Theterminology used to describe XML documents is defined in the body of thisspecification. The terms defined in the following list are used in buildingthose definitions and in describing the actions of an XML processor:
- may
[Definition: Conforming documentsand XML processors are permitted to but need not behave as described.]
- must
[Definition: Conforming documentsand XML processors are required to behave as described; otherwise they arein error. ]
- error
[Definition: A violation of therules of this specification; results are undefined. Conforming software maydetect and report an error and may recover from it.]
- fatal error
[Definition: An error whicha conformingXML processormust detect and report to the application. After encountering a fatal error,the processor may continue processing the data to search for further errorsand may report such errors to the application. In order to support correctionof errors, the processor may make unprocessed data from the document (withintermingled character data and markup) available to the application. Oncea fatal error is detected, however, the processor must not continue normalprocessing (i.e., it must not continue to pass character data and informationabout 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 ordisable the behavior described.]
- validity constraint
[Definition: A rulewhich applies to allvalid XML documents.Violations of validity constraints are errors; they must, at user option,be reported byvalidatingXML processors.]
- well-formedness constraint
[Definition:A rule which applies to allwell-formedXML documents. Violations of well-formedness constraints arefatal errors.]
- match
[Definition: (Of strings or names:)Two strings or names being compared must be identical. Characters with multiplepossible representations in ISO/IEC 10646 (e.g. characters with both precomposedand base+diacritic forms) match only if they have the same representationin both strings. No case folding is performed. (Of strings and rules in thegrammar:) A string matches a grammatical production if it belongs to the languagegenerated by that production. (Of content and content models:) An elementmatches its declaration when it conforms in the fashion described in the constraint[VC: Element Valid].]
- for compatibility
[Definition: Marksa sentence describing a feature of XML included solely to ensure that XMLremains compatible with SGML.]
- for interoperability
[Definition:Marks a sentence describing a non-binding recommendation included to increasethe chances that XML documents can be processed by the existing installedbase of SGML processors which predate the WebSGML Adaptations Annex to ISO8879.]
2Documents
[Definition: A data object is anXML document if it iswell-formed,as defined in this specification. A well-formed XML document may in additionbevalid if it meets certain furtherconstraints.]
Each XML document has both a logical and a physical structure.Physically, the document is composed of units calledentities.An entity mayrefer to otherentities to cause their inclusion in the document. A document begins in a"root" ordocument entity.Logically, the document is composed of declarations, elements, comments, characterreferences, and processing instructions, all of which are indicated in thedocument by explicit markup. The logical and physical structures must nestproperly, as described in4.3.2 Well-Formed ParsedEntities.
2.1Well-Formed XML Documents
[Definition: A textual object is awell-formed XML document if:]
Taken as a whole, it matches the production labeleddocument.
It meets all the well-formedness constraints given in this specification.
Each of theparsed entitieswhich is referenced directly or indirectly within the document iswell-formed.
Document
Matching thedocument productionimplies that:
It contains one or moreelements.
[Definition: There is exactlyone element, called theroot, or document element, no part of whichappears in thecontent of any otherelement.] For all other elements, if thestart-tagis in the content of another element, theend-tagis in the content of the same element. More simply stated, the elements, delimitedby start- and end-tags, nest properly within each other.
[Definition: Asa consequence of this, for each non-root elementC in the document,there is one other elementP in the document such thatCis in the content ofP, but is not in the content of any otherelement that is in the content ofP.P is referredto as theparent ofC, andC as achildofP.]
2.2Characters
[Definition: A parsedentity containstext, a sequence ofcharacters,which may represent markup or character data.] [Definition:Acharacter is an atomic unit of text as specified by ISO/IEC 10646[ISO/IEC 10646] (see also[ISO/IEC10646-2000]). Legal characters are tab, carriage return, line feed, andthe legal characters of Unicode and ISO/IEC 10646. The versions of these standardscited inA.1 Normative Referenceswere current at the time this document was prepared. New characters may beadded to these standards by amendments or new editions. Consequently, XMLprocessors must accept any character in the range specified forChar.The use of "compatibility characters", as defined in section 6.8 of[Unicode](see also D21 in section 3.6 of[Unicode3]), is discouraged.]
CharacterRange
| [2] | Char | ::= | #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF] | /* any Unicode character, excluding the surrogate blocks, FFFE, andFFFF. */ |
The mechanism for encoding character code points intobit patterns may vary from entity to entity. All XML processors must acceptthe UTF-8 and UTF-16 encodings of 10646; the mechanisms for signaling whichof the two is in use, or for bringing other encodings into play, are discussedlater, in4.3.3 Character Encoding in Entities.
2.3 Common SyntacticConstructs
This section defines some symbols used widely in the grammar.
S (white space) consists of one or more space (#x20) characters,carriage returns, line feeds, or tabs.
White Space
| [3] | S | ::= | (#x20 | #x9 | #xD | #xA)+ |
Characters are classified for convenience as letters,digits, or other characters. A letter consists of an alphabetic or syllabicbase character or an ideographic character. Full definitions of the specificcharacters in each class are given inB CharacterClasses.
[Definition: ANameis a token beginning with a letter or one of a few punctuation characters,and continuing with letters, digits, hyphens, underscores, colons, or fullstops, together known as name characters.] Names beginning with the string"xml", or any string which would match(('X'|'x') ('M'|'m')('L'|'l')), are reserved for standardization in this or future versionsof this specification.
Note:
TheNamespaces in XML Recommendation[XML Names] assignsa meaning to names containing colon characters. Therefore, authors shouldnot use the colon in XML names except for namespace purposes, but XML processorsmust accept the colon as a name character.
AnNmtoken(name token) is any mixture of name characters.
Names and Tokens
[4] NameChar ::= Letter |Digit| '.' | '-' | '_' | ':' |CombiningChar |Extender[5] Name ::= (Letter | '_' | ':') (NameChar)*[6] Names ::= Name (SName)*[7] Nmtoken ::= (NameChar)+[8] Nmtokens ::= Nmtoken (SNmtoken)*Literal data is any quoted string not containing the quotationmark used as a delimiter for that string. Literals are used for specifyingthe content of internal entities (EntityValue),the values of attributes (AttValue), and externalidentifiers (SystemLiteral). Note that aSystemLiteral can be parsed without scanningfor markup.
Literals
Note:
AlthoughtheEntityValue production allows the definitionof an entity consisting of a single explicit< in the literal(e.g.,<!ENTITY mylt "<">), it is strongly advised to avoidthis practice since any reference to that entity will cause a well-formednesserror.
2.4 CharacterData and Markup
Text consistsof intermingledcharacter dataand markup. [Definition:Markuptakes the form ofstart-tags,end-tags,empty-elementtags,entity references,character references,comments,CDATA section delimiters,document type declarations,processing instructions,XML declarations,text declarations, and any white space that is atthe top level of the document entity (that is, outside the document elementand not inside any other markup).]
[Definition:All text that is not markup constitutes thecharacter data of the document.]
Theampersand character (&) and the left angle bracket (<) may appear intheir literal formonly when used as markup delimiters, or withinacomment, aprocessing instruction, or aCDATAsection. If they are needed elsewhere, they must beescaped using eithernumeric character references or the strings "&"and "<" respectively. The right angle bracket (>) maybe represented using the string ">", and must,for compatibility, be escaped using ">"or a character reference when it appears in the string "]]>"in content, when that string is not marking the end of aCDATA section.
In the content of elements,character data is any string of characters which does not contain the start-delimiterof any markup. In a CDATA section, character data is any string of charactersnot including the CDATA-section-close delimiter, "]]>".
Toallow attribute values to contain both single and double quotes, the apostropheor single-quote character (') may be represented as "'",and the double-quote character (") as """.
CharacterData
| [14] | CharData | ::= | [^<&]* - ([^<&]* ']]>' [^<&]*) |
2.5Comments
[Definition:Commentsmay appear anywhere in a document outside othermarkup;in addition, they may appear within the document type declaration at placesallowed by the grammar. They are not part of the document'scharacter 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 are not recognizedwithin comments.
Comments
| [15] | Comment | ::= | '<!--' ((Char - '-') | ('-' (Char- '-')))* '-->' |
An example of a comment:
<!-- declarations for <head> & <body> --> |
Note that the grammar does not allow a comment ending in--->.The following example isnot well-formed.
2.6 ProcessingInstructions
[Definition:Processinginstructions (PIs) allow documents to contain instructions for applications.]
ProcessingInstructions
PIs are not part of the document'scharacter data, but must be passed through to theapplication. The PI begins with a target (PITarget)used to identify the application to which the instruction is directed. Thetarget names "XML", "xml", and so on are reservedfor standardization in this or future versions of this specification. TheXMLNotation mechanism may beused for formal declaration of PI targets. Parameter entity references arenot recognized within processing instructions.
2.7 CDATA Sections
[Definition:CDATA sections may occur anywherecharacter data may occur; they are used to escape blocks of text containingcharacters which would otherwise be recognized as markup. CDATA sections beginwith the string "<![CDATA[" and end with the string "]]>":]
CDATASections
Within a CDATA section, only theCDEndstring is recognized as markup, so that left angle brackets and ampersandsmay occur in their literal form; they need not (and cannot) be escaped using"<" and "&". CDATA sections cannotnest.
An example of a CDATA section, in which "<greeting>"and "</greeting>" are recognized ascharacter data, notmarkup:
<![CDATA[<greeting>Hello, world!</greeting>]]>
2.8 Prologand Document Type Declaration
[Definition:XML documents should begin with anXML declaration which specifiesthe version of XML being used.] For example, the following is a complete XMLdocument,well-formed butnotvalid:
<?xml version="1.0"?> <greeting>Hello, world!</greeting>
and so is this:
<greeting>Hello, world!</greeting> |
The version number "1.0" should be used to indicateconformance to this version of this specification; it is an error for a documentto use the value "1.0" if it does not conform to this versionof this specification. It is the intent of the XML working group to give laterversions of this specification numbers other than "1.0", butthis intent does not indicate a commitment to produce any future versionsof XML, nor if any are produced, to use any particular numbering scheme. Sincefuture versions are not ruled out, this construct is provided as a means toallow the possibility of automatic version recognition, should it become necessary.Processors may signal an error if they receive documents labeled with versionsthey do not support.
The function of the markup in an XML documentis to describe its storage and logical structure and to associate attribute-valuepairs with its logical structures. XML provides a mechanism, thedocument type declaration, to define constraints onthe logical structure and to support the use of predefined storage units.[Definition: An XML document isvalidif it has an associated document type declaration and if the document complieswith the constraints expressed in it.]
The document type declarationmust appear before the firstelementin the document.
Prolog
[Definition:The XMLdocument type declaration contains or points tomarkup declarations that provide a grammar for aclass of documents. This grammar is known as a document type definition, orDTD.The document type declaration can point to an external subset (a special kindofexternal entity) containingmarkup declarations, or can contain the markup declarations directly in aninternal subset, or can do both. The DTD for a document consists of both subsetstaken together.]
[Definition: Amarkup declaration is anelementtype declaration, anattribute-listdeclaration, anentitydeclaration, or anotationdeclaration.] These declarations may be contained in whole or in partwithinparameter entities, asdescribed in the well-formedness and validity constraints below. For furtherinformation, see4 Physical Structures.
DocumentType Definition
Note that it is possible to construct a well-formed documentcontaining adoctypedecl that neither pointsto an external subset nor contains an internal subset.
The markup declarationsmay be made up in whole or in part of thereplacementtext ofparameter entities.The productions later in this specification for individual nonterminals (elementdecl,AttlistDecl,and so on) describe the declarationsafter all the parameter entitieshave beenincluded.
Parameterentity references are recognized anywhere in the DTD (internal and externalsubsets and external parameter entities), except in literals, processing instructions,comments, and the contents of ignored conditional sections (see3.4Conditional Sections). They are also recognized in entity value literals.The use of parameter entities in the internal subset is restricted as describedbelow.
Validityconstraint: Root Element Type
TheName inthe document type declaration must match the element type of theroot element.
Validity constraint: Proper Declaration/PENesting
Parameter-entityreplacementtext 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 replacementtext for aparameter-entityreference, both must be contained in the same replacement text.
Well-formednessconstraint: PEs in Internal Subset
In the internal DTD subset,parameter-entity referencescan occur only where markup declarations can occur, not within markup declarations.(This does not apply to references that occur in external parameter entitiesor to the external subset.)
Well-formedness constraint: External Subset
Theexternal subset, if any, must match the production forextSubset.
Well-formednessconstraint: PE Between Declarations
The replacement text of a parameterentity reference in aDeclSep must match the productionextSubsetDecl.
Like the internalsubset, the external subset and any external parameter entities referencedin aDeclSep must consist of a series of completemarkup declarations of the types allowed by the non-terminal symbolmarkupdecl,interspersed with white space orparameter-entityreferences. However, portions of the contents of the external subset orof these external parameter entities may conditionally be ignored by usingtheconditional sectionconstruct; this is not allowed in the internal subset.