Creating a DTD

When creating your own DTD you'll need to define all the elements and
attributes that you'll be using in your XML documents.

In this case we'll be creating a DTD for our message XML document.  Some
syntax to remember when creating  DTDs are the following:

A comma equals the boolean operator AND

A horizontal bar equals the boolean operator OR

parenthenses means this item occurs only once

a plus following an item says that the item must occur at least once

a question mark following an item name means that it occurs either once
or not

and a asterick following an item name means that it can occur zero or
more times.

Elements are declared in the following manner.  You have the element
declaration, element name and then the element part(s).  Then you close
the element declaration.

Then for declaring attributes you have the attribute declaration, the
element name that the attribute is attached to, the attribute name, the
type of attribute it is and then if there is any default values for the
attribute and then close the attribute declaration.  Let's take a look
at the DTD I created for the message.xml file.  Here's the message DTD
we've created.

Since we have main element being... the root element being message have
to define that which we said has two parts either email or letter. 
Letter is composed of letterhead and text.

Email's composed of header, subject, or text.  Subject can or can not be
there and you can have more than one text area.

The attributes listed we have letter and email both having the attribute
reply which has the possible values of yes or no and the default value
is no.

Then we have the element header which has the elements in it sender,
recipient (which can be zero or more) and date (which can or can not be
there.)

We also have the element subject which just contains PC character data
which is what you would use if you were going to have plain text within
an element.

Then letterhead has sender, recipient, and date.  Sender has PC
character data, recipient has PC character data, date has PC character
data, and then text either has PC character data or salutations zero or
more times.  Then salutation would be PC character data.  After creating
this DTD we would save it as message.dtd.

Then we need to link our XML documents to this DTD so that they can be
validated against it.  To link to an external dtd you include
declaration doctype, the root element in the DTD, in this case message,
the location SYSTEM and then in ( ) the name of the file.

This could also be a hyper reference like http:// and then the url of
the location of the DTD if it was hosted somewhere online.  If this was
the case then you would want to change SYSTEM to PUBLIC.

Let's take a look what it would look like if you declared a DTD
internally.  The structure of declaring internally would be very
similar.

You would have the DOCTYPE declaration the name of the root element, you
would have an open [ all the element and attribute declarations and then
a closing ] and then a closing tag >.

So if we wrapped that email.xml file in this internal document
declaration this is what it would look like.  It would have the DOCTYPE
declaration, the message and then the element names and attribute lists
and then the closing bracket and then the closing tag. Now that you've
learned how to link a DTD to a document both internally and externally
let's take a look at validating an XML document.