[Mirrored from: http://www.textuality.com/sgml-erb/WD-xml-lang.html, April 02, 1997]


Extensible Markup Language (XML): Part I. Syntax

W3C Working Draft 31-Mar-97

This version:
Previous versions:
Latest version:
Tim Bray (Textuality) <tbray@textuality.com>
C. M. Sperberg-McQueen (University of Illinois at Chicago) <cmsmcq@uic.edu>

Status of this memo

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/pub/WWW/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/pub/WWW/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 Version 1.0 Part I: Syntax

Extensible Markup Language

Version 1.0

Part I: Syntax

Document W3C-SGML-ERB DD-1996-0004
31 March 1997
This draft is intended for public discussion.
It is subject to approval by the W3C SGML Editorial Review Board

Table of Contents

1. Introduction
    1.1 Origin and Goals
    1.2 Relationship to Existing Standards
    1.3 Terminology
    1.4 Notation
    1.5 Common Syntactic Constructs
2. Documents
    2.1 Logical and Physical Structure
    2.2 Well-Formed XML Documents
    2.3 Characters
    2.4 Character Data and Markup
    2.6 Processing Instructions
    2.7 CDATA Sections
    2.8 White Space Handling
    2.9 Prolog and Document Type Declaration
    2.10 Required Markup Declaration
3. Logical Structures
    3.1 Start- and End-Tags
    3.2 Element Declarations
        3.2.1 Element Content
        3.2.2 Mixed Content
    3.3 Attribute-List Declarations
        3.3.1 Attribute Types
        3.3.2 Attribute Defaults
        3.3.3 Attribute-Value Normalization
    3.4 Conditional Sections
4. Physical Structures
    4.1 Logical and Physical Structures
    4.2 Character and Entity References
    4.3 Entity Declarations
        4.3.1 Internal Entities
        4.3.2 External Entities
        4.3.3 Character Encoding in Entities
        4.3.4 Document Entity
    4.4 XML Processor Treatment of Entities
    4.5 Predefined Entities
    4.6 Notation Declarations
5. Conformance


B. Character Classes
C. Expansion of Entity and Character References
D. Deterministic Content Models
E. Autodetection of Character Sets
F. A Trivial Grammar for XML Documents
G. References
H. W3C SGML Editorial Review Board

1. 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.

1.1 Origin and Goals

XML was developed by an 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 SGML Working Group also organized by the W3C. The membership of the Editorial Review Board is given in an appendix. The ERB's contact with the W3C is Dan Connolly.

The design goals for XML are:

  1. XML shall be straightforwardly usable over the Internet.
  2. XML shall support a wide variety of applications.
  3. XML shall be compatible with SGML.
  4. It shall be easy to write programs which process XML documents.
  5. The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
  6. XML documents should be human-legible and reasonably clear.
  7. The XML design should be prepared quickly.
  8. The design of XML shall be formal and concise.
  9. XML documents shall be easy to create.
  10. Terseness in XML markup is of minimal importance.

This specification, together with the associated standards, provides all the information necessary to understand XML version 1.0 and construct computer programs to process it.

This version of the XML specification (31 March 1997) is for public review and discussion. It may be distributed freely, as long as all text and legal notices remain intact.

1.2 Relationship to Existing Standards

Standards relevant to users and implementors of XML include:

1.3 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.
reportable error
An error which conforming software must, at user option, report to the user.
validity constraint
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.
well-formedness constraint
A rule which applies to all well-formed XML documents. Violations of well-formedness constraints are reportable errors.
at user option
Conforming software may or must (depending on the modal verb in the sentence) behave as described; if they do, they must provide users a means to select, or to decline, 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).
exact(ly) match
Case-sensitive string match: two strings or names being compared must be identical. Characters with multiple possible representations in ISO 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.
for compatibility
A feature of XML included solely to ensure that XML remains compatible with SGML.
for interoperability
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 submission to ISO/IEC JTC1 SC18 WG8.

1.4 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 NNNN is a hexadecimal integer, the expression represents the character in ISO 10646 whose canonical (UCS-4) bit string, when interpreted as an unsigned binary number, has the value indicated.
[a-zA-Z], [#xNNNN-#xNNNN]
represents any character with a value in the range(s) indicated (inclusive).
[^a-z], [^#xNNNN-#xNNNN]
represents any character with a value outside the range indicated.
[^abc], [^#xNNNN#xNNNN#xNNNN]
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 b
a followed by b.
a | b
a or b but not both.
a - b
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 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?. Note that % has lower precedence than any of the suffix operators ?, *, or +; that is to say, %a* and %(a*) each mean that the result of including a parameter entity reference at the indicated location must match a*.
expression is treated as a unit, and may carry the % prefix operator, or a suffix operator: ?, *, or +.
/* ... */
[ WFC: ... ]
Well-formedness check; this identifies by name a check for well-formedness associated with a production.
[ VC: ... ]
Validity check; this identifies by name a check for validity associated with a production.

1.5 Common Syntactic Constructs

This section defines some symbols used widely in the grammar.

S (white space) consists of one or more space (#x0020) characters, carriage returns, line feeds, tabs, or ideographic space characters.

White space
[1] S ::= (#x0020 | #x0009 | #x000d | #x000a | #x3000)+

Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO 10646.

Character Range
[2] Char ::= #x09 | #x0A | #x0D | [#x20-#xFFFD] | [#x00010000-#x7FFFFFFF] /* any ISO 10646 31-bit code, FFFE and FFFF excluded */

Characters are classified for convenience as letters, digits, or other characters. Letters consist of an alphabetic base character possibly followed by one or more combining characters, or of an ideographic character. Certain layout and format-control characters defined by ISO 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). The use of any name beginning with a string which matches "XML" in a fashion other than those described in this and related specifications is an error.

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

Names and Tokens
[3] MiscName ::= '.' | '-' | '_' | CombiningChar | Ignorable | Extender
[4] NameChar ::= Letter | Digit | MiscName
[5] Name ::= (Letter | '_') (NameChar)*
[6] Names ::= Name (S Name)*
[7] Nmtoken ::= (NameChar)+
[8] Nmtokens ::= Nmtoken (S Nmtoken)*

Literal data is any quoted string containing neither a left angle bracket nor the quotation mark used as a delimiter for that string. It may contain entity 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):

[9] EntityValue ::= '"' ([^%&"] | PEReference | Reference)* '"'
|  "'" ([^%&'] | PEReference | Reference)* "'"
[10] AttValue ::= '"' ([^<&"] | Reference)* '"' | "'" ([^<&'] | Reference)* "'"
[11] SystemLiteral ::= '"' URLchar* '"' | "'" (URLchar - "'")* "'"
[12] URLchar ::= /* See RFC 1738 */
[13] PubidLiteral ::= '"' PubidChar* '"' | "'" (PubidChar - "'")* "'"
[14] PubidChar ::= #x0020 | #x0009 | #x000d | #x000a | #x3000 | [a-zA-Z0-9] | [-'()+,./:=?]
[15] SkipLit ::= ('"' [^"]* '"') | ("'" [^']* "'")

Within EntityValue, parameter-entity and character references are recognized and expanded immediately; general-entity references may occur, but they are not expanded until the entity containing them is referenced and expanded in the document itself. Within AttValue, general and character references are recognized and expanded; parameter-entity references are not recognized. Within SystemLiteral, no references of any kind are recognized.

2. Documents

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

2.1 Logical and Physical Structure

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

Physically, the document is composed of units called entities; it begins in a "root" or document entity, which may refer to other entities, and so on.

The logical structure contains declarations, elements, tags, 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.

2.2 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: amp, lt, gt, apos, quot.

Matching the document production implies that:

  1. It contains one or more elements.
  2. 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.

2.3 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 name and the 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 the specification. XML text data is a sequence of characters. A character is an atomic unit of text represented by a bit string; valid bit strings and their meanings are specified by ISO 10646. Users may extend the ISO 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 10646 character set may be referred to by the decimal or hexadecimal equivalent of its bit string.

2.4 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;".

Character data
[16] PCData ::= [^<&]*


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.

[17] Comment ::= '<!--' (Char* - (Char* '--' Char*)) '-->'

An example of a comment:

<!-- declarations for <head> & <body> -->

2.6 Processing Instructions

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

Processing Instructions
[18] PI ::= '<?' Name S (Char* - (Char* '?>' Char*)) '?>'

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. The use of PI targets with names beginning "XML" in any way other than those described in this specification is an error.

2.7 CDATA Sections

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

CDATA sections
[19] CDSect ::= CDStart CData CDEnd
[20] CDStart ::= '<![CDATA['
[21] CData ::= (Char* - (Char* ']]>' Char*))
[22] 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>]]>

2.8 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 or source code.

An XML processor which does not read the DTD must always pass all characters in a document that are not markup through to the application. An XML processor which does read the DTD must always pass all characters in mixed content that are not markup through to the application. It may also choose to pass white space ocurring in element content to the application; if it does so, it must signal to the application that

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.

The root element of any document, unless this attribute is provided or defaulted with a value of PRESERVE, is considered to have signaled no intentions as regards application space handling. Any space handling behavior specified for an element by the XML-SPACE attribute is inherited by that element's child elements as well.

2.9 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.

XML document
[23] document ::= Prolog element Misc*
[24] Prolog ::= XMLDecl? Misc* (doctypedecl Misc*)?
[25] XMLDecl ::= '<?XML' VersionInfo EncodingDecl? RMDecl? S? '?>'
[26] VersionInfo ::= S 'version' Eq ('"1.0"' | "'1.0'")
[27] Misc ::= Comment | PI | S

For example, the following is a complete XML document, well-formed but not valid:

<?XML version="1.0"?>
<greeting>Hello, world!</greeting>

and so is this:

<greeting>Hello, world!</greeting>

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. The external subset must obey the same grammatical constraints as the internal subset; i.e. it must match the production for markupdecl.

Document type definition
[28] doctypedecl ::= '<!DOCTYPE' S Name (S ExternalID)? S? ('[' markupdecl* ']' S?)? '>' [ VC: Root Element Type ]
[ VC: Non-null DTD ]
[29] markupdecl ::= %( ( %elementdecl | %AttlistDecl | %EntityDecl | %NotationDecl | %conditionalSect | %PI | %S | %Comment )* )

Validity Constraint - Root Element Type:
The Name in the document-type declaration must match the element type of the root element.

Validity Constraint - Non-null DTD:
The internal and external subsets of the DTD must not both be empty.

For example:

<?XML version="1.0"?>
<!DOCTYPE greeting SYSTEM "hello.dtd">
<greeting>Hello, world!</greeting>

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:

<?XML version="1.0" encoding="UTF-8" ?>
<!DOCTYPE greeting [
  <!ELEMENT greeting (#PCDATA)>
<greeting>Hello, world!</greeting>

The version label version="1.0" indicates that the document conforms to version 1.0 of the XML specification. The character-set label encoding="UTF-8" indicates that the document entity is encoded using the UTF-8 transformation of ISO 10646.

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.

2.10 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. 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:

Required markup declaration
[30] 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

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="1.0" RMD='INTERNAL'?>

3. 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, are those of the start-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.

3.1 Start- and End-Tags

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

[31] STag ::= '<' Name (S Attribute)* S? '>' [ WFC: Unique Att Spec ]
[32] Attribute ::= Name Eq AttValue [ VC: Attribute Value Type ]
[ WFC: No External Entity References ]
[33] 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.

Validity Constraint - Unique Att Spec:
No attribute may appear more than once in the same start-tag.

Validity Constraint - Attribute Value Type:
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.)

Well-Formedness Constraint - No External Entity References:
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 which is not empty is marked by an end-tag containing a name that echoes the element's type as given in the start-tag:

[34] ETag ::= '</' Name S? '>'

An example of an end-tag:


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

Content of elements
[35] content ::= (element | PCData | Reference | CDSect | PI | Comment)* [ VC: Content ]
[36] element ::= EmptyElement
| STag content ETag [ WFC: GI Match ]

Validity Constraint - Content:
Each element type used must be declared. The content of an element instance must match the content model declared for that element type.

Well-Formedness Constraint - GI Match:
The Name in an element's end-tag must match that in the start-tag.

If an element is empty, the start-tag constitutes the whole element. An empty element takes a special form:

Tags for empty elements
[37] EmptyElement ::= '<' Name (S Attribute)* S? '/>'

An example of an empty element:

<IMG align="left"
 src="http://www.w3.org/pub/WWW/Icons/WWW/w3c_48x48.gif" />

3.2 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:

Element declaration
[38] elementdecl ::= '<!ELEMENT' S %Name %S %contentspec S? '>' [ VC: Unique Element Declaration ]
[39] contentspec ::= 'EMPTY' | 'ANY' | Mixed | elements

where the Name gives the type of the element.

Validity Constraint - Unique Element Declaration:
No element type may be declared more than once.

The content of an element can be categorized as element content or mixed content , as explained below. An element declared using the keyword EMPTY must be empty 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 %name.para; %content.para; >
<!ELEMENT container ANY>

3.2.1 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:

Element-content models
[40] elements ::= (choice | seq) ('?' | '*' | '+')?
[41] cp ::= (Name | choice | seq) ('?' | '*' | '+')?
[42] cps ::= S? %cp S?
[43] choice ::= '(' S? %(cps ('|' cps)+) S? ')'
[44] seq ::= '(' S? %(cps (',' cps)*) S? ')'

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 respectively. 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.

Examples of element-content models:

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

3.2.2 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-content declaration
[45] Mixed ::= '(' S? %( %'#PCDATA' ( S? '|' S? %(%Name (S? '|' S? %Name)*) )* ) S? ')*'
| '(' S? %('#PCDATA') S? ')' '*'?

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)* >

3.3 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:

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

Attribute list declaration
[46] AttlistDecl ::= '<!ATTLIST' S %Name S? (%AttDef+)+ S? '>'
[47] 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 entity 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. 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.

3.3.1 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:

Attribute types
[48] AttType ::= StringType | TokenizedType | EnumeratedType
[49] StringType ::= 'CDATA'
[50] TokenizedType ::= 'ID' [ VC: ID ]
| 'IDREF' [ VC: Idref ]
| 'IDREFS' [ VC: Idref ]
| 'ENTITY' [ VC: Entity Name ]
| 'ENTITIES' [ VC: Entity Name ]
| 'NMTOKEN' [ VC: Name Token ]
| 'NMTOKENS' [ VC: Name Token ]

Validity Constraint - ID:
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.

Validity Constraint - Idref:
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.

Validity Constraint - Entity Name:
Values of this type must be 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.

Validity Constraint - Name token:
Values of this type must consist of a string matching the
Nmtoken (for NMTOKENS, 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:

Enumerated attribute types
[51] EnumeratedType ::= NotationType | Enumeration
[52] NotationType ::= %('NOTATION') S %( '(' S? %( %Name (S? '|' S? %Name)* ) S? ')' ) [ VC: Notation Attributes ]
[53] Enumeration ::= '(' S? %( %( %Nmtoken (S? '|' S? %Nmtoken)* S? ) ( '|' %( %Nmtoken (S? '|' S? %Nmtoken)* S? ) )* ) ')' [ VC: Enumeration ]

Validity Constraint - Notation Attributes:
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.

Validity Constraint - Enumeration:
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.

3.3.2 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:

Attribute defaults
[54] Default ::= '#REQUIRED' | '#IMPLIED' | ((%'#FIXED' S)? %AttValue) [ VC: Attribute Default Legal ]

#REQUIRED means that the document is invalid should the processor encounter a start-tag where this attribute is omitted, i.e. could occur but does not. #IMPLIED means that if an attribute is omitted, 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.

Validity Constraint - Attribute Default Legal:
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">

3.3.3 Attribute-Value Normalization

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

  1. Record-ends must be replaced by single space (#x0020) characters.
  2. Character references and references to internal text entities must be expanded. References to external entities are a reportable error.
  3. If the attribute is not of type CDATA, all strings of white space must be normalized to single space characters (#x0020), and leading and trailing white space must be removed.
  4. Values of type ID, IDREF, IDREFS, NMTOKEN, NMTOKENS, or of enumerated or notation types, must be folded to uppercase.

3.4 Conditional Sections

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

Conditional section
[55] conditionalSect ::= includeSect | ignoreSect
[56] includeSect ::= '<![' %'INCLUDE' '[' (%markupdecl*)* ']]>'
[57] ignoreSect ::= '<![' %'IGNORE' '[' ( ((SkipLit | Comment | PI) - (Char* ']]>' Char*)) | ignoreSect | (Char - ([<'"] | ']'))* | ('<!' (Char - ('-' | '['))*) ) ']]>'

Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, or nested conditional sections, intermingled with white space. Conditional sections must not occur within declarations, except the document-type declaration.

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 (recursively if necessary) before the processor decides whether to include or ignore the conditional section.

An example:

<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >
<!ELEMENT book (comments*, title, body, supplements?)>
<!ELEMENT book (title, body, supplements?)>

4. 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 be coterminous with, a single physical storage object such as a file or stream. 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 to be considered as an integral part of the document. A binary entity contains binary data with an associated notation. References to text and binary entities cannot be distinguished by syntax; their types are established in their declarations.

4.1 Logical and Physical Structures

The logical and physical structures 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.

4.2 Character and Entity References

A character reference refers to a specific character in the ISO 10646 character set, e.g. one not directly accessible from available input devices:

Character reference
[58] Hex ::= [0-9a-fA-F]
[59] CharRef ::= '&#' [0-9]+ ';' | '&#x' Hex+ ';'

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 within conditional sections; references to them use percent-sign (%) and semicolon (;) as delimiters.

Entity Reference
[60] Reference ::= EntityRef | CharRef
[61] EntityRef ::= '&' Name ';' [ WFC: Entity Declared ]
[ WFC: Text Entity ]
[ WFC: No Recursion ]
[62] PEReference ::= '%' Name ';' [ WFC: Entity Declared ]
[ WFC: Text Entity ]
[ WFC: No Recursion ]
[ WFC: In DTD ]

Well-Formedness Constraint - Entity Declared:
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: amp, lt, gt, apos, quot. 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.

Well-Formedness Constraint - Text Entity:
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.

Well-Formedness Constraint - No Recursion:
A text or parameter entity must not contain a recursive reference to itself, either directly or indirectly.

Well-Formedness Constraint - In DTD:
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.

Examples of character and entity references:

"&#x0020;" and "&#x0009;" are white-space characters.
"&Pub-Status;" is an entity reference.

Example of a parameter-entity reference:

         SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >

4.3 Entity Declarations

Entities are declared thus:

Entity declaration
[63] EntityDecl ::= '<!ENTITY' S %Name S %EntityDef S? '>' /* General entities */
| '<!ENTITY' S '%' S %Name S %EntityDef S? '>' /* Parameter entities */
[64] 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.

4.3.1 Internal Entities

If the entity definition is just an EntityValue, this 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 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

4.3.2 External Entities

If the entity is not internal, it is an external entity, declared as follows:

External entity declaration
[65] ExternalDef ::= ExternalID NDataDecl?
[66] ExternalID ::= 'SYSTEM' S SystemLiteral
|  'PUBLIC' S PubidLiteral S SystemLiteral
[67] NDataDecl ::= S %'NDATA' S %Name [ VC: Notation Declared ]

If the NDataDecl is present, this is a binary data entity, otherwise a text entity.

Validity Constraint - Notation Declared:
The Name must match the declared name of a

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"
<!ENTITY hatch-pic
         SYSTEM "../grafix/OpenHatch.gif"
         NDATA gif >

4.3.3 Character Encoding in Entities

Each 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, particularly work with Asian languages, the use of the UTF-16 transformation is required, and correct handling of this encoding is a desirable characteristic in XML processor implementations.

Entities encoded in UCS-2 must begin with the Byte Order Mark described by ISO 10646 Annex E and Unicode Appendix B (the ZERO WIDTH NO-BREAK SPACE character, U+FEFF). 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:

Encoding declaration
[68] EncodingDecl ::= S 'encoding' Eq QEncoding
[69] EncodingPI ::= '<?XML' S 'encoding' Eq QEncoding S? ?>
[70] QEncoding ::= '"' Encoding '"' | "'" Encoding "'"
[71] Encoding ::= LatinName
[72] LatinName ::= [A-Za-z] ([A-Za-z0-9._] | '-')* /* Name containing only Latin characters */

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 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 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, there is often difficulty in reliably signaling the encoding of an entity. Errors in this area fall into two categories:

  1. failing to read an entity because of inability to recognize its actual encoding, and
  2. reading an entity incorrectly because of an incorrect guess of its proper encoding.
The first class of error is extremely damaging, and 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: 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:


4.3.4 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.

4.4 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:

  1. 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).
  2. For both character and entity references, the processor must remove the reference itself from the text data before passing the data to the application.
  3. For character references, the processor must pass the indicated ISO 10646 bit pattern to the application in place of the reference.
  4. For an external entity, the processor must inform the application of the entity's system identifier.
  5. If the external entity is binary, the processor must inform the application of the associated notation name, and the notation's associated system identifier.
  6. 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 of entities used to escape markup delimiters (the entities amp, lt, gt, apos, quot) 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.
  7. 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.
  8. 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. While this behavior would not allow the application to validate the document, it is compliant with this specification.

Entity and character references can both be used to escape the left angle bracket, ampersand, and other delimiters. A set of general entities (amp, lt, gt, apos, quot) 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.

  1. In the replacement text of an internal entity, parameter-entity references and character references are resolved, but general-entity references are not resolved, before the replacement text is stored in the processor's symbol table.
  2. 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.

Since the replacement text of a general entity is parsed once when the general entity is declared and once when it is referred to, simple character references do not suffice to escape delimiters within the replacement text. Instead, delimiters must be escaped using a general entity reference (which is not expanded when the declaration is parsed) or a doubly-escaped character reference. See the appendix on expansion of entity references for detailed examples.

4.5 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 amp, lt, gt, apos, quot).

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.

<!ENTITY lt     "<"> 
<!ENTITY gt     ">"> 
<!ENTITY amp    "&"> 
<!ENTITY apos   "'"> 
<!ENTITY quot   '"'> 

4.6 Notation Declarations

Notations identify by name the format of external binary entities.

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.

Notation declarations
[73] 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.)

5. 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 systems 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 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.

  1. Comment declarations must have the delimiters <!-- comment text --> and can't have spaces within the markup of <!-- or -->.
  2. No comments (-- ... --) inside markup declarations.
  3. Comment declarations can't jump in and out of comments with -- --.
  4. No name groups for declaring multiple elements or making a single ATTLIST declaration apply to multiple elements.
  5. No CDATA or RCDATA declared content in element declarations.
  6. No exclusions or inclusions on content models.
  7. No minimization parameters on element declarations.
  8. Mixed content models must be optional-repeatable OR-groups, with #PCDATA first.
  9. No AND (&) content model groups.
  10. No NAME[S], NUMBER[S], or NUTOKEN[S] declared values for attributes.
  11. No #CURRENT or #CONREF declared values for attributes.
  12. Attribute default values must be quoted.
  13. Marked sections can't have spaces within the markup of <![keyword[ or ]]>.
  14. No RCDATA, TEMP, IGNORE, or INCLUDE marked sections in document instances.
  15. Marked sections in instances must use the CDATA keyword literally, not a parameter entity.
  16. No SDATA, CDATA, or bracketed internal entities.
  17. No SUBDOC, CDATA, or SDATA external entities.
  18. No public identifiers in ENTITY, DOCTYPE, and NOTATION declarations.
  19. No data attributes on NOTATIONs or attribute value specifications on ENTITY declarations.
  20. No SHORTREF declarations.
  21. No USEMAP declarations.
  22. No LINKTYPE declarations.
  23. No LINK declarations.
  24. No USELINK declarations.
  25. No IDLINK declarations.
  26. No SGML declarations.

The specific SGML declaration needed to enable SGML systems to process XML documents will vary from document to document, depending on the character set and the capacities and quantities required. Documents should be able to use the following SGML declaration:

<!SGML -- SGML Declaration for XML --
    "ISO 8879:1986 (ENR)"

            "ISO Registration Number 176//CHARSET
            ISO/IEC 10646-1:1993 UCS-2 with implementation 
            level 3//ESC 2/5 2/15 4/5"
            0       9 UNUSED
            9       2 9
            11      2 UNUSED
            13      1 13
            14     18 UNUSED
            32     95 32
            127     1 UNUSED
            128    32 UNUSED
            160 65376 160

        -- Capacities are not restricted in XML --
        TOTALCAP 99999999
        ENTCAP   99999999
        ENTCHCAP 99999999
        ELEMCAP  99999999
        GRPCAP   99999999
        EXGRPCAP 99999999
        EXNMCAP  99999999
        ATTCAP   99999999
        ATTCHCAP 99999999
        AVGRPCAP 99999999
        NOTCAP   99999999
        NOTCHCAP 99999999
        IDCAP    99999999
        IDREFCAP 99999999
        MAPCAP   99999999
        LKSETCAP 99999999
        LKNMCAP  99999999


        BASESET "ISO Registration Number 176//CHARSET
                ISO/IEC 10646-1:1993 UCS-2 with implementation 
                level 3//ESC 2/5 2/15 4/5"
            0 65536 0
            RE    13
            RS    10
            SPACE 32
            TAB   SEPCHAR 9
            ITAB  SEPCHAR 12288 -- ideographic space --

                224-246 248-255 257 259 261 263 265 267 269 271 273
                275 277 279 281 283 285 287 289 291 293 295 297 299
                301 303 305 307 309 311 314 316 318 320 322 324 326
                328 331 333 335 337 339 341 343 345 347 349 351 353
                355 357 359 361 363 365 367 369 371 373 375 378 380
                382 383 387 389 392 396 402 409 417 419 421 424 429
                432 436 438 441 445 453 454 456 457 459 460 462 464
                466 468 470 472 474 476 479 481 483 485 487 489 491
                493 495 498 499 501 507 509 511 513 515 517 519 521
                523 525 527 529 531 533 535 595 596 598-601 603 608
                611 616 617 623 626 643 648 650 651 658 940-943
                945-961 963-974 976 977 981 982 995 997 999 1001
                1003 1005 1007-1009 1072-1103 1105-1116 1118 1119
                1121 1123 1125 1127 1129 1131 1133 1135 1137 1139
                1141 1143 1145 1147 1149 1151 1153 1169 1171 1173
                1175 1177 1179 1181 1183 1185 1187 1189 1191 1193
                1195 1197 1199 1201 1203 1205 1207 1209 1211 1213
                1215 1218 1220 1224 1228 1233 1235 1237 1239 1241
                1243 1245 1247 1249 1251 1253 1255 1257 1259 1263
                1265 1267 1269 1273 1377-1414 7681 7683 7685 7687
                7689 7691 7693 7695 7697 7699 7701 7703 7705 7707
                7709 7711 7713 7715 7717 7719 7721 7723 7725 7727
                7729 7731 7733 7735 7737 7739 7741 7743 7745 7747
                7749 7751 7753 7755 7757 7759 7761 7763 7765 7767
                7769 7771 7773 7775 7777 7779 7781 7783 7785 7787
                7789 7791 7793 7795 7797 7799 7801 7803 7805 7807
                7809 7811 7813 7815 7817 7819 7821 7823 7825 7827
                7829 7841 7843 7845 7847 7849 7851 7853 7855 7857
                7859 7861 7863 7865 7867 7869 7871 7873 7875 7877
                7879 7881 7883 7885 7887 7889 7891 7893 7895 7897
                7899 7901 7903 7905 7907 7909 7911 7913 7915 7917
                7919 7921 7923 7925 7927 7929 7936-7943 7952-7957
                7968-7975 7984-7991 8000-8005 8017 8019 8021 8023
                8032-8039 8048-8061 8064-8071 8080-8087 8096-8103
                8112 8113 8115 8131 8144 8145 8160 8161 8165 8179
                8560-8575 65345-65370

                -- 305 and 383 should be 73 and 83 respectively, 
                but SGML does not allow a letter to be assigned 
                to UCNMSTRT --
                192-214 216-222 376 256 258 260 262 264 266 268 270
                272 274 276 278 280 282 284 286 288 290 292 294 296
                298 300 302 305 306 308 310 313 315 317 319 321 323
                325 327 330 332 334 336 338 340 342 344 346 348 350
                352 354 356 358 360 362 364 366 368 370 372 374 377
                379 381 383 386 388 391 395 401 408 416 418 420 423
                428 431 435 437 440 444 452 452 455 455 458 458 461
                463 465 467 469 471 473 475 478 480 482 484 486 488
                490 492 494 497 497 500 506 508 510 512 514 516 518
                520 522 524 526 528 530 532 534 385 390 393 394 398
                399 400 403 404 407 406 412 413 425 430 433 434 439
                902 904-906 913-929 931-939 908 910 911 914 920 934
                928 994 996 998 1000 1002 1004 1006 922 929
                1040-1071 1025-1036 1038 1039 1120 1122 1124 1126
                1128 1130 1132 1134 1136 1138 1140 1142 1144 1146
                1148 1150 1152 1168 1170 1172 1174 1176 1178 1180
                1182 1184 1186 1188 1190 1192 1194 1196 1198 1200
                1202 1204 1206 1208 1210 1212 1214 1217 1219 1223
                1227 1232 1234 1236 1238 1240 1242 1244 1246 1248
                1250 1252 1254 1256 1258 1262 1264 1266 1268 1272
                1329-1366 7680 7682 7684 7686 7688 7690 7692 7694
                7696 7698 7700 7702 7704 7706 7708 7710 7712 7714
                7716 7718 7720 7722 7724 7726 7728 7730 7732 7734
                7736 7738 7740 7742 7744 7746 7748 7750 7752 7754
                7756 7758 7760 7762 7764 7766 7768 7770 7772 7774
                7776 7778 7780 7782 7784 7786 7788 7790 7792 7794
                7796 7798 7800 7802 7804 7806 7808 7810 7812 7814
                7816 7818 7820 7822 7824 7826 7828 7840 7842 7844
                7846 7848 7850 7852 7854 7856 7858 7860 7862 7864
                7866 7868 7870 7872 7874 7876 7878 7880 7882 7884
                7886 7888 7890 7892 7894 7896 7898 7900 7902 7904
                7906 7908 7910 7912 7914 7916 7918 7920 7922 7924
                7926 7928 7944-7951 7960-7965 7976-7983 7992-7999
                8008-8013 8025 8027 8029 8031 8040-8047 8122 8123
                8136-8139 8154 8155 8184 8185 8170 8171 8186 8187
                8072-8079 8088-8095 8104-8111 8120 8121 8124 8140
                8152 8153 8168 8169 8172 8188 8544-8559 65313-65338

                170 181 186 223 304 312 329 384 397 405 410 411 414
                415 422 426 427 442 443 446-451 477 496 592-594 597
                602 604-607 609 610 612-615 618-622 624 625 627-642
                644-647 649 652-657 659-680 688-696 699-705 736-740
                768-837 864 865 890 912 944 962 978-980 986 988 990
                992 1010 1011 1155-1158 1216 1369 1415 1425-1441
                1443-1465 1467-1469 1471 1473 1474 1476 1488-1514
                1520-1522 1569-1594 1601-1618 1648-1719 1722-1726
                1728-1742 1744-1747 1749-1768 1770-1773 2305-2307
                2309-2361 2364-2381 2385-2388 2392-2403 2433-2435
                2437-2444 2447 2448 2451-2472 2474-2480 2482
                2486-2489 2492 2494-2500 2503 2504 2507-2509 2519
                2524 2525 2527-2531 2544 2545 2562 2565-2570 2575
                2576 2579-2600 2602-2608 2610 2611 2613 2614 2616
                2617 2620 2622-2626 2631 2632 2635-2637 2649-2652
                2654 2672-2676 2689-2691 2693-2699 2701 2703-2705
                2707-2728 2730-2736 2738 2739 2741-2745 2748-2757
                2759-2761 2763-2765 2784 2817-2819 2821-2828 2831
                2832 2835-2856 2858-2864 2866 2867 2870-2873
                2876-2883 2887 2888 2891-2893 2902 2903 2908 2909
                2911-2913 2946 2947 2949-2954 2958-2960 2962-2965
                2969 2970 2972 2974 2975 2979 2980 2984-2986
                2990-2997 2999-3001 3006-3010 3014-3016 3018-3021
                3031 3073-3075 3077-3084 3086-3088 3090-3112
                3114-3123 3125-3129 3134-3140 3142-3144 3146-3149
                3157 3158 3168 3169 3202 3203 3205-3212 3214-3216
                3218-3240 3242-3251 3253-3257 3262-3268 3270-3272
                3274-3277 3285 3286 3294 3296 3297 3330 3331
                3333-3340 3342-3344 3346-3368 3370-3385 3390-3395
                3398-3400 3402-3405 3415 3424 3425 3585-3630
                3632-3642 3648-3653 3655-3662 3713 3714 3716 3719
                3720 3722 3725 3732-3735 3737-3743 3745-3747 3749
                3751 3754 3755 3757 3758 3760-3769 3771-3773
                3776-3780 3784-3789 3804 3805 3864 3865 3893 3895
                3897 3902-3911 3913-3945 3953-3972 3974-3979
                3984-3989 3991 3993-4013 4017-4023 4025 4256-4293
                4304-4342 4352-4441 4447-4514 4520-4601 7830-7835
                8016 8018 8020 8022 8114 8116 8118 8119 8126 8130
                8132 8134 8135 8146 8147 8150 8151 8162-8164 8166
                8167 8178 8180 8182 8183 8319 8400-8412 8417 8450
                8455 8458-8467 8469 8472-8477 8484 8486 8488
                8490-8497 8499-8504 8576-8578 12295 12321-12335
                12353-12436 12441 12442 12449-12538 12549-12588
                12593-12686 19968-40869 44032-55203 63744-64045
                64256-64262 64275-64279 64286-64296 64298-64310
                64312-64316 64318 64320 64321 64323 64324
                64326-64433 64467-64829 64848-64911 64914-64967
                65008-65019 65056-65059 65136-65138 65140
                65142-65276 65382-65391 65393-65437 65440-65470
                65474-65479 65482-65487 65490-65495 65498-65500

            LCNMCHAR "-."
            UCNMCHAR "-."

                183 720 721 1600 1632-1641 1776-1785 2406-2415
                2534-2543 2662-2671 2790-2799 2918-2927 3047-3055
                3174-3183 3302-3311 3430-3439 3654 3664-3673 3782
                3792-3801 3872-3881 8204-8207 8234-8238 8298-8303
                12293 12337-12341 12443-12446 12540-12542 65279
                65296-65305 65392 65438 65439
                GENERAL YES
                ENTITY  NO

            NET "/>"
            PIC "?>"
            SHORTREF NONE

            -- Quantities are not restricted in XML --
            ATTCNT      99999999
             ATTSPLEN    99999999
            -- BSEQLEN  not used --
            -- DTAGLEN  not used --
            -- DTEMPLEN not used --
            ENTLVL      99999999
            GRPCNT      99999999
            GRPGTCNT    99999999
            GRPLVL      99999999
            LITLEN      99999999
            NAMELEN     99999999
            -- no need to change NORMSEP --
            PILEN       99999999
            TAGLEN      99999999
            TAGLVL      99999999

            DATATAG NO
            OMITTAG NO
            RANK NO
            SHORTTAG YES -- SHORTTAG is needed for NET --
            SIMPLE NO
            IMPLICIT NO
            EXPLICIT NO
            CONCUR NO
            SUBDOC NO
            FORMAL NO

B. 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.

[74] BaseChar ::= [#x0041-#x005A] | [#x0061-#x007A] /* Latin 1 upper and lowercase */
| #x00AA | #x00B5 | #x00BA | [#x00C0-#x00D6] | [#x00D8-#x00F6] | [#x00F8-#x00FF] /* 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 */
[75] Ideographic ::= [#x4E00-#x9FA5] | [#xF900-#xFA2D] | #x3007 | [#x3021-#x3029]
[76] 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]
[77] Letter ::= BaseCharIdeographic
[78] Digit ::= [#x0030-#x0039] /* 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 */
[79] Ignorable ::= [#x200C-#x200F] /* zw layout */
| [#x202A-#x202E] /* bidi formatting */
| [#x206A-#x206F] /* alt formatting */
| #xFEFF /* zw nonbreak space */
[80] Extender ::= #x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46 | #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309B-#x309E] | [#x30FC-#x30FE] | #xFF70 | #xFF9E | #xFF9F

C. 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

<!ENTITY example "<p>An ampersand (&#38;#38;) may be escaped
numerically (&#38;#38;#38;) or with a general entity
(&amp;amp;).</p>" >

then the XML processor will store the following string as the value of the entity example:

<p>An ampersand (&#38;) may be escaped
numerically (&#38;#38;) or with a general entity

and 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):

An ampersand (&) may be escaped
numerically (&#38;) or with a general entity

A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference.

1 <?XML version='1.0'?>
2 <!DOCTYPE test [
3 <!ELEMENT test (#PCDATA) >
4 <!ENTITY % xx '&#37;zz;'>
5 <!ENTITY % zz '&#60;!ENTITY tricky "error-prone" >' >
6 %xx;
7 ]>
8 <test>This sample shows a &tricky; method.</test>

This produces the following perhaps unexpected results:

  1. 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.)
  2. 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.
  3. 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.
  4. 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) string This sample shows a error-prone method.

D. 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.

E. Autodetection of Character Sets

The XML encoding declaration functions as an internal label on each entity, indicating which character set is in use. Before an XML processor can read the internal label, however, it apparently has to know what character set is in use&#151;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 four octets of input, which of the following cases apply (in reading this list, it may help to know that in Unicode, '<' is 0000 003C and '?' is 0000 003F, and the Byte Order Mark required of UCS-2 data streams is FEFF):

  1. 00 00 00 3C: UCS-4, big-endian machine (1234 order)
  2. 3C 00 00 00: UCS-4, little-endian machine (4321 order)
  3. 00 00 3C 00: UCS-4, unusual octet order (2143)
  4. 00 3C 00 00: UCS-4, unusual octet order (3412)
  5. FE FF 00 3C: UCS-2, big-endian
  6. FF FE 3C 00: UCS-2, little-endian
  7. 00 3C 00 3F: UCS-2, big-endian, no Byte Order Mark
  8. 3C 00 3F 00: UCS-2, little-endian, no Byte Order Mark
  9. 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
  10. 4C 6F E7 D4: EBCDIC (in some flavor; the full encoding declaration must be read to tell which code page is in use)
  11. 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-set routines should be careful to ensure the accuracy of the internal and external information used to label the entity.

F. 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 XML processor could parse a well-formed XML document, recognizing all element boundaries correctly (though not expanding entity references) using the following grammar:

Trivial text grammar
[81] Trivial ::= (PCData | Markup)*
[82] Eq ::= S? '=' S?
[83] Markup ::= '<' Name (S Name Eq AttValue)* S? '>' /* start-tags */
| '</' Name S? '>' /* end-tags */
| '<' Name (S Name Eq AttValue)* S? '/>' /* empty elements */
| '&' Name ';' /* entity references */
| '&#' [0-9]+ ';' /* character references */
| '&#x' Hex+ ';' /* character references */
| '<!--' (Char* - (Char* '--' Char*) '-->' /* comments */
| '<![CDATA[' CData ']]>' /* CDATA sections */
| '<!DOCTYPE' [^]]+ ('[' (Comment | TrivialLit | conditionalSect | [^]]* )* ']')? '>' /* doc type declaration */
| '<?' [^?]* ('?' [^>]+)* '?>' /* processing instructions */
[84] TrivialLit ::= ('"' [^"]* '"') | ("'" [^']* "'")

Most processors will require the more complex grammar given in the body of this specification.

G. 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/IEC 8879
ISO (International Organization for Standardization). ISO/IEC 8879-1986 (E). Information processing &#151; Text and Office Systems &#151; Standard Generalized Markup Language (SGML). First edition &#151; 1986-10-15. [Geneva]: International Organization for Standardization, 1986.
ISO/IEC 10646
ISO (International Organization for Standardization). ISO/IEC 10646-1993 (E). Information technology &#151; Universal Multiple-Octet Coded Character Set (UCS) &#151; Part 1: Architecture and Basic Multilingual Plane. [Geneva]: International Organization for Standardization, 1993.
ISO/IEC 10744
ISO (International Organization for Standardization). ISO/IEC 10744-1992 (E). Information technology &#151; 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.

H. W3C SGML Editorial Review Board

This specification was prepared and approved for publication by the W3C SGML Editorial Review Board (ERB). ERB approval of this specification does not necessarily imply that all ERB members voted for its approval. At the time it approved this specification, the SGML ERB comprised the following members:

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