We see big use of XML (eXtensible markup language) everywhere, especially in web programming and web services we do have to use XMLs. How is a simple XML file?
<?xml version="1.0"?> <user> <name>Tugrul</name> <lastname>ASLAN</lastname> <country>Turkey</country> </user>
The below example we are using the given XML data we have the tags like <user> </user> but how do we define those? Here we have the answer DTD(Document Type Definition). Simply we define a DTD file like this:
<!DOCTYPE user [ <!ELEMENT user(name,lastname,country)> <!ELEMENT name (#PCDATA)> <!ELEMENT lastname (#PCDATA)> <!ELEMENT country (#PCDATA)> ]>
here we have defined a definition for our xml schema. If I explain these attributes:
- !DOCTYPE means that user is the root element of the defition and xml schema,
- !ELEMENT user (name,lastname,country) defines that our schema will have three elements,
- !ELEMENT defines each element
- #PCDATA is last of all defines parse able text data
In xml the raw data has to be valid which does indicate that it needs to be a well formatted xml document which also identifies the rules of DTD. With the DTD you can identify your own XML tag format and add as many attributes as you wish.