Extensible Markup Language (XML) 1.1

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:

XML shall be straightforwardly usable over the Internet.
XML shall support a wide variety of applications.
XML shall be compatible with SGML.
It shall be easy to write programs which process XML documents.
The number of optional features in XML is to be kept to the absolute +minimum, ideally zero.
XML documents should be human-legible and reasonably clear.
The XML design should be prepared quickly.
The design of XML shall be formal and concise.
XML documents shall be easy to create.
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 1.1 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 Unicode (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.]

1.3 Rationale and list of changes for XML 1.1

The W3C's XML 1.0 Recommendation was first issued in 1998, and +despite the issuance of many errata culminating in a Third Edition +of 2004, has remained (by intention) unchanged with respect to what +is well-formed XML and what is not. This stability has been +extremely useful for interoperability. However, the Unicode +Standard on which XML 1.0 relies for character specifications has +not remained static, evolving from version 2.0 to version 4.0 and +beyond. Characters not present in Unicode 2.0 may already be used +in XML 1.0 character data. However, they are not allowed in XML +names such as element type names, attribute names, enumerated +attribute values, processing instruction targets, and so on. In +addition, some characters that should have been permitted in XML +names were not, due to oversights and inconsistencies in Unicode +2.0.

The overall philosophy of names has changed since XML 1.0. +Whereas XML 1.0 provided a rigid definition of names, wherein +everything that was not permitted was forbidden, XML 1.1 names are +designed so that everything that is not forbidden (for a specific +reason) is permitted. Since Unicode will continue to grow past +version 4.0, further changes to XML can be avoided by allowing +almost any character, including those not yet assigned, in +names.

In addition, XML 1.0 attempts to adapt to the line-end +conventions of various modern operating systems, but discriminates +against the conventions used on IBM and IBM-compatible mainframes. +As a result, XML documents on mainframes are not plain text files +according to the local conventions. XML 1.0 documents generated on +mainframes must either violate the local line-end conventions, or +employ otherwise unnecessary translation phases before parsing and +after generation. Allowing straightforward interoperability is +particularly important when data stores are shared between +mainframe and non-mainframe systems (as opposed to being copied +from one to the other). Therefore XML 1.1 adds NEL (#x85) to the +list of line-end characters. For completeness, the Unicode line +separator character, #x2028, is also supported. +

Finally, there is considerable demand to define a standard representation +of arbitrary Unicode characters in XML documents. Therefore, XML 1.1 +allows the use of character references to the control characters #x1 through +#x1F, most of which are forbidden in XML 1.0. For reasons of robustness, +however, these characters still cannot be used directly in documents. In +order to improve the robustness of character encoding detection, the additional +control characters #x7F through #x9F, which were freely allowed in XML 1.0 +documents, now must also appear only as character references. (Whitespace +characters are of course exempt.) The minor sacrifice of backward compatibility +is considered not significant. Due to potential problems with APIs, +#x0 is still forbidden both directly and as a character reference. +

Finally, XML 1.1 defines a set of constraints called "full +normalization" on XML documents, which document creators +SHOULD adhere to, and document processors +SHOULD verify. Using fully normalized documents +ensures that identity comparisons of names, attribute values, and +character content can be made correctly by simple binary comparison of +Unicode strings.

A new XML version, rather than a set of errata to XML 1.0, is +being created because the changes affect the definition of +well-formed documents. XML 1.0 processors must continue to reject +documents that contain new characters in XML names, new line-end +conventions, and references to control characters. The distinction between XML 1.0 and XML 1.1 documents +is indicated by the version number information in the XML +declaration at the start of each document. +

2 Documents

[Definition: A data object is an XML +document if it is well-formed, +as defined in this specification. A well-formed XML document MAY in addition +be 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:]

Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this specification.
Each of the parsed entities +which is referenced directly or indirectly within the document is well-formed.

Document

[1] document ::= prolog element Misc* - Char* RestrictedChar Char*

Matching the document production implies that:

It contains one or more elements.
[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 [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`	::=	`[#x1-#xD7FF] \| [#xE000-#xFFFD] \| [#x10000-#x10FFFF]`	/ any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. /
[2a]	`RestrictedChar`	::=	`[#x1-#x8] \| [#xB-#xC] \| [#xE-#x1F] \| [#x7F-#x84] \| [#x86-#x9F]`

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.

Note:

Document authors are encouraged to avoid +"compatibility characters", as defined +in Unicode [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-#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.

White Space

[3] S ::= (#x20 | #x9 | #xD | #xA)+

Note:

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.

[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.

Note:

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.

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

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 I 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)*`

Note:

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.

Literals

[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] \| [-'()+,./:=?;!*#@$_%]`

Note:

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 "&" 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 or 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 """.

Character Data

[14] CharData ::= [^<&]* - ([^<&]* ']]>' [^<&]*)

2.5 Comments

[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.

Comments

[15] Comment ::= ''

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.]

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 "<" +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 1.1 documents MUST +begin with an XML declaration which specifies the version of +XML being used.] For example, the following is a complete XML 1.1 document, well-formed but not valid:

<?xml version="1.1"?>
+<greeting>Hello, world!</greeting>

but the following is an XML 1.0 document because it +does not have an XML declaration:

<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.

Prolog

[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`	::=	`'1.1'`
[27]	`Misc`	::=	`Comment \| PI +\| S`

[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.)

Well-formedness constraint: External Subset

The external subset, if any, MUST match the production for extSubset.

Well-formedness constraint: PE Between Declarations

The replacement text of a parameter entity reference +in a DeclSep MUST match the production extSubsetDecl.

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="1.1"?>
+<!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="1.1" 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.

XML 1.1 processors SHOULD accept XML 1.0 +documents as well. If a document is well-formed or valid XML 1.0, and provided it +does not contain any control characters +in the range [#x7F-#x9F] other than as character escapes, it may be +made well-formed or valid XML 1.1 respectively simply by changing the +version number.

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 ::= #x20+ '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="1.1" 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 + +all of the following to a single #xA character:

the two-character sequence #xD #xA
the two-character sequence #xD #x85
the single character #x85
the single character #x2028
any #xD character that is not immediately followed by #xA or #x85.

The characters #x85 and #x2028 cannot be reliably recognized and +translated until an entity's encoding declaration (if present) has +been read. Therefore, it is a fatal error to use them within the XML +declaration or text declaration. +

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 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 hei&#xDF;em Bem&#xFC;h'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.

Note:

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

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'>

2.13 Normalization Checking

All XML parsed +entities (including document +entities) SHOULD be fully +normalized as per the definition of +B Definitions for Character Normalization supplemented by the following definitions of +relevant constructs for XML:

The +replacement text of all parsed +entities
All text matching, in context, one of the following +productions:

However, a document is still well-formed even if it is not +fully normalized. +XML processors SHOULD provide a user option to verify that the document being +processed is in fully normalized form, and report to the application whether +it is or not. The option to not verify SHOULD be chosen only when the +input text is certified, +as defined by B Definitions for Character Normalization.

The verification of full normalization MUST be carried out as if by +first verifying that the entity is in include-normalized +form as defined by B Definitions for Character Normalization and by then verifying that none of the relevant +constructs listed above begins (after character references are +expanded) with a composing character as defined by +B Definitions for Character Normalization. +Non-validating processors MUST ignore possible +denormalizations that would be caused by inclusion of external +entities that they do not read.

Note:

The composing character are all +Unicode characters of non-zero combining class, plus a small number +of class-zero characters that nevertheless take part as a +non-initial character in certain Unicode canonical +decompositions. Since these characters are meant to follow +base characters, restricting relevant constructs (including +content) from beginning with a composing character does not +meaningfully diminish the expressiveness of XML.

If, while verifying full normalization, a processor encounters +characters for which it cannot determine the normalization +properties (i.e., characters introduced in a version of Unicode [Unicode] +later than the one used in the implementation of the processor), +then the processor MAY, at user option, ignore any possible +denormalizations caused by these characters. The option to ignore +those denormalizations SHOULD NOT be chosen by applications when +reliability or security are critical.

XML processors MUST NOT transform the input to be in +fully normalized form. +XML applications that create XML 1.1 output +from either XML 1.1 or XML 1.0 input SHOULD ensure that the output +is fully normalized; it is not necessary for internal processing +forms to be fully normalized.

The purpose of this section is to strongly encourage XML +processors to ensure that the creators of XML documents have +properly normalized them, so that XML applications can make tests +such as identity comparisons of strings without having to worry +about the different possible "spellings" of strings which +Unicode allows. +

When entities are in a non-Unicode encoding, if the processor +transcodes them to Unicode, it SHOULD use a normalizing transcoder. +

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.

Element

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

This specification does not constrain the 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:

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

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: Unique Att Spec

An attribute name +MUST NOT appear more than once in the same start-tag or empty-element tag.

Validity constraint: Attribute Value Type

The attribute MUST +have been declared; the value MUST be of the type declared for it. (For attribute +types, see 3.3 Attribute-List Declarations.)

Well-formedness constraint: No External Entity References

Attribute +values MUST NOT contain direct or indirect entity references to external entities.

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:]

End-tag

[42] ETag ::= '</' Name S? +'>'

An example of an end-tag:

</termdef>

[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" />
+<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:]

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.

Validity constraint: Unique Element Type Declaration

An element +type MUST NOT be declared more than once.

Examples of element type declarations:

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

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 D 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."

Validity constraint: No Duplicate Types

The +same name MUST NOT appear more than once in a single mixed-content declaration.

Examples of mixed content declarations:

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

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 have varying lexical and semantic constraints. +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]
			`\| 'IDREF'`	[VC: IDREF]
			`\| 'IDREFS'`	[VC: IDREF]
			`\| 'ENTITY'`	[VC: Entity Name]
			`\| 'ENTITIES'`	[VC: Entity Name]
			`\| 'NMTOKEN'`	[VC: Name Token]
			`\| 'NMTOKENS'`	[VC: Name Token]

Validity constraint: ID

Values of 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: One ID per Element Type

An element +type MUST NOT have more than one ID attribute specified.

Validity constraint: ID Attribute Default

An ID attribute +MUST have a declared default of #IMPLIED or #REQUIRED.

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.

Validity constraint: Name Token

Values of type NMTOKEN MUST match the Nmtoken production; values of type NMTOKENS MUST match Nmtokens.

[Definition: Enumerated attributes MUST take one of a list of values +provided in the declaration]. There are two kinds of enumerated 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: One Notation Per Element Type

An element type MUST NOT have more than one NOTATION +attribute specified.

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.

Validity constraint: Enumeration

Values of this type MUST match +one of the Nmtoken tokens in the declaration.

For interoperability, the same Nmtoken SHOULD NOT occur more than once in the enumerated +attribute types of a single element type.

3.3.2 Attribute Defaults

An attribute declaration provides information +on whether the attribute's presence is REQUIRED, and if not, how an XML processor +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]

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: Required Attribute

If the default +declaration is the keyword #REQUIRED, then the attribute MUST be +specified for all elements of the type in the attribute-list declaration.

Validity constraint: Attribute +Default Value Syntactically Correct

The declared default value MUST meet the syntactic +constraints of the declared attribute type.

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) may come into play if the declared default value is actually used +(an element without a specification for this attribute occurs).

Validity constraint: Fixed Attribute Default

If an attribute +has a default value declared with the #FIXED keyword, instances of +that attribute MUST match the default value.

Examples of attribute-list declarations:

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

3.3.3 Attribute-Value Normalization

Before the value of an attribute is passed to the application 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.

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.
Begin with a normalized value consisting of the empty string.
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 specification	a is NMTOKENS	a is CDATA
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

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*)`

Validity constraint: Proper Conditional Section/PE Nesting

If any of the "<![", +"[", or "]]>" of a conditional section is contained +in the replacement text for a parameter-entity reference, all of them MUST +be contained in the same replacement text.

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 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 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]

Well-formedness constraint: Legal Character

Characters referred +to using character references MUST match the production for Char.

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]
				[WFC: In DTD]

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 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 external parameter entities with "standalone='no'", +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.

Well-formedness constraint: No Recursion

A parsed entity MUST NOT contain a recursive reference to itself, either directly or indirectly.

Well-formedness constraint: In DTD

Parameter-entity references MUST NOT appear outside +the DTD.

Examples of character and entity references:

Type <key>less-than</key> (&#x3C;) to save options.
+This document was prepared on &docdate; and
+is classified &security-level;.

Example of a parameter-entity reference:

<!-- 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:]

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
+specification.">

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 2396], updated by [IETF RFC 2732]), +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 2396] and [IETF RFC 2732], +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:

Each +character to be escaped +is represented in +UTF-8 [Unicode] +as one or more bytes.
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).
The original character is replaced by the resulting character sequence.

[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"
+"http://www.textuality.com/boilerplate/OpenHatch.xml">
+<!ENTITY hatch-pic
+SYSTEM "../grafix/OpenHatch.gif"
+NDATA gif >

4.3 Parsed Entities

4.3.1 The Text Declaration

External parsed entities SHOULD each begin with a text declaration.

Text Declaration

[77] TextDecl ::= '<?xml' VersionInfo? EncodingDecl S? '?>'

The text declaration MUST be provided literally, not by reference +to a parsed entity. The text declaration +MUST NOT appear at any +position other than the beginning of an external parsed entity. The text declaration +in an external parsed entity is not considered part of its replacement text.

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.

Well-Formed External Parsed Entity

[78] extParsedEnt ::= TextDecl? content - Char* RestrictedChar Char*

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 character +encodings with any other labels, even if the encodings or labels are very +similar to UTF-8 or UTF-16.

Entities encoded in UTF-16 MUST and entities +encoded in UTF-8 MAY begin with the Byte Order Mark described in +ISO/IEC 10646 [ISO/IEC 10646] or Unicode [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.

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 irregular code unit sequences, +as defined in 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'?>

4.3.4 Version Information in Entities

Each entity, including the document entity, +can be separately +declared as XML 1.0 or XML 1.1. The version declaration appearing +in the document entity determines the version of the document as a +whole. An XML 1.1 document may invoke XML 1.0 external entities, so +that otherwise duplicated versions of external entities, +particularly DTD external subsets, need not be maintained. However, +in such a case the rules of XML 1.1 are applied to the entire +document.

If an entity (including the document entity) is not labeled +with a version number, it is treated as if labeled as version +1.0.

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 +Type				Character
	Parameter	Internal General	External Parsed +General	Unparsed	Character
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&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 white space) 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 C 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.]

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.

XML 1.1 processors MUST be able to process both XML 1.0 +and XML 1.1 documents. Programs which generate XML SHOULD +generate XML 1.0, unless one of the specific features of XML 1.1 is required.

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 to normalize +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.

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

symbol ::= expression

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:

#xN: 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.
"string": matches a literal string matching that +given inside the double quotes.
'string': 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): expression is treated as a unit and may be combined as described +in this list.
A?: matches A or nothing; optional A.
A B: 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.
A+: matches one or more occurrences of A. Concatenation +has higher precedence than alternation; thus A+ | B+ is identical +to (A+) | (B+).
A*: 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:

/* ... */: comment.
[ 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.

`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
`EF BB BF`	UTF-8

`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)
Other	UTF-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