DTD tutorial

Introduction

This is a short tutorial about DTDs. It brievely shows how to read DTDs, then how to create these.

A note on its production: This spring I am teaching a little course at Webster, Geneva. I usually do my teaching slides with Framemaker (e.g. see my french slides. Now I wanted to know how long it takes to translate them into wiki. Answer: about an hour. I first export to HTML and then apply a html2wiki filter and then hand edit. That's too long. Of course, this result also needs some editing, slides are not meant for self-study .... - 17:06, 23 April 2007 (MEST).

DTD grammars are a set of rules that define:

• a set of elements (tags) and their attributes that can be used to create an XML document;
• how elements can be embedded ;
• different sorts of entities (reusable fragments, special characters).
• DTDs can't define what the contents look like, i.e. character data (element contents) and most attribute values.

Specification of a markup language

• The most important part is usually the DTD, but in addition other constraints can be added !
• The DTD does not identify the root element ! You have to tell the users what elements can be root elements.
• Since DTDs can't express data constraints, write them out in a specification document, e.g. "the value of length attribute is a string composed of a number plus "cm" or "inch" or "em"

 example: <size length="10cm">

Example 1: A simple DTD

<!ELEMENT page  (title, content, comment?)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT content (#PCDATA)>
<!ELEMENT comment (#PCDATA)>

• A DTD document contains just rules .... nothing else (see later for explanations)

Using a DTD with an XML document

A. Document type declarations

• A valid XML document includes a declaration that specifies the DTD used
• DTD is declared on top of the file after the XML declaration.
• XML declarations, DTD declaration etc. are part of the prologue
• So: The <!DOCTYPE...> declaration is part of the XML file, not the DTD ....

Example:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE hello SYSTEM "hello.dtd">
<hello>Here we <strong>go</strong> ... </hello>

Four ways of using a DTD

(1) No DTD (XML document will just be well-formed) (2) DTD rules are defined inside the XML document

• We get a "standalone" document (the XML document is self-sufficient)

(3) "Private/System" DTDs, the DTD is located on the system (own computer or the Internet)

1. • ... that's what you are going to use when you write your own DTDs

<!DOCTYPE hello SYSTEM "hello.dtd">

(4) Public DTDs, we use a name for the DTD. This means that both your XML editor and user software know the DTD. This is the strategy used for common Web DTDs like XHTML, SVG, MathML, etc.

Syntax of the DTD declaration in the XML document

• A DTD declaration starts with the keyword "DOCTYPE":

  <!DOCTYPE ....  >

• ... followed by the root element
• Remember that DTDs don't know their root element, root is defined in the XML document ! Also note that DTDs must define this root element just like any other element ! (you can have more than one)

 <!DOCTYPE hello .... >

• ... followed by the DTD definition or a reference to a DTD file

Syntax for internal DTDs (only !)

• DTD rules are inserted between brackets [ ... ]

   <!DOCTYPE hello [
       <!ELEMENT hello (#PCDATA)>
       ]>

Syntax to define "private" external DTDs:

• DTD is identified by the URL after the " SYSTEM " keyword

<!DOCTYPE hello  SYSTEM "hello.dtd" >

Syntax for public DTDs:

• after the " PUBLIC " keyword you have to specify an official name and a backup URL that a validator could use.

<!DOCTYPE rss PUBLIC "-//Netscape Communications//DTD RSS 0.91//EN"
 "http://my.netscape.com/publish/formats/rss-0.91.dtd"
>

Example 2: Hello XML without DTD

<?xml version="1.0" standalone="yes"?>
<hello> Hello XML et hello cher lecteur ! </hello>

Example 3: Hello XML with an internal DTD

<?xml version="1.0" standalone="yes"?>
<!DOCTYPE hello [
   <!ELEMENT hello (#PCDATA)>
   ]>
<hello> Hello XML et hello ch�re lectrice ! </hello>

Example 4: Hello XML with an external DTD

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE hello SYSTEM "hello.dtd">
<hello> This is a very simple XML document </hello>

• That's what you should with your own home-made DTDs

Example 5: XML with a public external DTD (RSS 0.91)

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE rss PUBLIC "-//Netscape Communications//DTD RSS 0.91//EN"
 "http://my.netscape.com/publish/formats/rss-0.91.dtd">
<rss version="0.91">
<channel> ...... </channel>
</rss>

4.2 Understanding DTDs by example

• Recall that DTDs define all the elements and attributes and the way they can be combined

Example 6: Hello text with XML

A simple XML document of type <page>

<page>
 <title>Hello friend</title>
 <content>Here is some content :)</content>
 <comment>Written by DKS/Tecfa, adapted from S.M./the Cocoon samples</comment>
</page>

A DTD that would validate the document

Example 7: A recipe list in XML

• init/simple_recipe.xml
• Source: Introduction to XML by Jay Greenspan (now dead URL)

 <?xml version="1.0"?>
 <!DOCTYPE list SYSTEM "simple_recipe.dtd">
 <list>
 <recipe>
   <author>Carol Schmidt</author>
   <recipe_name>Chocolate Chip Bars</recipe_name>
   <meal>Dinner</meal>
   <ingredients>
     <item>2/3 C butter</item>      <item>2 C brown sugar</item>
     <item>1 tsp vanilla</item>     <item>1 3/4 C unsifted all-purpose flour</item>
     <item>1 1/2 tsp baking powder</item>
     <item>1/2 tsp salt</item>      <item>3 eggs</item>
     <item>1/2 C chopped nuts</item>
     <item>2 cups (12-oz pkg.) semi-sweet choc. chips</item>
   </ingredients>
   <directions>
Preheat oven to 350 degrees. Melt butter; combine with brown sugar and vanilla in large mixing bowl. Set aside to cool.  Combine flour, baking powder, and salt; set aside. Add eggs to cooled sugar mixture; beat well. Stir in reserved dry  ingredients, nuts, and chips.
Spread in greased 13-by-9-inch pan. Bake for 25 to 30 minutes until golden brown; cool.  Cut into squares.
   </directions>
 </recipe>
</list>

Contents of the DTD

Example 8: A simple story grammar

Example 9: Lone family DTD

A valid XML file

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE family SYSTEM "family.dtd">
<family>
  <person name="Joe Miller" gender="male"
          type="father" id="123.456.789"/>
  <person name="Josette Miller" gender="female"
          type="girl" id="123.456.987"/>
</family>

Example 10: RSS

• complex/rss-0-92.dtd
• There are several RSS standards. RSS 0.91 is Netscape's original (still being used)

<!ELEMENT rss (channel)>

<!ATTLIST rss version CDATA #REQUIRED>
 <!-- must be "0.91">
 -->

 <!ELEMENT channel (title | description | link | language | item  | rating? | image? | textinput? | copyright? | pubDate? | lastBuildDate? | docs? | managingEditor? | webMaster? | skipHours? | skipDays?)*>
 <!ELEMENT title (#PCDATA)>
 <!ELEMENT description (#PCDATA)>
 <!ELEMENT link (#PCDATA)>
 <!ELEMENT image (title | url | link | width? | height? | description?)*>
 <!ELEMENT url (#PCDATA)>
 <!ELEMENT item (title | link | description)*>
 <!ELEMENT textinput (title | description | name | link)*>
 <!ELEMENT name (#PCDATA)>
 <!ELEMENT rating (#PCDATA)>
 <!ELEMENT language (#PCDATA)>
 <!ELEMENT width (#PCDATA)>
 <!ELEMENT height (#PCDATA)>
 <!ELEMENT copyright (#PCDATA)>
 <!ELEMENT pubDate (#PCDATA)>
 <!ELEMENT lastBuildDate (#PCDATA)>
 <!ELEMENT docs (#PCDATA)>
 <!ELEMENT managingEditor (#PCDATA)>
 <!ELEMENT webMaster (#PCDATA)>
 <!ELEMENT hour (#PCDATA)>
 <!ELEMENT day (#PCDATA)>
 <!ELEMENT skipHours (hour )>
 <!ELEMENT skipDays (day )>

Possible XML document for RSS

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE rss SYSTEM "rss-0.91.dtd">
<rss version="0.91">
   <channel>
     <title>Webster University</title>
     <description>Home Page of Webster University</description>
     <link>http://www.webster.edu</link>
     <item>
       <title>Webster Univ. Geneva</title>
       <description>Home page of Webster University Geneva</description>
       <link>http://www.webster.ch</link>
     </item>
     <item>
       <title>http://www.course.com/</title>
       <description>You can find Thomson text-books materials (exercise data) on this web site</description>
       <link>http://www.course.com/</link>
     </item>
   </channel>
 </rss>

Summary syntax of element definitions

• The purpose of this table is not to teach you how to write DTDs
• To understand how to use DTDs, you just need to know how to read a DTD

syntax element	short explanation	Example Element definitions Valid XML example
,	elements in that order	<!ELEMENT Name (First, Middle, Last)> Element Name must contain First, Middle and Last <Name> <First>D.</First><Middle>K.</Middle><Last>S.</Last> </Name>
?	optional element	<!ELEMENT Name (First,Middle?,Last)> Middle is optional <Name><First>D.</First><Last>S.</Last></Name>
	at least one element	<!ELEMENT list (movie ) <list><movie>Return of ...</movie> <movie>Comeback of ...</movie> </list>
*	zero or more elements	<!ELEMENT list (item*) almost as above, but list can be empty
|	pick one (or operator)	<!ELEMENT major (economics | law) <major> <economics> </economics> </major>
()	grouping construct, e.g. one can add ? or * or to a group.	<!ELEMENT text (para | list | title)* <text> <title>Story</title><para>Once upon a time</para> <title>The awakening</title> <list> ... </list> </text>

5.1 Definition of elements

Rough syntax of a DTD rule to define elements:

<!ELEMENT tag_name child_element_specification>

Child_element_specification may contain:

• A combination of child elements according to combination rules

<!ELEMENT page  (title, content, comment?)>

• Mixed contents, i.e. child elements plus #PCDATA or ANY

<!ELEMENT para (strong | #PCDATA )*>

• #PCDATA (Just data)

<!ELEMENT title (#PCDATA)>

• ANY (only used during development)

<!ELEMENT para (ANY)*>

• EMPTY (the element has no contents)

<!ELEMENT person EMPTY>

Tag names

• Each tag name must start with a letter or an underscore ('_')
  followed by letters, numbers or the following characters: '_' , '-', '.', ':'

BAD example:
  <!ELEMENT 1st ...>

BAD example:

 <!ELEMENT My Home ...>

5.2 Combination rules

A and B = tags	Explanation	DTD example	XML example
A , B	A followed by B	<!ELEMENT person ( name ,email? )>	<person> <name>Joe</name> <email>x@x.x</email> </person>
A?	A is optional, (it can be present or absent)	<!ELEMENT person (name, email? )>	<person> <name>Joe</name></person>
A	At least one A	<!ELEMENT person (name, email )>	<person> <name>Joe</name> <email>x@x.x</email></person> <person> <name>Joe</name> <email>x@x.x</email> <email>x@y.x</email> </person>
A*	Zero, one or several A	<!ELEMENT person (name, email* )>	<person> <name>Joe</name> </person>
A | B	Either A or B	<!ELEMENT person ( email | fax )>	<person> <name>Joe</name> <email>x@x.x</email></person> <person> <name>Joe</name> <fax>123456789</fax></person>
(A, B)	Parenthesis will group and you can apply the above combination rules to the whole group	<!ELEMENT list ( name, email ) >	<list> <person> <name>Joe</name> <email>x@x.x</email></person> </list>

Special contents

Special elements	Explanation	DTD examples	XML example
#PCDATA	"Parsed Character Data" Text contents of an element. It should not contain any <,>,& etc.	<!ELEMENT email (#PCDATA)>	<email>Daniel.Schneider@tecfa.unige.ch</email>
ANY	Allows any non-specified child elements and parsed character data (avoid this !!!)	<!ELEMENT person ANY>	<person> <c>text</c> <a>some <b>bbb</b> inside </a> </person>
EMPTY	No contents	<!ELEMENT br EMTPY>	<br/>

Note about Mixed Content

• Mixed element contents contain both text and tags.

<para> here is <a href="xx">link</a>. <b>Check</b> it out </para>

• You have to use the "|" construct for these
  • Good examples:

<!ELEMENT para (#PCDATA|a|ul|b|i|em)*>  
<!ELEMENT p (#PCDATA | a | abbr | acronym | br | cite | code | dfn | em | img | kbd |                         q | samp | span | strong | var )* >  
<!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* >

• • Bad examples:

<!ELEMENT p (name, first_name, #PCDATA)*>
<!ELEMENT p ( (#PCDATA) |a|ul|b|i|em)*>

Defining attributes

Rough syntax of Attribute rules:

<!ATTLIST element_name attr_name Attribute_type Type_Def Default >

Overview:

Type	Attribute types
CDATA	"Character Data" - Text data
NMTOKEN	A single word (no spaces or punctuations)
ID	Unique identifier of the element.
IDREF	Reference to an identifier.
IDREFS	Reference to one or more identifiers
(A|B|C|..)	List of values (from which the user must choose)

	Type Definition
#IMPLIED	Attribute is optional
#REQUIRED	Attribute is mandatory)
#FIXED Value	Attribute has a fixed value (user can't change it)

Illustrations:

DTD rule	example XML
<!ATTLIST person first_name CDATA #REQUIRED>	<person first_name="Joe">
<!ATTLIST person gender (male|female) #IMPLIED>	<person gender="male">
<!ATTLIST form method CDATA #FIXED "POST">	<form method="POST">
<!ATTLIST list type (bullets|ordered) "ordered">	<list type="bullets">
<!ATTLIST sibling type (brother|sister) #REQUIRED>	<sibling type="brother">
<!ATTLIST person id ID #REQUIRED>	<person id="N1004">

Shortcut to define multiple attributes for an element:

<!ATTLIST target_tag

attr1_nom TypeAttribut TypeDef Defaut

attr2_nom TypeAttribut TypeDef Defaut

...>

Shortcut illustrations:

<!ATTLIST person ident ID #REQUIRED 
                    gender male|female) #IMPLIED
                    nom CDATA            #REQUIRED   prenom       CDATA            #REQUIRED   relation     brother|sister)  #REQUIRED >  <!ATTLIST portable       owner   IDREF            #REQUIRED >

Example 5-1: Lone family DTD (file family.dtd)

A valid XML file

<?xml version="1.0" encoding="ISO-8859-1" ?>

<!DOCTYPE family SYSTEM "family.dtd">

<family>

  <person name="Joe Miller" gender="male"

          type="father" id="N123456789"/>

  <person name="Josette Miller" gender="female"

          type="girl" id="N123456987"/>

</family>

5.6 General entities

Consider entities as abbreviations for some other content. An entity must be defined in the DTD and its contents are substituted when encountered in the XML file.

• • Recall that XML initially only defines 5 entities and that HTML does many more...
  • Use the &lt; &amp; &gt; &aquot; &apos; entities to refer to <, &, >," and '

Syntax of an internal entity definition: <!ENTITY entity_name "content">

Syntax of an external entity definition: <!ENTITY entity_name SYSTEM URI>

Syntax of using an entity: &entity_name;

Illustrations of entity definitions:

DTD rule	XML example	Result
<!ENTITY jt "Joe Test">	<para> &jt; is here<para>	<para> Joe Test is here</para>
<!ENTITY space "&#160;">
<!ENTITY copyright "&#xA9;">	&copyright; D. Schneider
<!ENTITY explanation SYSTEM "project1a.xml">	<citation> &explanation; </citation>	<citation> contents of project1a.xml ... </citation>

5.7 Parameter entities

• Most professional DTDs use parameter entities.
• These are used to simplify DTD writing

<!ENTITY  % entity_name "content">
<!ENTITY  % entity_name SYSTEM "URI">

Example 5-2: DTD entities to define reusable child elements

• More complex DTD often use same structures all over. Instead of typing these several times for each element definition, one can use an ENTITY construction like this:

<!ENTITY % Content "(Para | List | Listing)*">

Later in the DTD we then can have element definitions like this:

<!ELEMENT Intro (Title, %Content; ) >
<!ELEMENT Goal (Title, %Content; ) >

The XML parser will then simply translate these %Content; and we get:

<!ELEMENT Intro (Title, (Para | List | Listing)*) >
<!ELEMENT Goal (Title, (Para | List | Listing)* ) >

Example 5-3: DTD entities to define reusable attribute definitions

• You may use the same procedure to define "bricks" for attribute definitions
• Entity example that defines part of an attribute definition

<!ENTITY % stamp   '
  id ID #IMPLIED
  creation-day NMTOKEN #IMPLIED
  .......
  mod-by NMTOKEN #IMPLIED
  version NMTOKEN #IMPLIED
  status (draft|final|obsolete) #IMPLIED
  approval (ok|not-ok|so-so) #IMPLIED
  main-author CDATA #IMPLIED
' >

ATTLIST definitions below use %stamp;

<!ELEMENT main-goal (title, content, (after-thoughts)?, (teacher-comments)?)>
<!ATTLIST main %stamp; >
<!ELEMENT title (...)>
<!ATTLIST main %stamp; >

Some advice for designing DTDs

Don't forget elements and be liberal

• Each element needs to be defined, but only once !
• Only make elements mandatory if they really are wanted, else use e.g. element  ?

Plan the global structure

• Before you start writing out DTDs, use some simple "language" to draft the structure, e.g. use a notation like:

name   ==>  family   given
family ==> "text"

• In most cases, each "object" of your "information domain" becomes an element
• Use child elements to model components
• Use attributes to describe properties of components

Start from the root element and work your way down:

1. Root element
2. Child elements of root element
3. Child elements of the other elements, etc.