XML DTD (document type definition)

A document type definition (DTD) defines a legitimate XML document building block. It uses a series of legitimate elements to define the structure of the document.

DTDs can be declared in an XML document in rows or as an external reference.

Declaring elements

In a DTD, an XML element is declared through an element declaration. The element declaration uses the following syntax:

<! Element name category > [category can Be: EMPTY, (#PCDATA), any]

Or

<! Element elements name (element content) >

An XML document instance with a DTD

<?xml version= "1.0"? ><! DOCTYPE note[  <! ELEMENT Note (to,from,heading,body) >  <! ELEMENT to      (#PCDATA) >  <! ELEMENT from    (#PCDATA) >  <! ELEMENT heading (#PCDATA) >  <! ELEMENT body    (#PCDATA) >]><note>  <to>George</to>  <from>john</from >  
  
! DOCTYPE Note (second line) defines this document as a type of note.
  

  
! Element Note (line three) defines a note element with four elements: "To, from, heading, body"

  
! element to (line fourth) defines the to element as "#PCDATA" type

  
! Element from (line fifth) defines the from element as the "#PCDATA" type

  
! Element heading (line sixth) defines the heading element as the "#PCDATA" type

  
! Element body (line seventh) defines the BODY element as the "#PCDATA" type

  
PCDATA means the parsed character data (Parsedcharacter), PCDATA is the text that will be parsed by the parser. The text will be examined by the parser entity and tagged

  
CDATA means character data (Characterdata). CDATA is text that will not be parsed by the parser

  
<! ELEMENT Note (#PCDATA |to|from|header|message) *[can be suffixed with:?, *, +]>

  
The above example declares that the "note" element can contain 0 or more occurrences of PCDATA, "to", "from", "header", or "message".
declaring properties
  
The property declaration uses the following syntax:

  
<! Attlist element Name Property Name property type default Value >

  
The following are the options for the property type:

  
Type description

  
CDATA value is character data (character)

  
(en1|en2|.) This value is one of the values in the enumeration list

  
ID value is a unique ID

  
IDREF value is the ID of another element

  
IDREFS value is a list of other IDs

  
NmToken value is a valid XML name

  
Nmtokens value is a list of valid XML names

  
Entity value is a solid

  
The entities value is a list of entities

  
NOTATION This value is the name of the symbol

  
XML: Value is a pre-defined XML value

  
The default value parameter can use the following values:

  
Value explanation

  
Default value of the Value property

  
#REQUIRED property value is required

  
#IMPLIED properties are not required

  
The value of the #FIXED Value property is fixed
Declaring entities
  
Grammar:

  
<! Entity entity name "Value of Entities" >

 

XML DTD (document type definition)