XML DTD

XML Document Type Declaration, commonly it is called DTD. It is used to describe XML language absolutely. When we create DTD, we are creating a syntax rules for any XML documents that uses the DTD. Syntax

<! DOCTYPE element DTD identifier   
[  
decleration1   
decleration2   
. . . . .  
]>
  • It start with <! DOCTYPE delimiter.
  • An element tells parser to parse document from specified root element.

Internal DTD

A DTD is referred to as an internal DTD if elements are declared within the XML files. Here declaration works independent of an external source. Syntax
<! DOCTYPE root-element [element-declarations]>
Where root-element is the name of root element and element declaration is where you declare the elements. Example
<?xml version=?1.0??>  
<! DOCTYPE student[  
<!ELEMENT students (name, class, section)>  
<!ELEMENT name (#PCDATA)>  
<!ELEMENT class (#PCDATA)>  
<!ELEMENT section (#PCDATA)>  
]>  
<student>  
    <name> Rohit sharma</name>  
    <class>10</class>     
    <section>?B?</section>  
</student>

External DTD

In external DTD elements are declared outside the xml file. The URL can point to either a local or remote file using relative and absolute refrencesrespectively. It means declaration includes information from the external source. Syntax
<! DOCTYPE root-element SYSTEM "file-name">
Where file name is the file with .dtd extension. Example Example shows external DTD usage:
<?xml version=?1.0??>  
<!DOCTYPE student SYSTEM "student.dtd">  
<student>  
    <name> Rohit sharma</name>  
    <class>10</class>     
    <section>?B?</section>  
</student>
The content of the DTD file student.dtd is as shown:
<!ELEMENT student (name, class, section)>  
<!ELEMENT name (#PCDATA)>  
<!ELEMENT class (#PCDATA)>  
<!ELEMENT section (#PCDATA)>
We use system identifiers or public identifiers to indicate an external DTD.

System identifiers

It enable us to specify the location of an external file containing DTD declarations.
<!DOCTYPE name SYSTEM "address.dtd" [. . . .]>
It contains keyword SYSTEM and a URI reference pointing to the location of the document.

Public identifiers

It provide the mechanism to locate DTD resources.
<!DOCTYPE name PUBLIC "-//Beginning  XML// DTD Address Example//EN">
Here we can see, it begins with keyword PUBLIC, followed by specialized identifier. Public identifiers are used to identify an entry in a catalog. It follow any format, commonly used format is called Formal public identifiers.



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