XML DTD(DOCUMENT TYPE DEFINITION)

XML DTD(DOCUMENT TYPE DEFINITION)

What is DTD?

·         DTD stands for document type definition

·         A DTD is a text file with .dtd extension

·         If XML file holds data, its corresponding DTD holds Meta data.

In a DTD legal building blocks of an XML documents are specified. i.e XML vocabulary is specified in A dtd.

What are the constituents of DTD file?

·         DTD point of view, all XML DOCUMENTs are made up by the following building blocks:

1.       Elements

2.       Attributes

3.       Entities

4.       PCDATA

5.       CDATA

Element:

this is the most important building block using which we can create tags. The tags can contain some text, it can contain someother elements (or) it can be empty.

<stud>

  <rollno>123</rollno>

<address/>

Element

2.       Attributes: attributes are used to provide additional information about an XML tag.

·         The attributes must be specified in the starting tags.

·         The attributes always come in name value pairs.

·         The attribute values must be specified either in single quotes or double quotes.

ENTITIES:

Some of the characters have a special meaning in XML , like the less than sign(<) that defines the start of a XML tag . most of you know the HTML Entity:”&nbsp;” this “no-breaking-space” entity is used in HTML to insert an extra space in a document. Entities are expanded when a document is parsed by an XML parser.

The following entities are predefined in XML:

ENTITIY REFERENCE	CHARACTER
&lt;	<
&gt	>
&amp	&
&quot	“
&apos;	‘

PCDATA:

Xml parsers normally parse all the text in an XML document. When an XML element is parsed, the text between the xml tags is also parsed:

Ex: <message>This text is also parsed<message>

The parser does this because XML elements can contain other element, as in this example, where the <name> element contains two other elements(first and last):

<name><first>Bill</first><last>Gates</last></name>

And the parser will break it up into sub-elements like this:

<name>

 <first>Bill</first>

<last>Gates</last>

</name>

Parsed Character Data (PCDATA) is a term used about text data that will be parsed by the XML parser.

CDATA-(Unparsed) Character Data

·         The term CDATA is used about text data that should not be parsed by the XML parser.

·         Characters like “<” and “&” are illegal in XML elements.

·         “<” will generate an error because the parser interprets it as the start of a new element

·         “&” will generate an error because the parser interprets it as the start of an character entity.

·         Some text , like JAVASCRIPT code, contains a lot of “<” or “&” characters. To avoid errors script code can be defined as CDATA.

·         Everything inside a CDATA section is ignored by the parser.

C DATA section starts with “<![CDATA[“and ends with “]]>”:

<script>

<![CDATA[function matchwo(a,b)

{

if(a<b&&a<0) then

{

 return l;

}

else

{

return 0;

}

]>

</script>

·         In the example above, everything inside the CDATA section is ignored by the parser.

Note: CDATA Sections: can not contain the string “]]>”. Nested CDATA sections are not allowed.

The “]]>” that marks the end of the CDATA section can not contain spaces or line breaks.

·         In the examples are declared with an element declaration. An element declaration has the following syntax:

XML DTD(DOCUMENT TYPE DEFINITION)

1.       

3.        Entities: these building blocks represent special characters.

4.       PCDATA: (Parsed character Data) : this data will not be parsed by the parser and it can not expand the entities.

5.       CDATA: (CHARACTER DATA): this data will not be parsed by the parser and it can not expand the entities.

Element