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 RFC 3066 [IETF RFC 3066] for language identification tags, ISO 639 [ISO 639] for language name codes, and ISO 3166 [ISO 3166] for country name codes), provides all the information necessary to understand XML Version 1.0 and construct computer programs to process it.

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

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:

error

[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·.

match

[Definition:]  (Of strings or names:) Two strings or names being compared MUST be identical. Characters with multiple possible representations in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic forms) match only if they have the same representation in both strings. 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.

2.1 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·.

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.

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 3.1 [Unicode3]; 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.

[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDDF],
[#1FFFE-#x1FFFF], [#2FFFE-#x2FFFF], [#3FFFE-#x3FFFF],
[#4FFFE-#x4FFFF], [#5FFFE-#x5FFFF], [#6FFFE-#x6FFFF],
[#7FFFE-#x7FFFF], [#8FFFE-#x8FFFF], [#9FFFE-#x9FFFF],
[#AFFFE-#xAFFFF], [#BFFFE-#xBFFFF], [#CFFFE-#xCFFFF],
[#DFFFE-#xDFFFF], [#EFFFE-#xEFFFF], [#FFFFE-#xFFFFF],
[#10FFFE-#x10FFFF].

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.

Characters are classified for convenience as letters, digits, or other characters. A letter consists of an alphabetic or syllabic base character or an ideographic character. Full definitions of the specific characters in each class are given in B Character Classes.

[Definition:]  A Name is a token beginning with a letter or one of a few punctuation characters, and continuing with letters, digits, hyphens, underscores, colons, or full stops, together known as name characters. Names beginning with the string "xml", or with any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.

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

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.

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 "&" and "<" respectively. The right angle bracket (>) MAY be represented using the string ">", and MUST, ·for compatibility·, be escaped using either">" 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 "'", and the double-quote character (") as """.

2.5 Comments

[Definition:]  CommentsMAY 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.

An example of a comment:

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

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

<!-- B+, B, or B--->

2.6 Processing Instructions

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

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 sectionsMAY 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 "]]>":

Within a CDATA section, only the CDEnd string is recognized as markup, so that left angle brackets and ampersands may occur in their literal form; they need not (and cannot) be escaped using "<" and "&". CDATA sections cannot nest.

An example of a CDATA section, in which "<greeting>" and "</greeting>" are recognized as ·character data·, not ·markup·:

<![CDATA[<greeting>Hello, world!</greeting>]]> 

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.

[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.

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.

Like the internal subset, the external subset and any external parameter entities referenced in a DeclSepMUST 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.

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).

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.

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:spaceMAY 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:langMAY be inserted in documents to specify the language used in the contents and attribute values of any element in an XML document. In valid documents, this attribute, like any other, MUST be ·declared· if it is used. The values of the attribute are language identifiers as defined by [IETF RFC 3066], Tags for the Identification of Languages, or its successor; 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>
</sp>

The intent declared with xml:lang is considered to apply to all attributes and content of the element where it is specified, unless overridden with an instance of xml:lang on another element within that content. 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.

A simple declaration for xml:lang might take the form

xml:lang CDATA #IMPLIED

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

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

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 spacedo 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·, ·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 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.

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.

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:

An example of an end-tag:

</termdef>

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

[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:

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" />
<br></br>
<br/>

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:

where the Name gives the element type being declared.

Examples of element type declarations:

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

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

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

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:

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.

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

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

a="

xyz"

x y z

#x20 #x20 x y z

a="&d;&d;A&a; &a;B&da;"

A #x20 B

#x20 #x20 A #x20 #x20 #x20 B #x20 #x20

a=
"

A

B
"

#xD #xD A #xA #xA B #xD #xA

#xD #xD A #xA #xA B #xD #xA

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.

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 considered part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section MUST be considered as not logically 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:

<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >

<![%draft;[
<!ELEMENT book (comments*, title, body, supplements?)>
]]>
<![%final;[
<!ELEMENT book (title, body, supplements?)>
]]>

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.

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.

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"... -->
<!ENTITY % ISOLat2
         SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >
<!-- ... now reference it. -->
%ISOLat2;

4.2 Entity Declarations

[Definition:]   Entities are declared thus:

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.

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

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

4.3 Parsed Entities

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

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

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 "<" and "&"MAY be used to escape < and & when they occur in character data.

All XML processors MUST recognize these entities whether they are declared or not. ·For interoperability·, valid XML documents SHOULD declare these entities, like any others, before using them. If the entities 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.

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.

5.1 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 processorsMUST, 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.

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:

• Certain well-formedness errors, specifically those that require reading external entities, may fail to be detected by a non-validating processor. Examples include the constraints entitled Entity Declared, Parsed Entity, and No Recursion, as well as some of the cases described as forbidden in 4.4 XML Processor Treatment of Entities and References.
• The information passed from the processor to the application may vary, depending on whether the processor reads parameter and external entities. For example, a non-validating processor may fail tonormalize attribute values, include the replacement text of internal entities, or supply default attribute values, where doing so depends on having read declarations in external or parameter entities.

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.

A.1 Normative References

IANA-CHARSETS

(Internet Assigned Numbers Authority) Official Names for Character Sets, ed. Keld Simonsen et al. (See http://www.iana.org/assignments/character-sets.)

IETF RFC 2119

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 RFC 2396

IETF (Internet Engineering Task Force). RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. 1998. (See http://www.ietf.org/rfc/rfc2396.txt.)

IETF RFC 2732

IETF (Internet Engineering Task Force). RFC 2732: Format for Literal IPv6 Addresses in URL's. R. Hinden, B. Carpenter, L. Masinter. 1999. (See http://www.ietf.org/rfc/rfc2732.txt.)

IETF RFC 3066

IETF (Internet Engineering Task Force). RFC 3066: Tags for the Identification of Languages, ed. H. Alvestrand. 2001. (See http://www.ietf.org/rfc/rfc3066.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.ch 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.

Unicode

The Unicode Consortium. The Unicode Standard, Version 2.0. Reading, Mass.: Addison-Wesley Developers Press, 1996.

Unicode3

The Unicode Consortium. The Unicode Standard, Version 3.2, defined by: The Unicode Standard, Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the Unicode Standard Annex #27: Unicode 3.1 (http://www.unicode.org/reports/tr27) and the Unicode Standard Annex #28: Unicode 3.2 (http://www.unicode.org/reports/tr28).

A.2 Other References

Aho/Ullman

Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. Compilers: Principles, Techniques, and Tools. Reading: Addison-Wesley, 1986, rpt. corr. 1988.

Brüggemann-Klein

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.

Clark

James Clark. Comparison of SGML and XML. (See http://www.w3.org/TR/NOTE-sgml-xml-971215.)

IANA-LANGCODES

(Internet Assigned Numbers Authority) Registry of Language Tags, ed. Keld Simonsen et al. (See http://www.iana.org/assignments/language-tags.)

IETF RFC 2141

IETF (Internet Engineering Task Force). RFC 2141: URN Syntax, ed. R. Moats. 1997. (See http://www.ietf.org/rfc/rfc2141.txt.)

IETF RFC 2781

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.)

IETF RFC 3023

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.)

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 639

(International Organization for Standardization). ISO 639:1988 (E). Code for the representation of names of languages. [Geneva]: International Organization for Standardization, 1988.

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.

WEBSGML

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/REC-xml-names/.)

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:

Without a Byte Order Mark:

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.