XML DTD
An XML document with correct syntax is called "Well Formed".
An XML document validated against a DTD is both "Well Formed" and "Valid".
What is a DTD?
DTD stands for Document Type Definition.
A DTD defines the structure and the legal elements and attributes of an XML document.
Valid XML Documents
A "Valid" XML document is "Well Formed", as well as it 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 above contains a reference to a DTD file. The content of the DTD file is shown and explained below.
XML DTD
The purpose of a DTD is to define the structure and the legal elements and attributes of an XML document:
Note.dtd:
<!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"
Tip: #PCDATA means parseable character data.
Using DTD for Entity Declaration
A DOCTYPE declaration can also be used to define special characters or 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: W3Schools.">
]>
<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 »
Tip: An entity has three parts: it starts with an ampersand (&), then comes the entity name, and it ends with a semicolon (;).
When to Use a DTD?
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 NOT to Use a DTD?
XML does not require a DTD.
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 DTD. Otherwise, your software might stop working because of validation errors.
