An XML document with correct syntax is called "Well Formed".
An XML document validated against a DTD is both "Well Formed" and "Valid".
Valid XML Documents
A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a DTD:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "Note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>The DOCTYPE declaration, in the example above, is a reference to an external DTD file. The content of the file is shown in the paragraph below.
XML DTD
The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:
<!DOCTYPE note
[
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>The DTD above is interpreted like this:
- !DOCTYPE note defines that the root element of the document is note
- !ELEMENT note defines that the note element must contain the elements: "to, from, heading, body"
- !ELEMENT to defines the to element to be of type "#PCDATA"
- !ELEMENT from defines the from element to be of type "#PCDATA"
- !ELEMENT heading defines the heading element to be of type "#PCDATA"
- !ELEMENT body defines the body element to be of type "#PCDATA"
#PCDATA means parse-able text data.
Using DTD for Entity Declaration
A doctype declaration can also be used to define special characters and character strings, used in the document:
Example
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ENTITY nbsp
" ">
<!ENTITY writer "Writer: Donald Duck.">
<!ENTITY copyright "Copyright: w3ii.">
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
<footer>&writer; ©right;</footer>
</note>
Try it Yourself »
An entity has three parts: an ampersand (&), an entity name, and a semicolon (;).
When to Use a DTD/Schema?
With a DTD, independent groups of people can agree to use a standard DTD for interchanging data.
With a DTD, you can verify that the data you receive from the outside world is valid.
You can also use a DTD to verify your own data.
If you want to study DTD, please read our DTD Tutorial.
When to NOT to Use a DTD/Schema?
XML does not require a DTD/Schema.
When you are experimenting with XML, or when you are working with small XML files, creating DTDs may be a waste of time.
If you develop applications, wait until the specification is stable before you add a document definition. Otherwise, your software might stop working because of validation errors.