DTD Introduction

• Document Type Definitions (DTDs) impose structure on an XML document
• Using DTDs, we can specify what a "valid" document should contain
• DTD specifications require more than being well-formed
• DTDs have limited expressive power, e.g., one cannot specify types
• DTDs can be used to define special languages of XML, i.e., restricted XML for special needs
• A document type declaration is placed in the XML document's preceding the root element
• To begins with <!DOCTYPE and ends with >

Internal Subset

To declaration that are inside the XML document, anythiing inside square brackets [].

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE myRoot [
 <!ELEMENT myElement ( #PCDATA ) >
]>

External Subset

To declration that are outside the XML document, physically exist in a different file that typically ends with the .dtd extension. External subsets are specified using either keyword SYSTEM or the keyword PUBLIC

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE SYSTEM "myDTD.dtd">

The PUBLIC keyword indicates that the DTD is widely used and may be available in well-known locations.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//NLM//DTD Journal Publishing DTD v3.0 20080202//EN" 
"https://jats.nlm.nih.gov/archiving/1.1d1/JATS-archivearticle1.dtd">

Examples

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE addressbook [ 
  <!ELEMENT addressbook (person*)> 
  <!ELEMENT person  (name, greet?, address*,                               
(fax | tel)*, email+)> 
  <!ELEMENT name   (#PCDATA)> 
  <!ELEMENT greet  (#PCDATA)> 
  <!ELEMENT address(#PCDATA)> 
  <!ELEMENT tel    (#PCDATA)> 
  <!ELEMENT fax    (#PCDATA)> 
  <!ELEMENT email  (#PCDATA)> 
]> 

<person>  
 <name>Mahendran</name> 
 <great>Mr.</great> 
 <addr>New Colony Road</addr> 
 <addr>Springfield USA, 98765</addr> 
 <tel>(321) 786 2543</tel> 
 <fax>(321) 786 2544</fax> 
 <tel>(321) 786 2544</tel> 
 <email>info@edumasktut.com</email>   
</person>  

DTD Elements

Elements are the primary building block used in XML documents and are declared in a DTD element type declaration.

An element can be declared as having mixed content like element and PCDATA. The comma (,) plus sign (+) and question mark (?) occurrence indicators cannot be used with mixed content element.

An element delcared as type ANY can contain any content including PCDATA, elements, or a combination of elements and PCDATA. Elements with ANY content can also be empty elements.

Syntax of a DTD rule to define elements:

Syntax: <!ELEMENT tag_name child_element_specification>

• A combination of child elements according to combination rules
  <!ELEMENT page (title, content, comment?)>
• Mixed contents, i.e. child elements plus #PCDATA or ANY
  <!ELEMENT para (strong | #PCDATA )*>
• #PCDATA (Just data)
  <!ELEMENT title (#PCDATA)>
• ANY (only used during development)
  <!ELEMENT para (ANY)*>
• EMPTY (the element has no contents)
  <!ELEMENT person EMPTY>

Regular Expression

name, greet?, addr*, (tel | fax)*, email+

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE addressbook [ 
  <!ELEMENT addressbook (person*)> 
  <!ELEMENT person  (name, greet?, address*, (fax | tel)*, email+)> 
  <!ELEMENT name   (#PCDATA)> 
  <!ELEMENT greet  (#PCDATA)> 
  <!ELEMENT address(#PCDATA)> 
  <!ELEMENT tel    (#PCDATA)> 
  <!ELEMENT fax    (#PCDATA)> 
  <!ELEMENT email  (#PCDATA)> 
]> 

• name = there must be a name element
• greet? = there is an optional greet element (i.e., 0 or 1 greet elements)
• name, greet? = the name element is followed by an optional greet element
• addr* = there are 0 or more address elements
• tel | fax = there is a tel or a fax element
• (tel | fax)* = there are 0 or more repeats of tel or fax
• email+ = there are 1 or more email elements

Combination rules

DTD Attributes

• An attribute list for an element is declared using the ATTLIST element type declaration.
• An attribute's default values: Keywords #IMPLIED, #REQUIRED, #FIXED are attribute default.
• Attributes type are classified as either string (CDATA), tokenized, or enumerated
• ID, IDREF, ENTITY, NMTOKEN are all type of tokenized attributes

<!ATTLIST element-name  
 attribute-name type default-value > 
 
 <!ATTLIST height  dim  CDATA  "cm"> 
 
 <height dim="cm" />
 

type is one of the following:

default-value is one of the following:

Attributes Examples

<!ELEMENT height (#PCDATA)> 
 
<!ATTLIST height  
      dimension  (cm|in)   #REQUIRED 
      accuracy    CDATA    #IMPLIED  
      resizable   CDATA    #FIXED "yes" 
>

DTD Entities

Entities are XML macros. Macros expanded when the document is processed.

• Character entities
• Named entities
• External entities
• Parameter entities

Character entities: stand for arbitrary Unicode characters (©)

<!ENTITY copy   "&#169;"> 

Named entities: Entities can reference other entities

<!ENTITY name "Mahendran"> 
<!ENTITY person "&name; Developer"> 

Using person in a document expands to:  Mahendran Developer

External entities: Represent the content of an external file. Useful when breaking a document down into parts.

[
<!ENTITY chap1 SYSTEM "chapter-1.xml"> 
<!ENTITY chap2 SYSTEM "chapter-2.xml">
]
<book>
&chap1;&chap2;
</book>

Parameter entities: distinguish themselves from other entities, in the entity declaration Using (%).

<!ENTITY % accept "INCLUDE" >

<![%accept;[
  <!ELEMENT message (appproved, sign)>
]]>

<!ELEMENT approved EMPTY>