need not read any part of the document other than the document entity. This has two effects that may be important to users of XML processors:

For maximum reliability in interoperating between different XML processors, applications which use non-validating processors SHOULD NOT rely on any behaviors not required of such processors. Applications which require DTD facilities not related to validation (such as the declaration of default attributes and internal entities that are or may be specified in external entities) SHOULD use validating XML processors.

6 Notation

The formal grammar of XML is given in this specification using a simple Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines one symbol, in the form

Symbols are written with an initial capital letter if they are the start symbol of a regular language, otherwise with an initial lowercase letter. Literal strings are quoted.

Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters:


where N is a hexadecimal integer, the expression matches the character whose number (code point) in ISO/IEC 10646 is N. The number of leading zeros in the #xN form is insignificant.

[a-zA-Z], [#xN-#xN]

matches any Char with a value in the range(s) indicated (inclusive).

[abc], [#xN#xN#xN]

matches any Char with a value among the characters enumerated. Enumerations and ranges can be mixed in one set of brackets.

[^a-z], [^#xN-#xN]

matches any Char with a value outside the range indicated.

[^abc], [^#xN#xN#xN]

matches any Char with a value not among the characters given. Enumerations and ranges of forbidden values can be mixed in one set of brackets.


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 simple expressions:


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. This operator has higher precedence than alternation; thus A B | C D is identical to (A B) | (C D).

A | B

matches A or B.

A - B

matches any string that matches A but does not match B.


matches one or more occurrences of A. Concatenation has higher precedence than alternation; thus A+ | B+ is identical to (A+) | (B+).


matches zero or more occurrences of A. Concatenation has higher precedence than alternation; thus A* | B* is identical to (A*) | (B*).

Other notations used in the productions are:

/* ... */


[ wfc: ... ]

well-formedness constraint; this identifies by name a constraint on well-formed documents associated with a production.

[ vc: ... ]

validity constraint; this identifies by name a constraint on valid documents associated with a production.

A References

A.1 Normative References

(Internet Assigned Numbers Authority) Official Names for Character Sets, ed. Keld Simonsen et al. (See http://www.iana.org/assignments/character-sets.)
IETF (Internet Engineering Task Force). RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. Scott Bradner, 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)
IETF (Internet Engineering Task Force). BCP 47, consisting of RFC 4646: Tags for Identifying Languages, and RFC 4647: Matching of Language Tags, A. Phillips, M. Davis. 2006.
IETF (Internet Engineering Task Force). RFC 3986: Uniform Resource Identifier (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. 2005. (See http://www.ietf.org/rfc/rfc3986.txt.)
ISO/IEC 10646
ISO (International Organization for Standardization). ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane and ISO/IEC 10646-2:2001. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 2: Supplementary Planes, as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. [Geneva]: International Organization for Standardization. (See http://www.iso.org/iso/home.htm for the latest version.)
ISO/IEC 10646:2000
ISO (International Organization for Standardization). ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane. [Geneva]: International Organization for Standardization, 2000.
The Unicode Consortium. The Unicode Standard, Version 5.0.0, defined by: The Unicode Standard, Version 5.0 (Boston, MA, Addison-Wesley, 2007. ISBN 0-321-48091-0).
The Unicode Consortium. Unicode normalization forms. Mark Davis and Martin Durst. 2008. (See http://unicode.org/reports/tr15/.)

A.2 Other 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. Formal Models in Document Processing. Habilitationsschrift. Faculty of Mathematics at the University of Freiburg, 1993. (See ftp://ftp.informatik.uni-freiburg.de/documents/papers/brueggem/habil.ps.)
Brüggemann-Klein and Wood
Brüggemann-Klein, Anne, and Derick Wood. Deterministic Regular Languages. Universität Freiburg, Institut für Informatik, Bericht 38, Oktober 1991. Extended abstract in A. Finkel, M. Jantzen, Hrsg., STACS 1992, S. 173-184. Springer-Verlag, Berlin 1992. Lecture Notes in Computer Science 577. Full version titled One-Unambiguous Regular Languages in Information and Computation 140 (2): 229-253, February 1998.
James Clark. Comparison of SGML and XML. (See http://www.w3.org/TR/NOTE-sgml-xml-971215.)
(Internet Assigned Numbers Authority) Registry of Language Tags (See http://www.iana.org/assignments/language-subtag-registry.)
IETF (Internet Engineering Task Force). RFC 2141: URN Syntax, ed. R. Moats. 1997. (See http://www.ietf.org/rfc/rfc2141.txt.)
IETF (Internet Engineering Task Force). RFC 3023: XML Media Types. eds. M. Murata, S. St.Laurent, D. Kohn. 2001. (See http://www.ietf.org/rfc/rfc3023.txt.)
IETF (Internet Engineering Task Force). RFC 2781: UTF-16, an encoding of ISO 10646, ed. P. Hoffman, F. Yergeau. 2000. (See http://www.ietf.org/rfc/rfc2781.txt.)
ISO 639
(International Organization for Standardization). ISO 639:1988 (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 8879
ISO (International Organization for Standardization). ISO 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 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.
ISO (International Organization for Standardization). ISO 8879:1986 TC2. Information technology — Document Description and Processing Languages. [Geneva]: International Organization for Standardization, 1998. (See http://www.sgmlsource.com/8879/n0029.htm.)
XML Names
Tim Bray, Dave Hollander, and Andrew Layman, editors. Namespaces in XML. Textuality, Hewlett-Packard, and Microsoft. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/xml-names/.)

B Character Classes

Because of changes to productions [4] and [5], the productions in this Appendix are now orphaned and not used anymore in determining name characters. This Appendix may be removed in a future edition of this specification; other specifications that wish to refer to the productions herein should do so by means of a reference to the relevant production(s) in the Fourth Edition of this specification.

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), ideographic characters, and combining characters (among others, this class contains most diacritics). Digits and extenders are also distinguished.

[84]   Letter   ::=    BaseChar | Ideographic
[85]   BaseChar   ::=   [#x0041-#x005A] | [#x0061-#x007A] | [#x00C0-#x00D6] | [#x00D8-#x00F6] | [#x00F8-#x00FF] | [#x0100-#x0131] | [#x0134-#x013E] | [#x0141-#x0148] | [#x014A-#x017E] | [#x0180-#x01C3] | [#x01CD-#x01F0] | [#x01F4-#x01F5] | [#x01FA-#x0217] | [#x0250-#x02A8] | [#x02BB-#x02C1] | #x0386 | [#x0388-#x038A] | #x038C | [#x038E-#x03A1] | [#x03A3-#x03CE] | [#x03D0-#x03D6] | #x03DA | #x03DC | #x03DE | #x03E0 | [#x03E2-#x03F3] | [#x0401-#x040C] | [#x040E-#x044F] | [#x0451-#x045C] | [#x045E-#x0481] | [#x0490-#x04C4] | [#x04C7-#x04C8] | [#x04CB-#x04CC] | [#x04D0-#x04EB] | [#x04EE-#x04F5] | [#x04F8-#x04F9] | [#x0531-#x0556] | #x0559 | [#x0561-#x0586] | [#x05D0-#x05EA] | [#x05F0-#x05F2] | [#x0621-#x063A] | [#x0641-#x064A] | [#x0671-#x06B7] | [#x06BA-#x06BE] | [#x06C0-#x06CE] | [#x06D0-#x06D3] | #x06D5 | [#x06E5-#x06E6] | [#x0905-#x0939] | #x093D | [#x0958-#x0961] | [#x0985-#x098C] | [#x098F-#x0990] | [#x0993-#x09A8] | [#x09AA-#x09B0] | #x09B2 | [#x09B6-#x09B9] | [#x09DC-#x09DD] | [#x09DF-#x09E1] | [#x09F0-#x09F1] | [#x0A05-#x0A0A] | [#x0A0F-#x0A10] | [#x0A13-#x0A28] | [#x0A2A-#x0A30] | [#x0A32-#x0A33] | [#x0A35-#x0A36] | [#x0A38-#x0A39] | [#x0A59-#x0A5C] | #x0A5E | [#x0A72-#x0A74] | [#x0A85-#x0A8B] | #x0A8D | [#x0A8F-#x0A91] | [#x0A93-#x0AA8] | [#x0AAA-#x0AB0] | [#x0AB2-#x0AB3] | [#x0AB5-#x0AB9] | #x0ABD | #x0AE0 | [#x0B05-#x0B0C] | [#x0B0F-#x0B10] | [#x0B13-#x0B28] | [#x0B2A-#x0B30] | [#x0B32-#x0B33] | [#x0B36-#x0B39] | #x0B3D | [#x0B5C-#x0B5D] | [#x0B5F-#x0B61] | [#x0B85-#x0B8A] | [#x0B8E-#x0B90] | [#x0B92-#x0B95] | [#x0B99-#x0B9A] | #x0B9C | [#x0B9E-#x0B9F] | [#x0BA3-#x0BA4] | [#x0BA8-#x0BAA] | [#x0BAE-#x0BB5] | [#x0BB7-#x0BB9] | [#x0C05-#x0C0C] | [#x0C0E-#x0C10] | [#x0C12-#x0C28] | [#x0C2A-#x0C33] | [#x0C35-#x0C39] | [#x0C60-#x0C61] | [#x0C85-#x0C8C] | [#x0C8E-#x0C90] | [#x0C92-#x0CA8] | [#x0CAA-#x0CB3] | [#x0CB5-#x0CB9] | #x0CDE | [#x0CE0-#x0CE1] | [#x0D05-#x0D0C] | [#x0D0E-#x0D10] | [#x0D12-#x0D28] | [#x0D2A-#x0D39] | [#x0D60-#x0D61] | [#x0E01-#x0E2E] | #x0E30 | [#x0E32-#x0E33] | [#x0E40-#x0E45] | [#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] | [#x0F40-#x0F47] | [#x0F49-#x0F69] | [#x10A0-#x10C5] | [#x10D0-#x10F6] | #x1100 | [#x1102-#x1103] | [#x1105-#x1107] | #x1109 | [#x110B-#x110C] | [#x110E-#x1112] | #x113C | #x113E | #x1140 | #x114C | #x114E | #x1150 | [#x1154-#x1155] | #x1159 | [#x115F-#x1161] | #x1163 | #x1165 | #x1167 | #x1169 | [#x116D-#x116E] | [#x1172-#x1173] | #x1175 | #x119E | #x11A8 | #x11AB | [#x11AE-#x11AF] | [#x11B7-#x11B8] | #x11BA | [#x11BC-#x11C2] | #x11EB | #x11F0 | #x11F9 | [#x1E00-#x1E9B] | [#x1EA0-#x1EF9] | [#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] | #x2126 | [#x212A-#x212B] | #x212E | [#x2180-#x2182] | [#x3041-#x3094] | [#x30A1-#x30FA] | [#x3105-#x312C] | [#xAC00-#xD7A3]
[86]   Ideographic   ::=   [#x4E00-#x9FA5] | #x3007 | [#x3021-#x3029]
[87]   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
[88]   Digit   ::=   [#x0030-#x0039] | [#x0660-#x0669] | [#x06F0-#x06F9] | [#x0966-#x096F] | [#x09E6-#x09EF] | [#x0A66-#x0A6F] | [#x0AE6-#x0AEF] | [#x0B66-#x0B6F] | [#x0BE7-#x0BEF] | [#x0C66-#x0C6F] | [#x0CE6-#x0CEF] | [#x0D66-#x0D6F] | [#x0E50-#x0E59] | [#x0ED0-#x0ED9] | [#x0F20-#x0F29]
[89]   Extender   ::=   #x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46 | #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309D-#x309E] | [#x30FC-#x30FE]

The character classes defined here can be derived from the Unicode 2.0 character database as follows:

C XML and SGML (Non-Normative)

XML is designed to be a subset of SGML, in that every XML document should also be a conforming SGML document. For a detailed comparison of the additional restrictions that XML places on documents beyond those of SGML, see [Clark].

D Expansion of Entity and Character References (Non-Normative)

This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion, as specified in 4.4 XML Processor Treatment of Entities and References.

If the DTD contains the declaration

<!ENTITY example "<p>An ampersand (&#38;#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:

In the following example

<!DOCTYPE foo [ 
<!ENTITY x "&lt;"> 
<foo attr="&x;"/>

the replacement text of x is the four characters "&lt;" because references to general entities in entity values are bypassed. The replacement text of lt is a character reference to the less-than character, for example the five characters "&#60;" (see 4.6 Predefined Entities). Since neither of these contains a less-than character the result is well-formed.

If the definition of x had been

<!ENTITY x "&#60;">

then the document would not have been well-formed, because the replacement text of x would be the single character "<" which is not permitted in attribute values (see WFC: No < in Attribute Values).

E Deterministic Content Models (Non-Normative)

As noted in 3.2.1 Element Content, it is required that content models in element type declarations be deterministic. This requirement is for compatibility with SGML (which calls deterministic content models "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 XML processor 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 processor doesn't need to look ahead to see what follows; either c or d would be accepted.

More formally: a finite state automaton may be constructed from the content model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi, and Ullman [Aho/Ullman]. In many such algorithms, a follow set is constructed for each position in the regular expression (i.e., each leaf node in the syntax tree for the regular expression); if any position has a follow set in which more than one following position is labeled with the same element type name, then the content model is in error and may be reported as an error.

Algorithms exist which allow many but not all non-deterministic content models to be reduced automatically to equivalent deterministic models; see Brüggemann-Klein 1991 [Brüggemann-Klein].

F Autodetection of Character Encodings (Non-Normative)

The XML encoding declaration functions as an internal label on each entity, indicating which character encoding is in use. Before an XML processor can read the internal label, however, it apparently has to know what character encoding is in use—which is what the internal label is trying to indicate. In the general case, this is a hopeless situation. It is not entirely hopeless in XML, however, because XML limits the general case in two ways: each implementation is assumed to support only a finite set of character encodings, and the XML encoding declaration is restricted in position and content in order to make it feasible to autodetect the character encoding in use in each entity in normal cases. Also, in many cases other sources of information are available in addition to the XML data stream itself. Two cases may be distinguished, depending on whether the XML entity is presented to the processor without, or with, any accompanying (external) information. We will consider these cases in turn.

F.1 Detection Without External Encoding Information

Because each XML entity not accompanied by external encoding information and not in UTF-8 or UTF-16 encoding 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 ". The notation ## is used to denote any byte value except that two consecutive ##s cannot be both 00.

With a Byte Order Mark:

00 00 FE FF UCS-4, big-endian machine (1234 order)
FF FE 00 00 UCS-4, little-endian machine (4321 order)
00 00 FF FE UCS-4, unusual octet order (2143)
FE FF 00 00 UCS-4, unusual octet order (3412)
FE FF ## ## UTF-16, big-endian
FF FE ## ## UTF-16, little-endian

Without a Byte Order Mark:

00 00 00 3C UCS-4 or other encoding with a 32-bit code unit and ASCII characters encoded as ASCII values, in respectively big-endian (1234), little-endian (4321) and two unusual byte orders (2143 and 3412). The encoding declaration must be read to determine which of UCS-4 or other supported 32-bit encodings applies.
3C 00 00 00
00 00 3C 00
00 3C 00 00
00 3C 00 3F UTF-16BE or big-endian ISO-10646-UCS-2 or other encoding with a 16-bit code unit in big-endian order and ASCII characters encoded as ASCII values (the encoding declaration must be read to determine which)
3C 00 3F 00 UTF-16LE or little-endian ISO-10646-UCS-2 or other encoding with a 16-bit code unit in little-endian order and ASCII characters encoded as ASCII values (the encoding declaration must be read to determine which)
3C 3F 78 6D UTF-8, ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any other 7-bit, 8-bit, or mixed-width encoding which ensures that the characters of ASCII have their normal positions, width, and values; the actual encoding declaration must be read to detect which of these applies, but since all of these encodings use the same bit patterns for the relevant ASCII characters, the encoding declaration itself may be read reliably
4C 6F A7 94 EBCDIC (in some flavor; the full encoding declaration must be read to tell which code page is in use)
OtherUTF-8 without an encoding declaration, or else the data stream is mislabeled (lacking a required encoding declaration), corrupt, fragmentary, or enclosed in a wrapper of some kind

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 characters from the ASCII repertoire (however encoded), a processor can reliably read the entire encoding declaration as soon as it has detected which family of encodings is in use. Since in practice, all widely used character encodings fall into one of the categories above, the XML encoding declaration allows reasonably reliable in-band labeling of character encodings, even when external sources of information at the operating-system or transport-protocol level are unreliable. Character encodings such as UTF-7 that make overloaded usage of ASCII-valued bytes may fail to be reliably detected.

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.2 Priorities in the Presence of External Encoding Information

The second possible case occurs when the XML entity is accompanied by encoding information, as in some file systems and some network protocols. When multiple sources of information are available, their relative priority and the preferred method of handling conflict should be specified as part of the higher-level protocol used to deliver XML. In particular, please refer to [IETF RFC 3023] or its successor, which defines the text/xml and application/xml MIME types and provides some useful guidance. In the interests of interoperability, however, the following rule is recommended.

  • If an XML entity is in a file, the Byte-Order Mark and encoding declaration are used (if present) to determine the character encoding.

G W3C XML Working Group (Non-Normative)

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 participants of the XML WG are:

H W3C XML Core Working Group (Non-Normative)

The fifth edition of this specification was prepared by the W3C XML Core Working Group (WG). The participants in the WG at the time of publication of this edition were:

I Production Notes (Non-Normative)

This edition was encoded in a slightly modified version of the XMLspec DTD, v2.10. The XHTML versions were produced with a combination of the xmlspec.xsl, diffspec.xsl, and REC-xml.xsl XSLT stylesheets.

J Suggestions for XML Names (Non-Normative)

The following suggestions define what is believed to be best practice in the construction of XML names used as element names, attribute names, processing instruction targets, entity names, notation names, and the values of attributes of type ID, and are intended as guidance for document authors and schema designers. All references to Unicode are understood with respect to a particular version of the Unicode Standard greater than or equal to 5.0; which version should be used is left to the discretion of the document author or schema designer.

The first two suggestions are directly derived from the rules given for identifiers in Standard Annex #31 (UAX #31) of the Unicode Standard, version 5.0 [Unicode], and exclude all control characters, enclosing nonspacing marks, non-decimal numbers, private-use characters, punctuation characters (with the noted exceptions), symbol characters, unassigned codepoints, and white space characters. The other suggestions are mostly derived from Appendix B in previous editions of this specification.

  1. The first character of any name should have a Unicode property of ID_Start, or else be '_' #x5F.

  2. Characters other than the first should have a Unicode property of ID_Continue, or be one of the characters listed in the table entitled "Characters for Natural Language Identifiers" in UAX #31, with the exception of "'" #x27 and "’" #x2019.

  3. Characters in names should be expressed using Normalization Form C as defined in [UnicodeNormal].

  4. Ideographic characters which have a canonical decomposition (including those in the ranges [#xF900-#xFAFF] and [#x2F800-#x2FFFD], with 12 exceptions) should not be used in names.

  5. Characters which have a compatibility decomposition (those with a "compatibility formatting tag" in field 5 of the Unicode Character Database -- marked by field 5 beginning with a "<") should not be used in names. This suggestion does not apply to characters which despite their compatibility decompositions are in regular use in their scripts, for example #x0E33 THAI CHARACTER SARA AM or #x0EB3 LAO CHARACTER AM.

  6. Combining characters meant for use with symbols only (including those in the ranges [#x20D0-#x20EF] and [#x1D165-#x1D1AD]) should not be used in names.

  7. The interlinear annotation characters ([#xFFF9-#xFFFB]) should not be used in names.

  8. Variation selector characters should not be used in names.

  9. Names which are nonsensical, unpronounceable, hard to read, or easily confusable with other names should not be employed.