DTD

• DTD (Document Type Definition) (.dtd)
• Specifies a relatively simple syntax for a set of XML files (validation criteria)
• Can't specify complex relationships or data formats
• Does not support XML namespaces
• The DTD is non-heirarchical (flat, linear)
• DTD is not an XML language (which makes it more difficult to parse or transform)

?     Optional (zero or one)
*     Zero or more
+     One or more

#PCDATA    Parsed character data
#CDATA     Unparsed character data

( #PCDATA [| element]* )*    Mixed-content mode; any number of elements
                                 can be interspersed with text

#REQUIRED             The attribute value must be specified in the document.
#IMPLIED              The value need not be specified in the document. If it isn't,
                          the application will have a default value it uses.
"defaultValue"        The default value to use, if a value is not specified
                          in the document.
#FIXED "fixedValue"   The value to use. If the document specifies any value
                          at all, it must be the same.

Examples

<!-- first line of .dtd file -->
<?xml version="1.0" encoding="UTF-8"?>

<!-- slideshow element contains only 1 or more slide elements -->
<!ELEMENT slideshow (slide+)>

<!-- picture element contains only 1 or more pairs of the specified items -->
<!ELEMENT picture ((image, title)+)>

<!-- slide element contains only a title followed by zero or more item elements -->
<!ELEMENT slide (title, item*)>

<!-- title element consists entirely of parsed character data (PCDATA) -->
<!ELEMENT title (#PCDATA)>

<!-- item element contains only zero or more occurrences of PCDATA or item elements
     (interspersed) (mixed-content model) -->
<!ELEMENT item (#PCDATA | item)* >

<!-- slide element's type attribute must be given as
     type="tech", type="exec", or type="all" -->
<!ATTLIST slide
    type (tech | exec | all) #IMPLIED
>

Sample DTD files

<?xml version="1.0" encoding="UTF-8"?>
<!-- DTD for a cd catalog -->
<!ELEMENT catalog (cd*)>
<!ELEMENT cd (title, artist)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT artist (#PCDATA) >

<?xml version="1.0" encoding="UTF-8"?>
<!-- DTD for a simple "slide show" -->
<!ELEMENT slideshow (slide+)>
<!ATTLIST slideshow     
      title   CDATA    #REQUIRED    
      date    CDATA    #IMPLIED    
      author  CDATA    "unknown"    
>
<!ELEMENT slide (title, item*)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT item (#PCDATA | item)* >

<!ENTITY ldquo  "&#147;"> <!-- Left Double Quote -->
<!ENTITY rdquo  "&#148;"> <!-- Right Double Quote -->
<!ENTITY trade  "&#153;"> <!-- Trademark Symbol (TM) -->
<!ENTITY rtrade "&#174;"> <!-- Registered Trademark (R) -->
<!ENTITY copyr  "&#169;"> <!-- Copyright Symbol -->

Sample usage in XML file

• After the XML declaration and before the root element

Internal DTD

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE catalog [
    <!ELEMENT catalog (cd*)>
    <!ELEMENT cd (title, artist)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT artist (#PCDATA)>
]>
<catalog>
  <cd>
    <title>...</title>
    <artist>...</artist>
  </cd>
  ...
</catalog>

External DTD

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE catalog SYSTEM "filename.dtd">
<catalog>
  <cd>
    <title>...</title>
    <artist>...</artist>
  </cd>
  ...
</catalog>