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 WG'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 3066 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.

The W3C's XML 1.0 Recommendation was first issued in 1998, and despite the issuance of many errata culminating in a Third Edition of 2003, has remained (by intention) unchanged with respect to what is well-formed XML and what is not. This stability has been extremely useful for interoperability. However, the Unicode Standard on which XML 1.0 relies for character specifications has not remained static, evolving from version 2.0 to version 4.0 and beyond. Characters not present in Unicode 2.0 may already be used in XML 1.0 character data. However, they are not allowed in XML names such as element type names, attribute names, enumerated attribute values, processing instruction targets, and so on. In addition, some characters that should have been permitted in XML names were not, due to oversights and inconsistencies in Unicode 2.0.

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

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

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

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

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 .

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

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

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 whitespace 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. 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 associateattribute 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.

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.

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

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. all of the following to a single #xA character:

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

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 intent declared with xml:lang is considered to apply to all attributes and content of the element where it is specified, unless overridden with an instance of xml:lang on another element within that content. In particular, the empty value of xml:lang is used on an element B to override a specification of xml:lang on an enclosing element A, without specifying another language. Within B, it is considered that there is no language information available, just as if xml:lang had not been specified on B or any of its ancestors.

A simple declaration for xml:lang might take the form

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:

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

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

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

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

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

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

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

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

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

An example:

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.