A formal definition of the XML language

About this document

This document contains the production rules, the well-formedness constraints and the validity constraints of the XML Recommendation 1.0 http://www.w3.org/TR/1998/REC-xml-19980210/. The rules, constraints, cross-references and numbering schemes have been automatically generated from the XML source of the XML Recommendation.

The production rules

The well-formedness constraints

(1) PEs in Internal Subset

In the internal DTD subset, parameter-entity references can occur only where markup declarations can occur, not within markup declarations. (This does not apply to references that occur in external parameter entities or to the external subset.)

(2) Element Type Match

The Name in an element's end-tag must match the element type in the start-tag.

(3) Unique Att Spec

No attribute name may appear more than once in the same start-tag or empty-element tag.

(4) No External Entity References

Attribute values cannot contain direct or indirect entity references to external entities.

(5) No < in Attribute Values

The replacement text of any entity referred to directly or indirectly in an attribute value (other than "&lt;") must not contain a <.

(6) Legal Character

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

(7) Entity Declared

In a document without any DTD, a document with only an internal DTD subset which contains no parameter entity references, or a document with "standalone='yes'", the Name given in the entity reference must match that in an entity declaration, except that well-formed documents need not declare any of the following entities: amp, lt, gt, apos, quot. The declaration of a parameter entity must precede any reference to it. Similarly, the declaration of a general entity must precede any reference to it which appears in a default value in an attribute-list declaration.

Note that if entities are declared in the external subset or in external parameter entities, a non-validating processor is not obligated to read and process their declarations; for such documents, the rule that an entity must be declared is a well-formedness constraint only if standalone='yes'.

(8) Parsed Entity

An entity reference must not contain the name of an unparsed entity. Unparsed entities may be referred to only in attribute values declared to be of type ENTITY or ENTITIES.

(9) No Recursion

A parsed entity must not contain a recursive reference to itself, either directly or indirectly.

(10) In DTD

Parameter-entity references may only appear in the DTD.

The validity constraints

(1) Root Element Type

The Name in the document type declaration must match the element type of the root element.

(2) Proper Declaration/PE Nesting

Parameter-entity replacement text must be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration (markupdecl above) is contained in the replacement text for a parameter-entity reference, both must be contained in the same replacement text.

(3) Standalone Document Declaration

The standalone document declaration must have the value "no" if any external markup declarations contain declarations of:

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

• entities (other than amp, lt, gt, apos, quot), if references to those entities appear in the document, or

• attributes with values subject to normalization, where the attribute appears in the document with a value which will change as a result of normalization, or

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

(4) Element Valid

An element is valid if there is a declaration matching elementdecl where the Name matches the element type, and one of the following holds:

1. The declaration matches EMPTY and the element has no content.

2. The declaration matches children and the sequence of child elements belongs to the language generated by the regular expression in the content model, with optional white space (characters matching the nonterminal S) between each pair of child elements.

3. The declaration matches Mixed and the content consists of character data and child elements whose types match names in the content model.

4. The declaration matches ANY, and the types of any child elements have been declared.

(5) Attribute Value Type

The attribute must have been declared; the value must be of the type declared for it. (For attribute types, see .)

(6) Unique Element Type Declaration

No element type may be declared more than once.

(7) Proper Group/PE Nesting

Parameter-entity replacement text must be properly nested with parenthetized groups. That is to say, if either of the opening or closing parentheses in a choice, seq, or Mixed construct is contained in the replacement text for a parameter entity, both must be contained in the same replacement text.

For interoperability, if a parameter-entity reference appears in a choice, seq, or Mixed construct, its replacement text should not be empty, and neither the first nor last non-blank character of the replacement text should be a connector (| or ,).

(8) No Duplicate Types

The same name must not appear more than once in a single mixed-content declaration.

(9) ID

Values of type ID must match the Name production. A name must not appear more than once in an XML document as a value of this type; i.e., ID values must uniquely identify the elements which bear them.

(10) One ID per Element Type

No element type may have more than one ID attribute specified.

(11) ID Attribute Default

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

(12) IDREF

Values of type IDREF must match the Name production, and values of type IDREFS must match Names; each Name must match the value of an ID attribute on some element in the XML document; i.e. IDREF values must match the value of some ID attribute.

(13) Entity Name

Values of type ENTITY must match the Name production, values of type ENTITIES must match Names; each Name must match the name of an unparsed entity declared in the DTD.

(14) Name Token

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

(15) Notation Attributes

Values of this type must match one of the notation names included in the declaration; all notation names in the declaration must be declared.

(16) Enumeration

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

(17) Required Attribute

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

(18) Attribute Default Legal

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

(19) Fixed Attribute Default

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

(20) Entity Declared

In a document with an external subset or external parameter entities with "standalone='no'", the Name given in the entity reference must match that in an entity declaration. For interoperability, valid documents should declare the entities amp, lt, gt, apos, quot, in the form specified in . The declaration of a parameter entity must precede any reference to it. Similarly, the declaration of a general entity must precede any reference to it which appears in a default value in an attribute-list declaration.

(21) Notation Declared

The Name must match the declared name of a notation.