XML was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the Working Group's contact with the W3C.

The design goals for XML are:

This specification, together with associated standards (Unicode and ISO/IEC 10646 for characters, Internet RFC 3066BCP 47 and the Language Subtag Registry for language identification tags, ISO 639 for language name codes, and ISO 3166 for country name codes), provides all the information necessary to understand XML Version &versionOfXML; and construct computer programs to process it.

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

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

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

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

Matching the document production implies that:

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

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

The mechanism for encoding character code points into bit patterns &may; vary from entity to entity. All XML processors MUST accept the UTF-8 and UTF-16 encodings of Unicode 3.1 ; the mechanisms for signaling which of the two is in use, or for bringing other encodings into play, are discussed later, in .

This section defines some symbols used widely in the grammar.

S (white space) consists of one or more space (#x20) characters, carriage returns, line feeds, or tabs.

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

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

A Name is a token beginning with a letter or one of a few punctuation characters, and continuing with letters, digits, hyphens, underscores, colons, or full stops, together known as name characters.

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

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

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

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

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

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

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

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

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

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

To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') &may; be represented as ' , and the double-quote character (") as " .

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

An example of a comment:

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

Processing instructions (PIs) allow documents to contain instructions for applications.

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

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

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

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

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

and so is this:

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

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

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

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

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

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

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

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

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

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

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

The system identifier hello.dtd gives the address (a URI reference) of a DTD for the document.

The declarations can also be given locally, as in this example:

If both the external and internal subsets are used, the internal subset MUST be considered to occur before the external subset. This has the effect that entity and attribute-list declarations in the internal subset take precedence over those in the external subset.

Markup declarations can affect the content of the document, as passed from an XML processor to an application; examples are attribute defaults and entity declarations. The standalone document declaration, which &may; appear as a component of the XML declaration, signals whether or not there are such declarations which appear external to the document entity or in parameter entities. An external markup declaration is defined as a markup declaration occurring in the external subset or in a parameter entity (external or internal, the latter being included because non-validating processors are not required to read them).

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

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

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

An example XML declaration with a standalone document declaration:

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

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

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

The value default signals that applications' default white-space processing modes are acceptable for this element; the value preserve indicates the intent that applications preserve all the white space. This declared intent is considered to apply to all elements within the content of the element where it is specified, unless overridden with another instance of the xml:space attribute. This specification does not give meaning to any value of xml:space other than default and preserve. It is an error for other values to be specified; the XML processor &MAY; report the error or &MAY; recover by ignoring the attribute specification or by reporting the (erroneous) value to the application. Applications may ignore or reject erroneous values.

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

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

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

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

(Productions 33 through 38 have been removed.)

For example:

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

A simple declaration for xml:lang might take the form

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

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

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

An example of a start-tag:

The end of every element that begins with a start-tag MUST be marked by an end-tag containing a name that echoes the element's type as given in the start-tag:

An example of an end-tag:

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

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

Empty-element tags &may; be used for any element which has no content, whether or not it is declared using the keyword EMPTY. For interoperability, the empty-element tag SHOULD be used, and SHOULD only be used, for elements which are declared EMPTY.

Examples of empty elements:

The element structure of an XML document &may;, for validation purposes, be constrained using element type and attribute-list declarations. An element type declaration constrains the element's content.

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

An element type declaration takes the form:

where the Name gives the element type being declared.

Examples of element type declarations:

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

Attribute-list declarations specify the name, data type, and default value (if any) of each attribute associated with a given element type:

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

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

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

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

If the keyword of the conditional section is INCLUDE, then the contents of the conditional section MUST be processed as part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section MUST NOT be processed as part of the DTD. If a conditional section with a keyword of INCLUDE occurs within a larger conditional section with a keyword of IGNORE, both the outer and the inner conditional sections MUST be ignored. The contents of an ignored conditional section MUST be parsed by ignoring all characters after the "[" following the keyword, except conditional section starts "<![" and ends "]]>", until the matching conditional section end is found. Parameter entity references MUST NOT be recognized in this process.

If the keyword of the conditional section is a parameter-entity reference, the parameter entity MUST be replaced by its content before the processor decides whether to include or ignore the conditional section.

An example:

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

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

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

Examples of character and entity references:

Example of a parameter-entity reference:

Entities are declared thus:

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

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

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

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

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

then the replacement text for the entity book is:

The general-entity reference &rights; would be expanded should the reference &book; appear in the document's content or an attribute value.

These simple rules may have complex interactions; for a detailed discussion of a difficult example, see .

Entity and character references &may; 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 < and & &may; be used to escape < and & when they occur in character data.

All XML processors MUST recognize these entities whether they are declared or not. For interoperability, valid XML documents SHOULD declare these entities, like any others, before using them. If the entities lt or amp are declared, they MUST be declared as internal entities whose replacement text is a character reference to the respective character (less-than sign or ampersand) being escaped; the double escaping is REQUIRED for these entities so that references to them produce a well-formed result. If the entities gt, apos, or quot are declared, they MUST be declared as internal entities whose replacement text is the single character being escaped (or a character reference to that character; the double escaping here is OPTIONAL but harmless). For example:

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

Notation declarations provide a name for the notation, for use in entity and attribute-list declarations and in attribute specifications, and an external identifier for the notation which may allow an XML processor or its client application to locate a helper application capable of processing data in the given notation.

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

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

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

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

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

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

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

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

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