Extensible Markup Language (XML) From: http://www.w3.org/TR/WD-xml-970807.xml tags numeric char refs #252 and #228 replaced by latin-8 codes "Part 1. Syntax" removed "XML vs. SGML" list spacing changed to "compact" --> "> '"> "> amp, lt, gt, apos, quot"> ]> Extensible Markup Language (XML) WD-xml-&iso6.doc.date; W3C Working Draft &w3c.doc.date; http://www.w3.org/TR/WD-xml-&iso6.doc.date; http://www.w3.org/TR/WD-xml-961114 http://www.w3.org/TR/WD-xml-lang-970331 http://www.w3.org/TR/WD-xml-lang-970630 http://www.w3.org/TR/WD-xml Tim Bray Textuality and Netscape tbray@textuality.com Jean Paoli Microsoft jeanpa@microsoft.com C. M. Sperberg-McQueen University of Illinois at Chicago cmsmcq@uic.edu

This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at http://www.w3.org/TR.

Note: Since working drafts are subject to frequent change, you are advised to reference the above URL, rather than the URLs for working drafts themselves.

This work is part of the W3C SGML Activity (for current status, see http://www.w3.org/MarkUp/SGML/Activity).

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

Extensible Markup Language

Chicago, Vancouver, Mountain View, et al.: World-Wide Web Consortium, XML Working Group, 1996, 1997.

Created in electronic form.

English Extended Backus-Naur Form (formal grammar)

1997-07-24 : CMSMcQ : correct error (lost *) in definition of ignoreSectContents (thanks to Makoto Murata)

Allow all empty elements to have end-tags, consistent with SGML TC (as per JJC).

1997-07-23 : CMSMcQ : pre-emptive strike on pending corrections: introduce the term 'empty-element tag', note that all empty elements may use it, and elements declared EMPTY must use it. Add WFC requiring encoding decl to come first in an entity. Redefine notations to point to PIs as well as binary entities. Change autodetection table by removing bytes 3 and 4 from examples with Byte Order Mark. Add content model as a term and clarify that it applies to both mixed and element content.

1997-06-30 : CMSMcQ : change date, some cosmetic changes, changes to productions for choice, seq, Mixed, NotationType, Enumeration. Follow James Clark's suggestion and prohibit conditional sections in internal subset. TO DO: simplify production for ignored sections as a result, since we don't need to worry about parsers which don't expand PErefs finding a conditional section.

1997-06-29 : TB : various edits

1997-06-29 : CMSMcQ : further changes:

suppress old FINAL EDIT comments and some dead material.

revise occurrences of % in grammar to exploit Henry Thompson's pun, especially markupdecl and attdef

remove RMD requirement relating to element content (?)

1997-06-28 : CMSMcQ : Various changes for 1 July draft:

add text for draconian error handling (introduce the term Fatal Error)

RE deleta est (changing wording from original announcement to restrict the requirement to validating parsers)

tag definition of validating processor and link to it

add colon as name character

change def of %operator

change standard definitions of lt, gt, amp

strip leading zeros from #x00nn forms.

1997-04-02 : CMSMcQ : final corrections of editorial errors found in last night's proofreading. Reverse course once more on well-formed: Webster's Second hyphenates it, and that's enough for me.

1997-04-01 : CMSMcQ : corrections from JJC, EM, HT, and self

1997-03-31 : Tim Bray : many changes

1997-03-29 : CMSMcQ : some Henry Thompson (on entity handling), some Charles Goldfarb, some ERB decisions (PE handling in miscellaneous declarations. Changed Ident element to accept def attribute. Allow normalization of Unicode characters. move def of systemliteral into section on literals.

1997-03-28 : CMSMcQ : make as many corrections as possible, from Terry Allen, Norbert Mikula, James Clark, Jon Bosak, Henry Thompson, Paul Grosso, and self. Among other things: give in on "well formed" (Terry is right), tentatively rename QuotedCData as AttValue and Literal as EntityValue to be more informative, since attribute values are the only place QuotedCData was used, and vice versa for entity text and Literal. (I'd call it Entity Text, but 8879 uses that name for both internal and external entities.)

1997-03-26 : CMSMcQ : resynch the two forks of this draft, reapply my changes dated 03-20 and 03-21. Normalize old 'may not' to 'must not' except in the one case where it meant 'may or may not'.

1997-03-21 : TB : massive changes on plane flight from Chicago to Vancouver

1997-03-21 : CMSMcQ : correct as many reported errors as possible.

1997-03-20 : CMSMcQ : correct typos listed in CMSMcQ hand copy of spec.

1997-03-20 : CMSMcQ : cosmetic changes preparatory to revision for WWW conference April 1997: restore some of the internal entity references (e.g. to docdate, etc.), change character xA0 to   and define nbsp as  , and refill a lot of paragraphs for legibility.

1996-11-12 : CMSMcQ : revise using Tim's edits.

Add list type of NUMBERED and change most lists either to BULLETS or to NUMBERED.

suppress QuotedNames, Names (not used)

correct trivial-grammar doc type decl

rename 'marked section' as 'CDATA section' passim

Also edits from James Clark:

define the set of characters from which [^abc] subtracts

charref should use just [0-9] not Digit

location info needs cleaner treatment: remove? (ERB question)

one example of a PI has wrong pic.

clarify discussion of encoding names

encoding failure should lead to unspecified results; don't prescribe error recovery

don't require exposure of entity boundaries

ignore white space in element content

reserve entity names of the form u-NNNN

clarify relative URLs

And some of my own:

correct productions for content model: model cannot consist of a name, so "elements ::= cp" is no good.

1996-11-11 : CMSMcQ : revise for style. Add new rhs to entity declaration, for parameter entities.

1996-11-10 : CMSMcQ : revise for style. Fix / complete section on names, characters. Add sections on parameter entities, conditional sections. Still to do: Add compatibility note on deterministic content models. Finish stylistic revision.

1996-10-31 : TB : Add Entity Handling section

1996-10-30 : TB : Clean up term & termdef. Slip in ERB decision re EMPTY.

1996-10-28 : TB : Change DTD. Implement some of Michael's suggestions. Change comments back to //. Introduce language for XML namespace reservation. Add section on white-space handling. Lots more cleanup.

1996-10-24 : CMSMcQ : quick tweaks, implement some ERB decisions. Characters are not integers. Comments are /* */ not //. Add bibliographic refs to 10646, HyTime, Unicode. Rename old Cdata as MsData since it's only seen in marked sections. Call them attribute-value pairs not name-value pairs, except once. Internal subset is optional, needs '?'. Implied attributes should be signaled to the app, not have values supplied by processor.

1996-10-16 : TB : track down & excise all DSD references; introduce some EBNF for entity declarations.

1996-10-?? : TB : consistency check, fix up scraps so they all parse, get formatter working, correct a few productions.

1996-10-10/11 : CMSMcQ : various maintenance, stylistic, and organizational changes:

replace a few literals with xmlpio and pic entities, to make them consistent and ensure we can change pic reliably when the ERB votes

drop paragraph on recognizers from notation section

add match, exact match to terminology

move old 2.2 XML Processors and Apps into intro

mention comments, PIs, and marked sections in discussion of delimiter escaping

streamline discussion of doctype decl syntax

drop old section of 'PI syntax' for doctype decl, and add section on partial-DTD summary PIs to end of Logical Structures section

revise DSD syntax section to use Tim's subset-in-a-PI mechanism

1996-10-10 : TB : eliminate name recognizers (and more?)

1996-10-09 : CMSMcQ : revise for style, consistency through 2.3 (Characters)

1996-10-09 : CMSMcQ : re-unite everything for convenience, at least temporarily, and revise quickly

1996-10-08 : TB : first major homogenization pass

1996-10-08 : TB : turn "current" attribute on div type into CDATA

1996-10-02 : TB : remould into skeleton + entities

1996-09-30 : CMSMcQ : add a few more sections prior to exchange with Tim.

1996-09-20 : CMSMcQ : finish transcribing notes.

1996-09-19 : CMSMcQ : begin transcribing notes for draft.

1996-09-13 : CMSMcQ : made outline from notes of 09-06, do some housekeeping

Extensible Markup Language Version &XML.version; Document W3C-SGML-ERB DD-1996-0004 &doc.date; This draft is intended for public discussion. It is subject to approval by the XML Working Group. Introduction

The Extensible Markup Language, abbreviated XML, describes a class of data objects called XML documents and partially describes the behavior of computer programs which process them. XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879].

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

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

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 and chaired by Jon Bosak of Sun Microsystems with the very active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the WG's contact with the W3C.

The design goals for XML are:

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 the associated standards, provides all the information necessary to understand XML version &XML.version; and construct computer programs to process it.

This version of the XML specification (&doc.date;) is for &doc.audience;. It &doc.distribution;.

Relationship to Existing Standards

Standards relevant to users and implementors of XML include:

SGML (ISO 8879:1986). By definition, valid XML documents are conformant SGML documents in the sense described in ISO standard 8879. The current draft of this specification presupposes the successful completion of the current work on a technical corrigendum to ISO 8879 now being prepared by ISO/IEC JTC1/SC18/WG8. If the corrigendum is not adopted in the expected form, some clauses of this specification may change, and some recommendations now labeled for interoperability will become requirements labeled for compatibility.

Unicode and ISO/IEC 10646. This specification depends on the international standard ISO/IEC 10646 (with amendments AM 1 through AM 5) and the Unicode Standard, Version 2.0, which define the encodings and meanings of the characters which make up XML text data. All the characters in ISO/IEC 10646 are present, at the same code points, in Unicode.

IETF RFC 1738 and RFC 1808. RFC 1738 and RFC 1808 define the syntax and semantics of Uniform Resource Locators, or URLs.

Terminology

Some terms used with special meaning in this specification are:

Conforming data and XML processors are permitted to but need not behave as described.

Conforming data and XML processors are required to behave as described; otherwise they are in error.

A violation of the rules of this specification; results are undefined. Conforming software may detect and report an error and may recover from it.

An error which conforming software must detect and report to the application. After encountering a fatal error, an XML processor may continue processing the data to search for further errors and may report such errors to the application. In order to support correction of errors, the processor may make unprocessed text from the document (with intermingled character data and markup) available to the application. Once a fatal error is detected, however, the processor must not continue normal processing (i.e. it must not continue to pass character data and information about the document's logical structure to the application in the normal way).

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.

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

Conforming software may or must (depending on the modal verb in the sentence) behave as described; if it does, it must provide users a means to enable or disable the behavior described.

(Of strings or names:) Case-insensitive match: two strings or names being compared match if they are identical after case-folding. (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:) The content of a parent element in a document matches the content model for that element if (a) the content model matches the rule for Mixed and the content consists of character data and elements whose names match names in the content model, or if (b) the content model matches the rule for elements, and the sequence of child elements belongs to the language generated by the regular expression in the content model.

a process applied to a sequence of characters, in which those identified as non-uppercase (in scripts which have case distinctions) are replaced by their uppercase equivalents, as specified in The Unicode Standard, Version 2.0, section 4.1. Note that Unicode recommends folding to lowercase; for compatibility reasons, XML processors must fold to uppercase. Case-folding, as described here, neither requires nor forbids the normalization of Unicode character sequences into canonical form (e.g. as described in The Unicode Standard, section 5.9).

Case-sensitive string match: two strings or names being compared must be identical. Characters with multiple possible representations in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic forms) match only if they have the same representation in both strings. At user option, processors may normalize such characters to their canonical form.

A feature of XML included solely to ensure that XML remains compatible with SGML.

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 technical corrigendum to ISO 8879 now in the process of preparation by ISO/IEC JTC1/SC18/WG8.

Notation

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

Symbols are written with an initial capital letter if they are defined by a regular expression, or with an initial lowercase letter if a recursive grammar is required for recognition. Literal strings are quoted; unless otherwise noted they are case-insensitive. The distinction between symbols which can and cannot be recognized using simple regular expressions may be used to set the boundary between an implementation's lexical scanner and its parser, but this specification neither constrains the placement of that boundary nor presupposes that all implementations will have one.

Within the expression on the right-hand side of a rule, the meaning of symbols is as shown below.

where N is a hexadecimal integer, the expression represents the character in ISO/IEC 10646 whose canonical (UCS-4) bit string, when interpreted as an unsigned binary number, has the value indicated. The number of leading zeroes in the #xN form is insignificant; the number of leading zeroes in the corresponding bit string is governed by the character encoding in use and is not significant for XML.

represents any character with a value in the range(s) indicated (inclusive).

represents any character with a value outside the range indicated.

represents any character with a value not among the characters given.

represents a literal string matching that given inside the double quotes.

represents a literal string matching that given inside the single quotes.

a followed by b.

a or b but not both.

the set of strings represented by a but not represented by b

a or nothing; optional a.

one or more occurrences of a.

zero or more occurrences of a.

specifies that in the external DTD subset a parameter entity may occur in the text at the position where a may occur; if so, its replacement text must match S? a S?. If the expression a is governed by a suffix operator, then the suffix operator determines both the maximum number of parameter-entity references allowed and the number of occurrences of a in the replacement text of the parameter entities: %a* means that a must occur zero or more times, and that some of its occurrences may be replaced by references to parameter entities whose replacement text must contain zero or more occurrences of a; it is thus a more compact way of writing %(a*)*. Similarly, %a+ means that a must occur one or more times, and may be replaced by parameter entities with replacement text matching S? (a S?)+. The recognition of parameter entities in the internal subset is much more highly constrained.

expression is treated as a unit, and may carry the % prefix operator, or a suffix operator: ?, *, or +.

comment.

Well-formedness check; this identifies by name a check for well-formedness associated with a production.

Validity check; this identifies by name a check for validity associated with a production.

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. S (#x20 | #x9 | #xd | #xa)+

Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646. Char #x9 | #xA | #xD | [#x20-#xFFFD] | [#x00010000-#x7FFFFFFF] any ISO/IEC 10646 UCS-4 code, FFFE and FFFF excluded Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic or syllabic base character possibly followed by one or more combining characters, or of an ideographic character. Certain layout and format-control characters defined by ISO/IEC 10646 should be ignored when recognizing identifiers; these are defined by the classes Ignorable and Extender. Full definitions of the specific characters in each class are given in the appendix on character classes.

A Name is a token beginning with a letter or underscore character and continuing with letters, digits, hyphens, underscores, or full stops (together known as name characters). Names beginning with the string XML are reserved for standardization in this or future versions of this specification.

Note: the colon character is also allowed within XML names; it is reserved for experimentation with name spaces and schema scoping. Its meaning is expected to be standardized at some future point, at which point those documents using colon for experimental purposes will need to be updated. (Note: there is no guarantee that any name-space mechanism adopted for XML will in fact use colon as a name-space delimiter.) In practice, this means that authors should not use colon in XML names except as part of name-space experiments, but that implementors should accept colon as a name character.

An Nmtoken (name token) is any mixture of name characters. MiscName '.' | '-' | '_' | ':' | CombiningChar | Ignorable | Extender NameChar Letter | Digit | MiscName Name (Letter | '_' | ':') (NameChar)* Names Name (S Name)* Nmtoken (NameChar)+ Nmtokens Nmtoken (S Nmtoken)*

Literal data is any quoted string not containing the quotation mark used as a delimiter for that string; different forms of literal data may or may not contain angle brackets, entity references, and character references. Literals are used for specifying the replacement text of internal entities (EntityValue), the values of attributes (AttValue), and external identifiers (SystemLiteral); for some purposes, the entire literal can be skipped without scanning for markup within it (SkipLit): EntityValue '"' ([^%&"] | PEReference | Reference)* '"' |  "'" ([^%&'] | PEReference | Reference)* "'" AttValue '"' ([^<&"] | Reference)* '"' |  "'" ([^<&'] | Reference)* "'" SystemLiteral '"' URLchar* '"' | "'" (URLchar - "'")* "'" URLchar See RFC 1738 and 1808 PubidLiteral '"' PubidChar* '"' | "'" (PubidChar - "'")* "'" PubidChar #x20 | #x9 | #xd | #xa | #x&IDEOSPACE; | [a-zA-Z0-9] | [-'()+,./:=?] SkipLit ('"' [^"]* '"') | ("'" [^']* "'") Note that entity references and character references are recognized and processed within EntityValue and AttValue, but not within SystemLiteral.

Documents

A textual object is an XML document if it is either valid or well-formed, as defined in this specification.

Logical and Physical Structure

Each XML document has both a logical and a physical structure.

Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a "root" or document entity.

The logical structure contains declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.

The two structures must be synchronous: see section 4.1.

Well-Formed XML Documents

A textual object is said to be a well-formed XML document if, first, it matches the production labeled document, and if for each entity reference which appears in the document, either the entity has been declared in the document type declaration or the entity name is one of: &magicents;.

Matching the document production implies that:

It contains one or more elements.

It meets all the well-formedness constraints (WFCs) given in the grammar.

There is exactly one element, called the root, or document element, for which neither the start-tag nor the end-tag is in the content of any other element. For all other elements, if the start-tag is in the content of another element, the end-tag is in the content of the same element. More simply stated, the elements, delimited by start- and end-tags, nest within each other.

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

Characters

The data stored in an XML entity is either text or binary. Binary data has an associated notation, identified by name; beyond a requirement to make available the notation's name and the associated system identifier, XML places no constraints on the contents or use of binary entities. So-called binary data might in fact be textual; its identification as binary means that an XML processor need not parse it in the fashion described by this specification. XML text data is a sequence of characters. A character is an atomic unit of text; valid characters are specified by ISO/IEC 10646. Users may extend the ISO/IEC 10646 character repertoire by exploiting the private use areas.

The mechanism for encoding character values into bit patterns may vary from entity to entity. All XML processors must accept the UTF-8 and UCS-2 encodings of 10646; the mechanisms for signaling which of the two are in use, or for bringing other encodings into play, are discussed later, in the discussion of character encodings.

Regardless of the specific encoding used, any character in the ISO/IEC 10646 character set may be referred to by the decimal or hexadecimal equivalent of its bit string.

Character Data and Markup

XML text consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty elements, entity references, character references, comments, CDATA sections, document type declarations, and processing instructions.

All text that is not markup constitutes the character data of the document.

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

In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup. In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, "]]>".

To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') may be represented as "&apos;", and the double-quote character (") as "&quot;". PCData [^<&]*

Comments

Comments may appear anywhere except in a CDATA section, i.e. within element content, in mixed content, or in a DTD. They must not occur within declarations or tags. They are not part of the document's character data; an XML processor may, but need not, make it possible for an application to retrieve the text of comments. For compatibility, the string -- (double-hyphen) must not occur within comments. Comment '<!&como;' (Char* - (Char* '&comc;' Char*)) '&comc;>'

An example of a comment: <!&como; declarations for <head> & <body> &comc;>

Processing Instructions

Processing instructions (PIs) allow documents to contain instructions for applications. PI '<?' Name S (Char* - (Char* &pic; Char*)) &pic; PIs are not part of the document's character data, but must be passed through to the application. The Name is called the PI target; it is used to identify the application to which the instruction is directed. XML provides an optional mechanism, NOTATION, for formal declaration of such names. PI targets with names beginning with the string "XML" are reserved for standardization in this or future versions of this specification.

CDATA Sections

CDATA sections can occur anywhere character data may occur; they are used to escape blocks of text containing characters which would otherwise be recognized as markup. CDATA sections begin with the string <![CDATA[ and end with the string ]]>: CDSect CDStart CData CDEnd CDStart '<![CDATA[' CData (Char* - (Char* ']]>' Char*)) CDEnd ']]>' Within a CDATA section, only the CDEnd string is recognized, so that left angle brackets and ampersands may occur in their literal form; they need not (and cannot) be escaped using &lt; and &amp;. CDATA sections cannot nest.

An example of a CDATA section: <![CDATA[<greeting>Hello, world!</greeting>]]>

White Space Handling

In editing XML documents, it is often convenient to use "white space" (spaces, tabs, and blank lines, denoted by the nonterminal S in this specification) to set apart the markup for greater readability. Such white space is typically not intended for inclusion in the delivered version of the document. On the other hand, "significant" white space that must be retained in the delivered version is common, for example in poetry and source code.

An XML processor must always pass all characters in a document that are not markup through to the application. A validating XML processor must distinguish white space in element content from other non-markup characters and signal to the application that white space in element content is not significant.

A special attribute may be inserted in documents to signal an intention that the element to which this attribute applies requires all white space to be treated as significant by applications.

In valid documents, this attribute must be declared as follows, if used:

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 this element, unless overriden with another instance of the XML-SPACE attribute.

The root element of any document is considered to have signaled no intentions as regards application space handling, unless it provides a value for this attribute or the attribute is declared with a default value.

Prolog and Document Type Declaration

XML documents may, and should, begin with an XML declaration which specifies, among other things, the version of XML being used.

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

The document type declaration must appear before the first start-tag in the document. document prolog element Misc* prolog XMLDecl? Misc* (doctypedecl Misc*)? XMLDecl &xmlpio; VersionInfo EncodingDecl? RMDecl? S? &pic; VersionInfo S 'version' Eq ('"&XML.version;"' | "'&XML.version;'") Misc Comment | PI | S

The identification of the XML version as "1.0" does not indicate a commitment to produce any future versions of XML, nor if any are produced, to use any particular numbering scheme. Since future versions are not ruled out, this construct is provided as a means to allow the possibility of automatic version recognition, should it become necessary.

For example, the following is a complete XML document, well-formed but not valid: Hello, world! ]]> and so is this: Hello, world! ]]>

The XML document type declaration may include a pointer to an external entity containing a subset of the necessary markup declarations, and may also directly include another, internal, subset.

These two subsets make up the document type definition, abbreviated DTD. The DTD, in effect, provides a grammar which defines a class of documents. Properly speaking, the DTD consists of both subsets taken together, but it is a common practice for the bulk of the markup declarations to appear in the external subset, and for this subset, usually contained in a file, to be referred to as "the DTD" for a class of documents. doctypedecl '<!DOCTYPE' S Name (S ExternalID)? S? ('[' %markupdecl* ']' S?)? '>' Root Element Type Non-null DTD markupdecl ( %elementdecl | %AttlistDecl | %EntityDecl | %NotationDecl | %PI | %S | %Comment | InternalPERef )* InternalPERef PEReference Integral Declarations The Name in the document-type declaration must match the element type of the root element. The internal and external subsets of the DTD must not both be empty. A parameter-entity reference recognized in this context must have replacement text consisting of zero or more complete declarations, i.e. matching the production for the non-terminal markupdecl.

The external subset must obey substantially the same grammatical constraints as the internal subset; i.e. it must match the production for the non-terminal symbol markupdecl. In the external subset, however, parameter-entity references can be used to replace constructs prefixed by % in a production of the grammar, and conditional sections may occur. In the internal subset, by contrast, conditional sections may not occur and the only parameter-entity references allowed are those which match the non-terminal InternalPERef within the rule for markupdecl. extSubset ( %markupdecl | %conditionalSect )*

For example: Hello, world! ]]> The system identifier hello.dtd indicates the location of a DTD for the document.

The declarations can also be given locally, as in this example: ]> Hello, world! ]]>

If both the external and internal subsets are used, an XML processor must read the internal subset first, then the external subset. This has the effect that entity and attribute declarations in the internal subset take precedence over those in the external subset.

Required Markup Declaration

In some cases, an XML processor can read an XML document and accomplish useful tasks without having first processed the entire DTD. However, certain declarations can substantially affect the actions of an XML processor. It is desirable, therefore, to be able to specify whether a document contains any such declarations. A document author can communicate whether or not DTD processing is necessary using a required markup declaration (abbreviated RMD), which appears as a component of the XML declaration: RMDecl S 'RMD' Eq "'" ('NONE' | 'INTERNAL' | 'ALL') "'" | S 'RMD' Eq '"' ('NONE' | 'INTERNAL' | 'ALL') '"'

In an RMD, the value NONE indicates that an XML processor can parse the containing document correctly without first reading any part of the DTD. The value INTERNAL indicates that the XML processor must read and process the internal subset of the DTD, if provided, to parse the containing document correctly. The value ALL indicates that the XML processor must read and process the declarations in both the subsets of the DTD, if provided, to parse the containing document correctly.

The RMD must indicate that the entire DTD is required if the external subset contains any declarations of

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

entities (other than &magicents;), if references to those entities appear in the document, or

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

If such declarations occur in the internal but not the external subset, the RMD must take the value INTERNAL. It is an error to specify INTERNAL if the external subset is required, or to specify NONE if the internal or external subset is required.

If no RMD is provided, an XML processor must behave as though an RMD had been provided with the value ALL.

An example XML declaration with an RMD:<?XML version="&XML.version;" RMD='INTERNAL'?>

Logical Structures

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

This specification does not constrain the semantics, use, or (beyond syntax) names of the elements and attributes, except that names beginning with the string XML are reserved for standardization in this or future versions of this specification.

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

The beginning of every non-empty XML element is marked by a start-tag. STag '<' Name (S Attribute)* S? '>' Unique Att Spec Attribute Name Eq AttValue Attribute Value Type No External Entity References Eq S? '=' S? The Name in the start- and end-tags gives the element's type. The Name-AttValue pairs are referred to as the attribute specifications of the element, with the Name referred to as the attribute name and the content of the AttValue (the characters between the ' or " delimiters) as the attribute value.

No attribute may appear more than once in the same start-tag or empty-element tag. The attribute must have been declared; the value must be of the type declared for it. (For attribute types, see the discussion of attribute declarations.) Attribute values cannot contain entity references to external entities.

An example of a start-tag: <termdef id="dt-dog" term="dog">

The end of every element may (for elements which are not empty, must) be marked by an end-tag containing a name that echoes the element's type as given in the start-tag: ETag '</' Name S? '>'

An example of an end-tag:</termdef>

The text between the start-tag and end-tag is called the element's content: content (element | PCData | Reference | CDSect | PI | Comment)* Content element EmptyElement | STag content ETagGI Match

Each element type used must be declared. The content of an element instance must match the content model declared for that element type. The Name in an element's end-tag must match that in the start-tag.

If an element is empty, it may be represented either by a start-tag immediately followed by an end-tag, or by an empty-element tag. An Empty-element tag takes a special form: EmptyElement '<' Name (S Attribute)* S? '/>' Unique Att Spec

Empty-element tags may be used for any element which has no content, whether or not they are declared using the keyword EMPTY.

Examples of empty elements: <IMG align="left" src="http://www.w3.org/Icons/WWW/w3c_home" /> <br></br> <br/>

Element Declarations

The element structure of an XML document may, for validation purposes, be constrained using element and attribute declarations.

An element declaration constrains the element's type and its content.

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

An element declaration takes the form: elementdecl '<!ELEMENT' S %Name S (%S S)? %contentspec S? '>' Unique Element Declaration contentspec 'EMPTY' | 'ANY' | Mixed | elements where the Name gives the type of the element.

No element type may be declared more than once.

An element can declared using a content model, in which case its content can be categorized as element content or mixed content, as explained below.

An element declared using the keyword EMPTY must be empty and may be tagged using an empty-element tag when it appears in the document.

If an element type is declared using the keyword ANY, then there are no validity constraints on its content: it may contain child elements of any type and number, interspersed with character data.

Examples of element declarations: <!ELEMENT br EMPTY> <!ELEMENT %name.para; %content.para; > <!ELEMENT container ANY>

Element Content

An element type may be declared to have element content, which means that elements of that type may only contain other elements (no character data). In this case, the constraint includes a content model, a simple grammar governing the allowed types of the child elements and the order in which they appear. The grammar is built on content particles (CPs), which consist of names, choice lists of content particles, or sequence lists of content particles: elements (choice | seq) ('?' | '*' | '+')? cp (Name | choice | seq) ('?' | '*' | '+')? cps S? %cp S? choice '(' S? %ctokplus (S? '|' S? %ctoks)* S? ')' ctokplus cps ('|' cps)+ ctoks cps ('|' cps)* seq '(' S? %stoks (S? ',' S? %stoks)* S? ')' stoks cps (',' cps)* where each Name gives the type of an element which may appear as a child. Any content particle in a choice list may appear in the element content at the appropriate location; content particles occurring in a sequence list must each appear in the element content in the order given. The optional character following a name or list governs whether the element or the content particles in the list may occur one or more (+), zero or more (*), or zero or one times (?). The syntax and meaning are identical to those used in the productions in this specification.

The content of an element matches a content model if and only if it is possible to trace out a path through the content model, obeying the sequence, choice, and repetition operators and matching each element in the content against an element name in the content model. For compatibility reasons, it is an error if an element in the document can match more than one occurrence of an element name in the content model. More formally: a finite state automaton may be constructed from the content model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi, and Ullman. In many such algorithms, a follow set is constructed for each position in the regular expression (i.e., each leaf node in the syntax tree for the regular expression); if any position has a follow set in which more than one following position is labeled with the same element type name, then the content model is in error and may be reported as an error. For more information, see the appendix on deterministic content models.

Examples of element-content models: <!ELEMENT spec (front, body, back?)> <!ELEMENT div1 (head, (p | list | note)*, div2*)> <!ELEMENT head (%head.content; | %head.misc;)*>

Mixed Content

An element type may be declared to contain mixed content, that is, text comprising character data optionally interspersed with child elements. In this case, the types of the child elements are constrained, but not their order nor their number of occurrences: Mixed '(' S? %( %'#PCDATA' (S? '|' S? %Mtoks)* ) S? ')*' | '(' S? %('#PCDATA') S? ')' Mtoks %Name (S? '|' S? %Name)* where the Names give the types of elements that may appear as children. The same name must not appear more than once in a single mixed-content declaration.

Examples of mixed content declarations: <!ELEMENT p (#PCDATA|a|ul|b|i|em)*> <!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* > <!ELEMENT b (#PCDATA)>

Attribute-List Declarations

Attributes are used to associate name-value pairs with elements. Attributes may appear only within start-tags; thus, the productions used to recognize them appear in the discussion of start-tags. Attribute-list declarations may be used:

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

To establish a set of type constraints on these attributes.

To provide default values for attributes.

Attribute-list declarations specify the name, data type, and default value (if any) of each attribute associated with a given element type: AttlistDecl '<!ATTLIST' S %Name S? %AttDef+ S? '>' AttDef S %Name S %AttType S %Default The Name in the AttlistDecl rule is the type of an element. At user option, an XML processor may issue a warning if attributes are declared for an element type not itself declared, but this is not an error. The Name in the AttDef rule is the name of the attribute.

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

Attribute Types

XML attribute types are of three kinds: a string type, a set of tokenized types, and enumerated types. The string type may take any literal string as a value; the tokenized types have varying lexical and semantic constraints, as noted: AttType StringType | TokenizedType | EnumeratedType StringType 'CDATA' TokenizedType 'ID' ID | 'IDREF' Idref | 'IDREFS' Idref | 'ENTITY' Entity Name | 'ENTITIES' Entity Name | 'NMTOKEN' Name Token | 'NMTOKENS' Name Token

Values of this type must be valid Name symbols. 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. Values of this type must match the Name (for IDREFS, the Names) production; each Name must match the value of an ID attribute on some element in the XML document; i.e. IDREF values must match some ID. Values of this type must match the production for Name (for ENTITIES, Names); each Name must exactly match the name of an external binary general entity declared in the DTD. Values of this type must consist of a string matching the Nmtoken nonterminal (for NMTOKENS, the Nmtokens nonterminal) of the grammar defined in this specification.

The XML processor must normalize attribute values before passing them to the application, as described in the section on attribute-value normalization.

Enumerated attributes can take one of a list of values provided in the declaration; there are two types: EnumeratedType NotationType | Enumeration NotationType %'NOTATION' S '(' S? %Ntoks (S? '|' S? %Ntoks)* S? ')' Notation Attributes Ntoks %Name (S? '|' S? %Name)* Enumeration '(' S? %Etoks (S? '|' S? %Etoks)* S? ')' Enumeration Etoks %Nmtoken (S? '|' S? %Nmtoken)*

The names in the declaration of NOTATION attributes must be names of declared notations (see the discussion of notations). Values of this type must match one of the notation names included in the declaration. 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.
Attribute Defaults

An attribute declaration provides information on whether the attribute's presence is required, and if not, how an XML processor should react if a declared attribute is absent in a document: Default '#REQUIRED' | '#IMPLIED' Attribute Default Legal | ((%'#FIXED' S)? %AttValue) #REQUIRED means that the document is invalid should the processor encounter a start-tag for the element type in question which specifies no value for this attribute. #IMPLIED means that if the attribute is omitted from an element of this type, the XML processor must inform the application that no value was specified; no constraint is placed on the behavior of the application.

If the attribute is neither #REQUIRED nor #IMPLIED, then the AttValue value contains the declared default value. If the #FIXED is present, the document is invalid if the attribute is present with a different value from the default. If a default value is declared, when an XML processor encounters an omitted attribute, it is to behave as though the attribute were present with its value being the declared default value.

The declared default value must meet the constraints of the declared attribute type.

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

Attribute-Value Normalization

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

Line-end characters (or, on some systems, record boundaries) must be replaced by single space (#x20) characters.

Character references and references to internal text entities must be expanded. References to external entities are an error.

If the attribute is not of type CDATA, all strings of white space must be normalized to single space characters (#x20), and leading and trailing white space must be removed.

Values of type ID, IDREF, IDREFS, NMTOKEN, NMTOKENS, or of enumerated or notation types, must be folded to uppercase.

If no DTD is present, attributes should be treated as CDATA.

Conditional Sections

Conditional sections are portions of the document type declaration external subset which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them. conditionalSect includeSect | ignoreSect includeSect '<![' %'INCLUDE' '[' (%markupdecl*)* ']]>' ignoreSect '<![' %'IGNORE' '[' ignoreSectContents* ']]>' ignoreSectContents ((SkipLit | Comment | PI) - (Char* ']]>' Char*)) | ('<![' ignoreSectContents* ']]>') | (Char - (']' | [<'"])) | ('<!' (Char - ('-' | '[')))

Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, processing instructions, or nested conditional sections, intermingled with white space.

If the keyword of the conditional section is INCLUDE, then the conditional section is read and processed in the normal way. If the keyword is IGNORE, then the declarations within the conditional section are ignored; the processor must read the conditional section to detect nested conditional sections and ensure that the end of the outermost (ignored) conditional section is properly detected. If a conditional section with a keyword of INCLUDE occurs within a larger conditional section with a keyword of IGNORE, both the outer and the inner conditional sections are ignored.

If the keyword of the conditional section is a parameter entity reference, the parameter entity is replaced by its value 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?)> ]]>

Physical Structures

An XML document may consist of one or many virtual storage units. These are called entities; they are identified by name and have content. An entity may be stored in, but need not comprise the whole of, a single physical storage object such as a file or database field. Each XML document has one entity called the document entity, which serves as the starting point for the XML processor (and may contain the whole document).

Entities may be either binary or text. A text entity contains text data which is considered as an integral part of the document. A binary entity contains binary data with an associated notation. Only text entities may be referred to using entity references; only the names of binary entities may be given as the value of ENTITY attributes.

Logical and Physical Structures

The logical and physical structures (elements and entities) in an XML document must be synchronous. Tags and elements must each begin and end in the same entity, but may refer to other entities internally; comments, processing instructions, character references, and entity references must each be contained entirely within a single entity. Entities must each contain an integral number of elements, comments, processing instructions, and references, possibly together with character data not contained within any element in the entity, or else they must contain non-textual data, which by definition contains no elements.

Character and Entity References

A character reference refers to a specific character in the ISO/IEC 10646 character set, e.g. one not directly accessible from available input devices: CharRef '&#' [0-9]+ ';' | '&hcro;' [0-9a-fA-F]+ ';'

An entity reference refers to the content of a named entity. General entities are text entities for use within the document itself; references to them use ampersand (&) and semicolon (;) as delimiters. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity. Parameter entities are text entities for use within the DTD, or to control processing of conditional sections; references to them use percent-sign (%) and semicolon (;) as delimiters. Reference EntityRef | CharRef EntityRef '&' Name ';' Entity Declared Text Entity No Recursion PEReference '%' Name ';' Entity Declared Text Entity No Recursion In DTD

The Name given in the entity reference must exactly match the name given in the declaration of the entity, except that well-formed documents need not declare any of the following entities: &magicents;. In valid documents, these entities must be declared, in the form specified in the section on predefined entities. In the case of parameter entities, the declaration must precede the reference. An entity reference must not contain the name of a binary entity. Binary entities may be referred to only in attribute values declared to be of type ENTITY or ENTITIES. A text or parameter entity must not contain a recursive reference to itself, either directly or indirectly. In the external DTD subset, a parameter-entity reference is recognized only at the locations where the nonterminal PEReference or the special operator % appears in a production of the grammar. In the internal subset, parameter-entity references are recognized only when they match the InternalPERef non-terminal in the production for markupdecl.

Examples of character and entity references: Type <key>less-than</key> (&hcro;3C;) to save options. This document was prepared on &docdate; and is classified &security-level;.

Example of a parameter-entity reference: <!ENTITY % ISOLat2 SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" > %ISOLat2;

Entity Declarations

Entities are declared thus: EntityDecl '<!ENTITY' S %Name S %EntityDef S? '>' General entities | '<!ENTITY' S '%' S %Name S %EntityDef S? '>' Parameter entities EntityDef EntityValue | ExternalDef The Name is that by which the entity is invoked by exact match in an entity reference. If the same entity is declared more than once, the first declaration encountered is binding; at user option, an XML processor may issue a warning if entities are declared multiple times.

Internal Entities

If the entity definition is an EntityValue, the defined entity is called an internal entity. There is no separate physical storage object, and the replacement text of the entity is given in the declaration. Within the EntityValue, parameter-entity references and character references are recognized and expanded immediately. General-entity references within the replacement text are not recognized at the time the entity declaration is parsed, though they may be recognized when the entity itself is referred to.

An internal entity is a text entity.

Example of an internal entity declaration: <!ENTITY Pub-Status "This is a pre-release of the specification.">

External Entities

If the entity is not internal, it is an external entity, declared as follows: ExternalDef ExternalID %NDataDecl? ExternalID 'SYSTEM' S SystemLiteral |  'PUBLIC' S PubidLiteral S SystemLiteral NDataDecl S %'NDATA' S %Name Notation Declared If the NDataDecl is present, this is a binary data entity, otherwise a text entity.

The Name must match the declared name of a notation.

The SystemLiteral that follows the keyword SYSTEM is called the entity's system identifier. It is a URL, which may be used to retrieve the entity. Unless otherwise provided by information outside the scope of this specification (e.g. a special XML element defined by a particular DTD, or a processing instruction defined by a particular application specification), relative URLs are relative to the location of the entity or file within which the entity declaration occurs. Relative URLs in entity declarations within the internal DTD subset are thus relative to the location of the document; those in entity declarations in the external subset are relative to the location of the files containing the external subset.

In addition to a system literal, an external identifier may include a public identifier. An XML processor may use the public identifier to try to generate an alternative URL. If the processor is unable to do so, it must use the URL specified in the system literal.

Examples of external entity declarations: <!ENTITY open-hatch 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 >

Character Encoding in Entities

Each external text entity in an XML document may use a different encoding for its characters. All XML processors must be able to read entities in either UTF-8 or UCS-2. It is recognized that for some purposes, the use of additional ISO/IEC 10646 planes other than the Basic Multilingual Plane may be required. A facility for handling characters in these planes is therefore a desirable characteristic in XML processors and applications.

Entities encoded in UCS-2 must begin with the Byte Order Mark described by ISO/IEC 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or character data of the XML document. XML processors must be able to use this character to differentiate between UTF-8 and UCS-2 encoded documents.

Although an XML processor is only required to read entities in the UTF-8 and UCS-2, it is recognized that many other encodings are in daily use around the world, and it may be advantageous for XML processors to read entities that use these encodings. For this purpose, XML provides an encoding declaration processing instruction, which, if it occurs, must appear at the beginning of a system entity, before any other character data or markup. In the document entity, the encoding declaration is part of the XML declaration; in other entities, it is part of an encoding processing instruction: EncodingDecl S 'encoding' Eq QEncoding Start of System Entity EncodingPI &xmlpio; S 'encoding' Eq QEncoding S? &pic; QEncoding '"' Encoding '"' | "'" Encoding "'" Encoding LatinName LatinName [A-Za-z] ([A-Za-z0-9._] | '-')* Name containing only Latin characters

An XML encoding declaration may occur at the beginning of a system entity; it must not occur within the body of any entity.

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-8859-9 should be used for the parts of ISO 8859, and the values ISO-2022-JP, Shift_JIS, and EUC-JP should be used for the various encoded forms of JIS X-0208. XML processors may recognize other encodings; it is recommended that character encodings registered (as charsets) with the Internet Assigned Numbers Authority (IANA), other than those just listed, should be referred to using their registered names.

It is an error for an entity including an encoding declaration to be presented to the XML processor in an encoding other than that named in the declaration.

An entity which begins with neither a Byte Order Mark nor an encoding declaration must be in the UTF-8 encoding.

While XML provides mechanisms for distinguishing encodings, it is recognized that in a heterogeneous networked environment, it may be difficult to signal the encoding of an entity reliably. Errors in this area fall into two categories:

failing to read an entity because of inability to recognize its actual encoding, and

reading an entity incorrectly because of an incorrect guess of its proper encoding.

The first class of error is extremely damaging, and given a correct encoding declaration, the second class is extremely unlikely. For these reasons, XML processors should make an effort to use all available information, internal and external, to aid in detecting an entity's correct encoding. Such information may include, but is not limited to:

Using information from an HTTP header

Using a MIME header obtained other than through HTTP

Metadata provided by the native OS file system or by document management software

Analysing the bit patterns at the front of an entity to determine if the application of any known encoding yields a valid encoding declaration. See the appendix on autodetection of character sets for a fuller description.

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

Examples of encoding declarations: <?XML ENCODING='UTF-8'?> <?XML ENCODING='EUC-JP'?>

Document Entity

The document entity serves as the root of the entity tree and a starting-point for an XML processor. This specification does not specify how the document entity is to be located by an XML processor; unlike other entities, the document entity might well appear on an input stream of the processor without any identification at all.

XML Processor Treatment of Entities

XML allows character and general-entity references in two places: the content of elements (content) and attribute values (AttValue). When an XML processor encounters such a reference, or the name of an external binary entity as the value of an ENTITY or ENTITIES attribute, then:

In all cases, the XML processor may inform the application of the reference's occurrence and its identifier (for an entity reference, the name; for a character reference, the character number in decimal, hexadecimal, or binary form).

For both character and entity references, the processor must remove the reference itself from the text data before passing the data to the application.

For character references, the processor must pass the character indicated to the application in place of the reference.

For an external entity, the processor must inform the application of the entity's system identifier and public identifier if any.

If the external entity is binary, the processor must inform the application of the associated notation name, and the notation's associated system and public (if any) identifiers.

For an internal (text) entity, the processor must include the entity; that is, retrieve its replacement text and process it as a part of the document (i.e. as content or AttValue, whichever was being processed when the reference was recognized), passing the result to the application in place of the reference. The replacement text may contain both text and markup, which must be recognized in the usual way, except that the replacement text of entities used to escape markup delimiters (the entities &magicents;) is always treated as data. (The string AT&amp;T expands to AT&T; the remaining ampersand is not recognized as an entity-reference delimiter.)

Since the entity may contain other entity references, an XML processor may have to repeat the inclusion process recursively.

If the entity is an external text entity, then in order to validate the XML document, the processor must include the content of the entity.

If the entity is an external text entity, and the processor is not attempting to validate the XML document, the processor may, but need not, include the entity's content.

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

Entity and character references can both be used to escape the left angle bracket, ampersand, and other delimiters. A set of general entities (&magicents;) is specified for this purpose. Numeric character references may also be used; they are expanded immediately when recognized, and must be treated as character data, so the numeric character references &#60; and &#38; may be used to escape < and & when they occur in character data.

XML allows parameter-entity references in a variety of places within the DTD. Parameter-entity references are always expanded immediately upon being recognized, and the DTD must match the relevant rules of the grammar after all parameter-entity references have been expanded. In addition, parameter entities referred to in specific contexts are required to satisfy certain constraints in their replacement text; for example, a parameter entity referred to within the internal DTD subset must match the rule for markupdecl.

Implementors of XML processors need to know the rules for expansion of references in more detail. These rules only come into play when the replacement text for an internal entity itself contains other references.

In the replacement text of an internal entity, parameter-entity references and character references in the replacement text are recognized and resolved when the entity declaration is parsed, before the replacement text is stored in the processor's symbol table. General-entity references in the replacement text are not resolved when the entity declaration is parsed.

In the document, when a general-entity reference is resolved, its replacement text is parsed. Character references encountered in the replacement text are resolved immediately; general-entity references encountered in the replacement text may be resolved or left unresolved, as described above. Character and general-entity references must be contained entirely within the entity's replacement text.

Simple character references do not suffice to escape delimiters within the replacement text of an internal entity: they will be expanded when the entity declaration is parsed, before the replacement text is stored in the symbol table. When the entity itself is referred to, the replacement text will be parsed again, and the delimiters (no longer character references) will be recognized as delimiters. To escape the characters &magicents; in an entity replacement text, use a general-entity reference or a doubly-escaped character reference. See the appendix on expansion of entity references for detailed examples.

Predefined Entities

As mentioned in the discussion of Character Data and Markup, the characters used as markup delimiters by XML may all be escaped using entity references (for the entities &magicents;).

All XML processors must recognize these entities whether they are declared or not. Valid XML documents must declare these entities, like any others, before using them.

If the entities in question are declared, they must be declared as internal entities whose replacement text is the single character being escaped, as shown below. ]]>

Notation Declarations

Notations identify by name the format of external binary entities, or the application to which processing instructions are addressed.

Notation declarations provide a name for the notation, for use in entity and attribute-list declarations and in attribute-value specifications, and an external identifier for the notation which may allow an XML processor or its client application to locate a helper application capable of processing data in the given notation. NotationDecl '<!NOTATION' S %Name S %ExternalID S? '>'

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

Conformance

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

Validating and non-validating systems alike must report violations of the well-formedness constraints given in this specification.

Validating processors must report locations in which the document does not comply with the constraints expressed by the declarations in the DTD. They must also report all failures to fulfill the validity constraints given in this specification.

XML and SGML

XML is designed to be a subset of SGML, in that every valid XML document should also be a conformant SGML document, using the same DTD, and that the parse trees produced by an SGML parser and an XML processor should be the same. To achieve this, XML was defined by removing features and options from the specification of SGML.

The following list describes syntactic characteristics which XML does not allow but which are legal in SGML. The list may not be complete.

No tag omission.

Special tag-form for empty elements.

Comment declarations must have the delimiters <!&como; comment text &comc;> and can't have spaces within the markup of <!&como; or &comc;>.

No comments (&como; ... &comc;) inside markup declarations.

Comment declarations can't jump in and out of comments with &como; &comc;.

No name groups for declaring multiple elements or making a single ATTLIST declaration apply to multiple elements.

No RANK feature.

No CDATA or RCDATA declared content in element declarations. (Use CDATA sections instead.)

No exclusions or inclusions on content models.

No minimization parameters on element declarations.

Mixed content models must be optional-repeatable OR-groups, with #PCDATA first.

No AND (&) content model groups.

No NAME[S], NUMBER[S], or NUTOKEN[S] declared values for attributes. (Use NMTOKEN[S] or CDATA with application-specific validation instead.)

No #CURRENT or #CONREF declared values for attributes.

Attribute default values must be quoted.

Marked sections can't have spaces within the markup of <![keyword[ or ]]>.

No RCDATA, TEMP, IGNORE, or INCLUDE marked sections in document instances.

Marked sections in document instances must use the CDATA keyword literally, not a parameter entity.

No CDATA, RCDATA, or TEMP marked sections in the DTD.

Some restrictions on the content of ignored marked-sections: comments, literals, and processing instructions in ignored sections may not contain the delimiter string ]]>; this helps ensure that the end-point of the conditional section does not change when the section is changed from IGNORE to INCLUDE.

No SDATA, CDATA, or bracketed internal entities.

No SUBDOC, CDATA, or SDATA external entities.

External entities must have a system identifier.

Parameter-entity references in the internal DTD subset may occur only between declarations.

Parameter-entity references in the external DTD subset are restricted to certain positions in the grammar, and must replace whole non-terminals of the grammar; this ensures that all valid XML documents are valid SGML, and makes the restrictions on parameter-entity replacement easier to understand and implement.

No data attributes on NOTATIONs or attribute value specifications on ENTITY declarations.

No SHORTREF declarations.

No USEMAP declarations.

No LINKTYPE declarations.

No LINK declarations.

No USELINK declarations.

No IDLINK declarations.

No SGML declarations.

In most current SGML systems, XML documents should be able to use the following SGML declaration. Some systems (those which take the document character set to be a description of the input stream) may require different declarations, depending on the character set and the capacities and quantities required. " PIC "?>" SHORTREF NONE NAMES SGMLREF ]]> ]]>

Character Classes

Following the characteristics defined in the Unicode standard, characters are classed as base characters (among others, these contain the alphabetic characters of the Latin alphabet, without diacritics), ideographic characters, combining characters (among others, this class contains most diacritics); these classes combine to form the class of letters. Digits, extenders, and characters which should be ignored for purposes of recognizing identifiers are also distinguished. BaseChar [#x41-#x5A] | [#x61-#x7A] Latin 1 upper and lowercase | #xAA | #xB5 | #xBA | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#xFF] Latin 1 supplementary | [#x0100-#x017F] | [#x0180-#x01F5] | [#x01FA-#x0217] Extended Latin-A and B | [#x0250-#x02A8] IPA Extensions | [#x02B0-#x02B8] | [#x02BB-#x02C1] | [#x02E0-#x02E4] Spacing Modifiers | #x037A | #x0386 | [#x0388-#x038A] | #x038C | [#x038E-#x03A1] | [#x03A3-#x03CE] | [#x03D0-#x03D6] | #x03DA | #x03DC | #x03DE | #x03E0 | [#x03E2-#x03F3] Greek and Coptic | [#x0401-#x040C] | [#x040E-#x044F] | [#x0451-#x045C] | [#x045E-#x0481] | [#x0490-#x04C4] | [#x04C7-#x04C8] | [#x04CB-#x04CC] | [#x04D0-#x04EB] | [#x04EE-#x04F5] | [#x04F8-#x04F9] Cyrillic | [#x0531-#x0556] | #x0559 | [#x0561-#x0587] Armenian | [#x05D0-#x05EA] | [#x05F0-#x05F2] Hebrew | [#x0621-#x063A] | [#x0641-#x064A] | [#x0671-#x06B7] | [#x06BA-#x06BE] | [#x06C0-#x06CE] | [#x06D0-#x06D3] | #x06D5 | [#x06E5-#x06E6] Arabic | [#x0905-#x0939] | #x093D | [#x0958-#x0961] Devanagari | [#x0985-#x098C] | [#x098F-#x0990] | [#x0993-#x09A8] | [#x09AA-#x09B0] | #x09B2 | [#x09B6-#x09B9] | [#x09DC-#x09DD] | [#x09DF-#x09E1] | [#x09F0-#x09F1] Bengali | [#x0A05-#x0A0A] | [#x0A0F-#x0A10] | [#x0A13-#x0A28] | [#x0A2A-#x0A30] | [#x0A32-#x0A33] | [#x0A35-#x0A36] | [#x0A38-#x0A39] | [#x0A59-#x0A5C] | #x0A5E | [#x0A72-#x0A74] | [#x0A85-#x0A8B] | #x0A8D Gurmukhi | [#x0A8F-#x0A91] | [#x0A93-#x0AA8] | [#x0AAA-#x0AB0] | [#x0AB2-#x0AB3] | [#x0AB5-#x0AB9] | #x0ABD | #x0AE0 Gujarati | [#x0B05-#x0B0C] | [#x0B0F-#x0B10] | [#x0B13-#x0B28] | [#x0B2A-#x0B30] | [#x0B32-#x0B33] | [#x0B36-#x0B39] | #x0B3D | [#x0B5C-#x0B5D] | [#x0B5F-#x0B61] Oriya | [#x0B85-#x0B8A] | [#x0B8E-#x0B90] | [#x0B92-#x0B95] | [#x0B99-#x0B9A] | #x0B9C | [#x0B9E-#x0B9F] | [#x0BA3-#x0BA4] | [#x0BA8-#x0BAA] | [#x0BAE-#x0BB5] | [#x0BB7-#x0BB9] Tamil | [#x0C05-#x0C0C] | [#x0C0E-#x0C10] | [#x0C12-#x0C28] | [#x0C2A-#x0C33] | [#x0C35-#x0C39] | [#x0C60-#x0C61] Telugu | [#x0C85-#x0C8C] | [#x0C8E-#x0C90] | [#x0C92-#x0CA8] | [#x0CAA-#x0CB3] | [#x0CB5-#x0CB9] | #x0CDE | [#x0CE0-#x0CE1] Kannada | [#x0D05-#x0D0C] | [#x0D0E-#x0D10] | [#x0D12-#x0D28] | [#x0D2A-#x0D39] | [#x0D60-#x0D61] Malayalam | [#x0E01-#x0E2E] | #x0E30 | [#x0E32-#x0E33] | [#x0E40-#x0E45] Thai | [#x0E81-#x0E82] | #x0E84 | [#x0E87-#x0E88] | #x0E8A | #x0E8D | [#x0E94-#x0E97] | [#x0E99-#x0E9F] | [#x0EA1-#x0EA3] | #x0EA5 | #x0EA7 | [#x0EAA-#x0EAB] | [#x0EAD-#x0EAE] | #x0EB0 | [#x0EB2-#x0EB3] | #x0EBD | [#x0EC0-#x0EC4] | [#x0EDC-#x0EDD] Lao | [#x0F40-#x0F47] | [#x0F49-#x0F69] Tibetan | [#x10A0-#x10C5] | [#x10D0-#x10F6] Georgian | [#x1100-#x1159] | [#x115F-#x11A2] | [#x11A8-#x11F9] Hangul Jamo | [#x1E00-#x1E9B] | [#x1EA0-#x1EF9] Add'l Extended Latin | [#x1F00-#x1F15] | [#x1F18-#x1F1D] | [#x1F20-#x1F45] | [#x1F48-#x1F4D] | [#x1F50-#x1F57] | #x1F59 | #x1F5B | #x1F5D | [#x1F5F-#x1F7D] | [#x1F80-#x1FB4] | [#x1FB6-#x1FBC] | #x1FBE | [#x1FC2-#x1FC4] | [#x1FC6-#x1FCC] | [#x1FD0-#x1FD3] | [#x1FD6-#x1FDB] | [#x1FE0-#x1FEC] | [#x1FF2-#x1FF4] | [#x1FF6-#x1FFC] Greek Extensions | #x207F Super-, subscripts | #x2102 | #x2107 | [#x210A-#x2113] | #x2115 | [#x2118-#x211D] | #x2124 | #x2126 | #x2128 | [#x212A-#x2131] | [#x2133-#x2138] Letterlike Symbols | [#x2160-#x2182] Number forms | [#x3041-#x3094] Hiragana | [#x30A1-#x30FA] Katakana | [#x3105-#x312C] Bopomofo | [#x3131-#x318E] Hangul Jamo | [#xAC00-#xD7A3] Hangul syllables | [#xFB00-#xFB06] | [#xFB13-#xFB17] | [#xFB1F-#xFB28] | [#xFB2A-#xFB36] | [#xFB38-#xFB3C] | #xFB3E | [#xFB40-#xFB41] | [#xFB43-#xFB44] | [#xFB46-#xFB4F] Alphabetic presentation forms | [#xFB50-#xFBB1] | [#xFBD3-#xFD3D] | [#xFD50-#xFD8F] | [#xFD92-#xFDC7] | [#xFDF0-#xFDFB] | [#xFE70-#xFE72] | #xFE74 | [#xFE76-#xFEFC] Arabic presentation forms | [#xFF21-#xFF3A] | [#xFF41-#xFF5A] | [#xFF66-#xFF6F] | [#xFE71-#xFF9D] | [#xFFA0-#xFFBE] | [#xFFC2-#xFFC7] | [#xFFCA-#xFFCF] | [#xFFD2-#xFFD7] | [#xFFDA-#xFFDC] Half- and fullwidth forms Ideographic [#x4E00-#x9FA5] | [#xF900-#xFA2D] | #x3007 | [#x3021-#x3029] CombiningChar [#x0300-#x0345] | [#x0360-#x0361] | [#x0483-#x0486] | [#x0591-#x05A1] | [#x05A3-#x05B9] | [#x05BB-#x05BD] | #x05BF | [#x05C1-#x05C2] | #x05C4 | [#x064B-#x0652] | #x0670 | [#x06D6-#x06DC] | [#x06DD-#x06DF] | [#x06E0-#x06E4] | [#x06E7-#x06E8] | [#x06EA-#x06ED] | [#x0901-#x0903] | #x093C | [#x093E-#x094C] | #x094D | [#x0951-#x0954] | [#x0962-#x0963] | [#x0981-#x0983] | #x09BC | #x09BE | #x09BF | [#x09C0-#x09C4] | [#x09C7-#x09C8] | [#x09CB-#x09CD] | #x09D7 | [#x09E2-#x09E3] | #x0A02 | #x0A3C | #x0A3E | #x0A3F | [#x0A40-#x0A42] | [#x0A47-#x0A48] | [#x0A4B-#x0A4D] | [#x0A70-#x0A71] | [#x0A81-#x0A83] | #x0ABC | [#x0ABE-#x0AC5] | [#x0AC7-#x0AC9] | [#x0ACB-#x0ACD] | [#x0B01-#x0B03] | #x0B3C | [#x0B3E-#x0B43] | [#x0B47-#x0B48] | [#x0B4B-#x0B4D] | [#x0B56-#x0B57] | [#x0B82-#x0B83] | [#x0BBE-#x0BC2] | [#x0BC6-#x0BC8] | [#x0BCA-#x0BCD] | #x0BD7 | [#x0C01-#x0C03] | [#x0C3E-#x0C44] | [#x0C46-#x0C48] | [#x0C4A-#x0C4D] | [#x0C55-#x0C56] | [#x0C82-#x0C83] | [#x0CBE-#x0CC4] | [#x0CC6-#x0CC8] | [#x0CCA-#x0CCD] | [#x0CD5-#x0CD6] | [#x0D02-#x0D03] | [#x0D3E-#x0D43] | [#x0D46-#x0D48] | [#x0D4A-#x0D4D] | #x0D57 | #x0E31 | [#x0E34-#x0E3A] | [#x0E47-#x0E4E] | #x0EB1 | [#x0EB4-#x0EB9] | [#x0EBB-#x0EBC] | [#x0EC8-#x0ECD] | [#x0F18-#x0F19] | #x0F35 | #x0F37 | #x0F39 | #x0F3E | #x0F3F | [#x0F71-#x0F84] | [#x0F86-#x0F8B] | [#x0F90-#x0F95] | #x0F97 | [#x0F99-#x0FAD] | [#x0FB1-#x0FB7] | #x0FB9 | [#x20D0-#x20DC] | #x20E1 | [#x302A-#x302F] | #x3099 | #x309A | #xFB1E | [#xFE20-#xFE23] Letter BaseChar Ideographic Digit [#x30-#x39] ISO 646 digits | [#x0660-#x0669] Arabic-Indic digits | [#x06F0-#x06F9] Eastern Arabic-Indic digits | [#x0966-#x096F] Devanagari digits | [#x09E6-#x09EF] Bengali digits | [#x0A66-#x0A6F] Gurmukhi digits | [#x0AE6-#x0AEF] Gujarati digits | [#x0B66-#x0B6F] Oriya digits | [#x0BE7-#x0BEF] Tamil digits (no zero) | [#x0C66-#x0C6F] Telugu digits | [#x0CE6-#x0CEF] Kannada digits | [#x0D66-#x0D6F] Malayalam digits | [#x0E50-#x0E59] Thai digits | [#x0ED0-#x0ED9] Lao digits | [#x0F20-#x0F29] Tibetan digits | [#xFF10-#xFF19] Fullwidth digits Ignorable [#x200C-#x200F] zw layout | [#x202A-#x202E] bidi formatting | [#x206A-#x206F] alt formatting | #xFEFF zw nonbreak space Extender #xB7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46 | #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309B-#x309E] | [#x30FC-#x30FE] | #xFF70 | #xFF9E | #xFF9F

Expansion of Entity and Character References

This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion.

If the DTD contains the declaration An ampersand (&#38;) may be escaped numerically (&#38;#38;) or with a general entity (&amp;).

" > ]]> then the XML processor will recognize the character references when it parses the entity declaration, and resolve them before storing the following string as the value of the entity example: An ampersand (&) may be escaped numerically (&#38;) or with a general entity (&amp;).

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

A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference. 2 4 5 ' > 6 %xx; 7 ]> 8 This sample shows a &tricky; method. ]]> This produces the following

in line 4, the reference to character 37 is expanded immediately, and the parameter entity xx is stored in the symbol table with the value %zz;. Since the replacement text is not rescanned, the reference to parameter entity zz is not recognized. (And it would be an error if it were, since zz is not yet declared.)

in line 5, the character reference &#60; is expanded immediately and the parameter entity zz is stored with the replacement text <!ENTITY tricky "error-prone" >, which is a well-formed entity declaration.

in line 6, the reference to xx is recognized, and the replacement text of xx (namely %zz;) is parsed. The reference to zz is recognized in its turn, and its replacement text (<!ENTITY tricky "error-prone" >) is parsed. The general entity tricky has now been declared, with the replacement text error-prone.

in line 8, the reference to the general entity tricky is recognized, and it is expanded, so the full content of the test element is the self-describing (and ungrammatical) string This sample shows a error-prone method.

Deterministic Content Models

For compatibility, it is required that content models in element declarations be deterministic. SGML requires deterministic content models (it calls them `unambiguous'); XML processors built using SGML systems may flag non-deterministic content models as errors.

For example, the content model ((b, c) | (b, d)) is non-deterministic, because given an initial b the parser cannot know which b in the model is being matched without looking ahead to see which element follows the b. In this case, the two references to b can be collapsed into a single reference, making the model read (b, (c | d)). An initial b now clearly matches only a single name in the content model. The parser doesn't need to look ahead to see what follows; either c or d would be accepted.

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

Autodetection of Character Encodings

The XML encoding declaration functions as an internal label on each entity, indicating which character encoding is in use. Before an XML processor can read the internal label, however, it apparently has to know what character encoding is in use—which is what the internal label is trying to indicate. In the general case, this is a hopeless situation. It is not entirely hopeless in XML, however, because XML limits the general case in two ways: each implementation is assumed to support only a finite set of character encodings, and the XML encoding declaration is restricted in position and content in order to make it feasible to autodetect the character encoding in use in each entity in normal cases.

Because each XML entity not in UTF-8 or UCS-2 format must begin with an XML encoding declaration, in which the first characters must be '<?XML', any conforming processor can detect, after two to four octets of input, which of the following cases apply (in reading this list, it may help to know that in UCS-4, '<' is #x0000003C and '?' is #x0000003F, and the Byte Order Mark required of UCS-2 data streams is #xFEFF):

00 00 00 3C: UCS-4, big-endian machine (1234 order)

3C 00 00 00: UCS-4, little-endian machine (4321 order)

00 00 3C 00: UCS-4, unusual octet order (2143)

00 3C 00 00: UCS-4, unusual octet order (3412)

FE FF: UCS-2, big-endian

FF FE: UCS-2, little-endian

00 3C 00 3F: UCS-2, big-endian, no Byte Order Mark (and thus, strictly speaking, in error)

3C 00 3F 00: UCS-2, little-endian, no Byte Order Mark (and thus, strictly speaking, in error)

3C 3F 58 4D: UTF-8, ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any other 7-bit, 8-bit, or mixed-width encoding which ensures that the characters of ASCII have their normal positions, width, and values; the actual encoding declaration must be read to detect which of these applies, but since all of these encodings use the same bit patterns for the ASCII characters, the encoding declaration itself may be read reliably

4C 6F E7 D4: EBCDIC (in some flavor; the full encoding declaration must be read to tell which code page is in use)

other: UTF-8 without an encoding declaration, or else the data are corrupt, fragmentary, or enclosed in a wrapper of some kind

This level of autodetection is enough to read the XML encoding declaration and parse the character-encoding identifier, which is still necessary to distinguish the individual members of each family of encodings (e.g. to tell UTF-8 from 8859, and the parts of 8859 from each other, or to distinguish the specific EBCDIC code page in use, and so on).

Because the contents of the encoding declaration are restricted to ASCII characters, a processor can reliably read the entire encoding declaration as soon as it has detected which family of encodings is in use. Since in practice, all widely used character encodings fall into one of the categories above, the XML encoding declaration allows reasonably reliable in-line labeling of character encodings, even when external sources of information at the operating-system or transport-protocol level are unreliable.

Once the processor has detected the character encoding in use, it can act appropriately, whether by invoking a separate input routine for each case, or by calling the proper conversion function on each character of input.

Like any self-labeling system, the XML encoding declaration will not work if any software changes the entity's character set or encoding without updating the encoding declaration. Implementors of character-encoding routines should be careful to ensure the accuracy of the internal and external information used to label the entity.

A Trivial Grammar for XML Documents

The grammar given in the body of this specification is relatively simple, but for some purposes it is convenient to have an even simpler one. A very simple, though non-conforming, XML processor could parse a well-formed XML document using the following simplified grammar, recognizing all element boundaries correctly, though not expanding entity references and not detecting all errors: simpleDoc (SimpleData | Markup)* SimpleData [^<&]* cf. PCData SimpleLit ('"' [^"]* '"') | ("'" [^']* "'") cf. SkipLit Markup '<' Name (S Name S? '=' S? SimpleLit)* S? '>'start-tags | '<' Name (S Name S? '=' S? SimpleLit)* S? '/>'empty elements | '</' Name S? '>' end-tags | '&' Name ';' entity references | '&#' [0-9]+ ';' decimal character references | '&hcro;' [0-9a-fA-F]+ ';' hexadecimal character references | '<!&como;' (Char* - (Char* '&comc;' Char*)) '&comc;>' comments | '<?' (Char* - (Char* &pic; Char*)) '&pic;' processing instructions | '<![CDATA[' (Char* - (Char* ']]>' Char*)) ']]>' CDATA sections | '<!DOCTYPE' (Char - ('[' | ']'))+ ('[' simpleDTD* ']')? '>' doc type declaration simpleDTD '<!&como;' (Char* - (Char* '&comc;' Char*)) '&comc;>' comment | '<?' (Char* - (Char* &pic; Char*)) '&pic;' processing instruction SimpleLit (Char - (']' | '<' | '"' | "'"))+ '<!' (Char - ('-'))+ declarations other than comment Most processors will require the more complex grammar given in the body of this specification.

References Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. Compilers: Principles, Techniques, and Tools. Reading: Addison-Wesley, 1986, rpt. corr. 1988. Brüggemann-Klein, Anne. Regular Expressions into Finite Automata. Universität Freiburg, Institut für Informatik, Bericht 33, Juli 1991. Brüggemann-Klein, Anne, and Derick Wood. Deterministic Regular Languages. Universität Freiburg, Institut für Informatik, Bericht 38, Oktober 1991. ISO (International Organization for Standardization). ISO/IEC 8879-1986 (E). Information processing — Text and Office Systems — Standard Generalized Markup Language (SGML). First edition — 1986-10-15. [Geneva]: International Organization for Standardization, 1986. ISO (International Organization for Standardization). ISO/IEC 10646-1993 (E). Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane. [Geneva]: International Organization for Standardization, 1993 (plus amendments AM 1 through AM 5). ISO (International Organization for Standardization). ISO/IEC 10744-1992 (E). Information technology — Hypermedia/Time-based Structuring Language (HyTime). [Geneva]: International Organization for Standardization, 1992. Extended Facilities Annexe. [Geneva]: International Organization for Standardization, 1996. IETF (Internet Engineering Task Force). RFC 1738: Uniform Resource Locators. 1991. The Unicode Consortium. The Unicode Standard, Version 2.0. Reading, Mass.: Addison-Wesley Developers Press, 1996. W3C XML Working Group

This specification was prepared and approved for publication by the W3C XML Working Group (WG). WG approval of this specification does not necessarily imply that all WG members voted for its approval. At the time it approved this specification, the XML WG comprised the following members:

Jon Bosak, Sun (Chair) James Clark (Technical Lead) Tim Bray, Textuality and Netscape (XML Co-editor) Jean Paoli, Microsoft (XML Co-editor) C. M. Sperberg-McQueen, U. of Ill. (XML Co-editor) Steve DeRose, INSO Dave Hollander, HP Eliot Kimber, Highland Tom Magliery, NCSA Eve Maler, ArborText Murray Maloney, Grif Peter Sharpe, SoftQuad