XML - DTDs

The XML Document Type Declaration, commonly known as DTD, is a way to describe XML language precisely. DTDs check vocabulary and validity of the structure of XML documents against grammatical rules of appropriate XML language.
An XML DTD can be either specified inside the document, or it can be kept in a separate document and then liked separately.
Basic syntax of a DTD is as follows −
<!DOCTYPE element DTD identifier
[
   declaration1
   declaration2
   ........
]>
In the above syntax,
The DTD starts with <!DOCTYPE delimiter.
The DTD starts with <!DOCTYPE delimiter.
An element tells the parser to parse the document from the specified root element.
An element tells the parser to parse the document from the specified root element.
DTD identifier is an identifier for the document type definition, which may be the path to a file on the system or URL to a file on the internet. If the DTD is pointing to external path, it is called External Subset.
DTD identifier is an identifier for the document type definition, which may be the path to a file on the system or URL to a file on the internet. If the DTD is pointing to external path, it is called External Subset.
The square brackets [ ] enclose an optional list of entity declarations called Internal Subset.
The square brackets [ ] enclose an optional list of entity declarations called Internal Subset.
A DTD is referred to as an internal DTD if elements are declared within the XML files. To refer it as internal DTD, standalone attribute in XML declaration must be set to yes. This means, the declaration works independent of an external source.
Following is the syntax of internal DTD −
<!DOCTYPE root-element [element-declarations]>

where root-element is the name of root element and element-declarations is where you declare the elements.
Following is a simple example of internal DTD −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>
<!DOCTYPE address [
   <!ELEMENT address (name,company,phone)>
   <!ELEMENT name (#PCDATA)>
   <!ELEMENT company (#PCDATA)>
   <!ELEMENT phone (#PCDATA)>
]>

<address>
   <name>Tanmay Patil</name>
   <company>TutorialsPoint</company>
   <phone>(011) 123-4567</phone>
</address>
Let us go through the above code −
Start Declaration − Begin the XML declaration with the following statement.
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>

DTD − Immediately after the XML header, the document type declaration follows, commonly referred to as the DOCTYPE −
<!DOCTYPE address [

The DOCTYPE declaration has an exclamation mark (!) at the start of the element name. The DOCTYPE informs the parser that a DTD is associated with this XML document. 
DTD Body − The DOCTYPE declaration is followed by body of the DTD, where you declare elements, attributes, entities, and notations.
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone_no (#PCDATA)>
Several elements are declared here that make up the vocabulary of the <name> document. <!ELEMENT name (#PCDATA)> defines the element name to be of type "#PCDATA". Here #PCDATA means parse-able text data.
End Declaration − Finally, the declaration section of the DTD is closed using a closing bracket and a closing angle bracket (]>). This effectively ends the definition, and thereafter, the XML document follows immediately.
The document type declaration must appear at the start of the document (preceded only by the XML header) − it is not permitted anywhere else within the document.
The document type declaration must appear at the start of the document (preceded only by the XML header) − it is not permitted anywhere else within the document.
Similar to the DOCTYPE declaration, the element declarations must start with an exclamation mark.
Similar to the DOCTYPE declaration, the element declarations must start with an exclamation mark.
The Name in the document type declaration must match the element type of the root element.
The Name in the document type declaration must match the element type of the root element.
In external DTD elements are declared outside the XML file. They are accessed by specifying the system attributes which may be either the legal .dtd file or a valid URL. To refer it as external DTD, standalone attribute in the XML declaration must be set as no. This means, declaration includes information from the external source.
Following is the syntax for external DTD −
<!DOCTYPE root-element SYSTEM "file-name">

where file-name is the file with .dtd extension.
The following example shows external DTD usage −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
<!DOCTYPE address SYSTEM "address.dtd">
<address>
   <name>Tanmay Patil</name>
   <company>TutorialsPoint</company>
   <phone>(011) 123-4567</phone>
</address>
The content of the DTD file address.dtd is as shown −
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
You can refer to an external DTD by using either system identifiers or public identifiers.
A system identifier enables you to specify the location of an external file containing DTD declarations. Syntax is as follows −
<!DOCTYPE name SYSTEM "address.dtd" [...]>

As you can see, it contains keyword SYSTEM and a URI reference pointing to the location of the document.
Public identifiers provide a mechanism to locate DTD resources and is written as follows −
<!DOCTYPE name PUBLIC "-//Beginning XML//DTD Address Example//EN">

As you can see, it begins with keyword PUBLIC, followed by a specialized identifier. Public identifiers are used to identify an entry in a catalog. Public identifiers can follow any format, however, a commonly used format is called Formal Public Identifiers, or FPIs.

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