Editing XML tutorial

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beginner

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Introduction

This is a beginners tutorial for XML editing made from slides

Learning goals

Be able to somewhat understand Document Type Definition (DTD) schemas
Understand the necessity of using an XML editor
Be able to edit XML without hand-editing tags, profit from DTD and Schema awareness
Be able to check well-formedness and validate
Be able to fix well-formedness and validity errors

Prerequisites

Some idea what XML is about
XML principles (important !)
XML namespace (optional)
HTML and CSS (some)

Next steps

CSS for XML tutorial
DTD tutorial
XML namespace (more details about XML namespaces)
XML Schema tutorial - Basics
XSLT Tutorial - Basics
XPath tutorial - basics
XQuery tutorial - basics (if you have interest in XML databases)
PHP - MySQL - XML tutorial - basics (shows how to display an XML result-set retrieved form MySQL with XSLT)

Recall of XML principles

Let us recall some principles that you also may have read in the XML principles article. In particular:

An XML document is a hierarchical structure
Syntax must be well-formed (all tags closed, etc.)
Special XML characters like the < and the > must be dealt with in a special way
Content may be validated by a schema (aka grammar)
Often, more than one XML language appears in a document. In that case, so-called namespaces must be used

Defining XML languages

Many XML languages are defined with so-called schemas, i.e. some sort of grammars that define tags and attributes and how they can be combined. There exist several schema formalism. Other languages are defined with text, e.g. the well-known RSS 0.9 syndication language. Often a language is defined using both schemas and text, e.g. HTML and SVG.

There are four more or less popular schema languages:

(1) Document Type Definitions (DTDs)

DTD uses a terse formal syntax that declares in particular which elements and attributes may appear in a document and how they should be nested. On can define how elements can be nested within other elements, what attributes can be used within an element and finally it is possible to declare some very simple data types for attributes.

(2) XML Schema

XML Schema has the same purpose as DTDs, but allows to add additional constraints, e.g. you could require that and element should include only a number and that this number should be in the range of 1:10. XML Schemas are mostly used to describe complex document and data formats, e.g. e-learning standards or Microsoft "dotX" formats.

(3) Relax NG

is an XML Schema language that represents a sort of compromise between the simplicity of DTDs and the complexity of W3C XML Schema

(4) Schematron

is a rule-based validation language for making assertions about the presence or absence of patterns in XML trees. It can be used in addition to other Schema languages.

Using DTDs (Document Type Definitions)

Principles

DTD grammars are just a set of rules that define:

a set of elements (tags) and their attributes;
how elements can be combined/embedded;
different sorts of entities (reusable fragments, special characters).

The most important part in a formal XML specification making use of DTDs, is usually the DTD. In addition, other constraints can be added ! In particular:

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, you may write out additional ones in a specification document

e.g. "the value of length attribute is a string composed of a number plus one of "inch", "em", "cm".

 <size length="10cm">

DTD file association with an XML file

XML grammars like DTDs and XML Schemas can be directly associated with an XML file. This way, the XML carries information about its content structure that allows any client to verify if it is valid.

A simple DTD example (file "page.dtd")

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

The following XML document is a valid with respect to the grammar defined in "page.dtd" (just above)

 <?xml version="1.0"?>
 <!DOCTYPE page SYSTEM "page.dtd">
 <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 document contains just definition of rules .... nothing else (see later for explanations). The "page" DTD defines the following:

a page element, that must include a title followed by a content element and optionally a comment element.
the title, content and comment elements only can include tags, i.e. no other tags.

Specification of a markup language. Is a DTD enough ?

DTDs can’t define what the character data (element contents) and most attribute values should look like. For example, if you require that the user enters a number between 10 and 15 or the name of 15 different capitals, then you would have to use another formalism than DTD.

We introduce some of the DTD "language" below, but details are explained in the DTD tutorial. But let us now first systematically describe how a DTD file can be associated with an XML document.

Associating a DTD with an XML document

There are four ways of using a DTD with an XML file:

(1) No DTD

XML document will just be well-formed, or validation takes place in some other contexts, e.g. there exist tools that allow you to find out if a given XML document is valid with respect to a given DTD file)

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

(2) DTD rules are defined inside the XML document

We get a "standalone" document (the XML document is self-sufficient)
Notice the use of brackets [....]

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

(3) Private/System DTDs

the DTD is located on the system (own computer or the Internet).
That's what you are going to use when you write your own DTDs.

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

(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.
The naming convention also allows for a fallback URI that should include the physical DTD file.

 <?xml version="1.0" "?>
 <!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>

Syntax of the DTD declaration in the XML document

The syntax rules are fairly simple and can be understood from looking at the example above, and you may skip this section ....

(1) Every DTD declaration must start with

<!DOCTYPE .... >

(2) Then, the root element must be specified next. Remember that DTDs don’t know their root element, root is defined in the XML document ! DTDs must define this root element just like any other element ! In some cases, DTDs are meant to be used in different ways, i.e. several elements could be used as root elements.

 <!DOCTYPE hello .... >

(3) The next elements of the DTD declaration are different according to the DTD type (public or private)

(a) Syntax for internal DTDs (only !). DTD rules are inserted between brackets [ ... ]

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

(b) Syntax to define "private" external DTDs: The DTD is identified by the URL after the "SYSTEM" keyword

 <!DOCTYPE hello SYSTEM "hello.dtd">

Example using an URL

 <!DOCTYPE hello SYSTEM "http://tecfa.unige.ch/guides/xml/examples/simple/hello-page.dtd">

(c) Syntax for public DTDs: After the "PUBLIC" keyword you have to specify an official name and a backup URL that a validator could use. For example:

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

Understanding DTDs by example

Below we will present a few DTDs in increasing complexity.

Hello text with XML

Below is a simple XML document of type <page>:

<?xml version="1.0"?>
 <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>

The following DTD could validate the document:

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

Firstly it defines a page element that must include a title element, a content element, and optionally a comment element. Second, each of these sub-elements can only include text data, i.e. no other text.

Simple page DTD explained

Schemas for recipes

Recipes are very popular in XML education.

Take one Let's first look at a quite simple example, originally published by Jay Greenspan (dead link)

<?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
<course>Dessert</course>
</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>

The DTD would look like this

A simple recipe DTD

Take two

Below is half-filled in example of a slightly more complex recipe list in XML. As you can see, this example uses a more nested structure. For example, author, date, and version are children of a meta element. Directions includes a para element, i.e. a kind of formatting instruction which is meant to produce more legible text.

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE list SYSTEM "recipe-2.dtd">
<?xml-stylesheet href="recipe-2.css" type="text/css"?>
<list>
  <recipe>
    <meta>
      <author>Joe</author>
      <date></date>
      <version></version>
    </meta>
    <recipe_name>Vegetable soup</recipe_name>
    <meal>dinner</meal>
    <ingredients>
      <item>4 Carrots</item>
      <item>2 Onions</item>
      <item>Garlic</item>
      <itme>1/2 Cabbage</item>
      <item>Salt</item>
      <item>Pepper</item>
    </ingredients>
    <directions>
     <para>Cut the vegies into little pieces. Then boil with
     water. Add some salt and pepper</para>
    </directions>
  </recipe>
</list>

Contents of the DTD (simple_recipe.dtd)

<!-- Simple recipe DTD -->

<!-- This DTD will allow to write simple recipees
  list = a list of recipees
  recipee = container for a recipee
  meta = Metainformation: must include author of this file, 
         date, version in this order
  recipee_author = optional name of recipee author
  mail = title of meal
  ingredients = list of items you need
  directions = How to cook, may include either para's or bullet's.
-->

 <!ELEMENT list (recipe+)>
 <!ELEMENT recipe (meta, recipe_author?, recipe_name, meal, 
                  ingredients, directions)>
 <!ELEMENT meta (author, date, version)>
 <!ELEMENT version (#PCDATA)>
 <!ELEMENT date (#PCDATA)>
 <!ELEMENT author (#PCDATA)>
 <!ELEMENT recipe_author (#PCDATA)>
 <!ELEMENT recipe_name (#PCDATA)>
 <!ELEMENT meal (#PCDATA)>
 <!ELEMENT ingredients (item+)>
 <!ELEMENT item (#PCDATA)>
 <!ELEMENT directions (para | bullet)* >
 <!ELEMENT bullet (#PCDATA|strong)*>
 <!ELEMENT para (#PCDATA|strong)*>
 <!ELEMENT strong (#PCDATA)>

A simple story grammar

Let's present the grammar first

<?xml version="1.0"?>

<!-- DTD to write simple stories
     Made by Daniel K. Schneider / TECFA / University of Geneva
     VERSION 1.0
     30/10/2003
-->

<!ELEMENT STORY (title, context, problem, goal, THREADS, moral, INFOS)>
<!ATTLIST STORY xmlns:xlink CDATA #FIXED "http://www.w3.org/1999/xlink">

<!ELEMENT THREADS (EPISODE+)>
<!ELEMENT EPISODE (subgoal, ATTEMPT+, result) >
<!ELEMENT ATTEMPT (action | EPISODE) >
<!ELEMENT INFOS ( ( date | author | a )* ) >

<!ELEMENT title (#PCDATA) >
<!ELEMENT context (#PCDATA) >
<!ELEMENT problem (#PCDATA) >
<!ELEMENT goal (#PCDATA) >
<!ELEMENT subgoal (#PCDATA) >
<!ELEMENT result (#PCDATA) >
<!ELEMENT moral (#PCDATA) >
<!ELEMENT action (#PCDATA) >
<!ELEMENT date (#PCDATA) >
<!ELEMENT author (#PCDATA) >

<!ELEMENT a (#PCDATA)>
<!ATTLIST a
     xlink:href CDATA #REQUIRED
     xlink:type CDATA #FIXED "simple"
>

Below is a short story

<?xml version="1.0"?>
<!DOCTYPE STORY SYSTEM "story-grammar.dtd">
<?xml-stylesheet href="story-grammar.css" type="text/css"?>
<STORY xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>The little Flexer</title>
  <context>Once upon a time, in a dark small office.</context>
  <problem>Kaspar was trying to learn Flex but didn't have a real project. 
   He then decided that it would be a good idea to look at Data-Driven Controls. 
   These are most useful in combination with an external datasources in XML format.</problem>
  <goal>So he decided how to write a mx:Tree application that imports XML data.</goal>
  <THREADS>

    <EPISODE>
      <subgoal>He decided to play with a little example.</subgoal>
      <ATTEMPT>
	<action>So he went to see the LiveDocs and copied an example.</action>
      </ATTEMPT>
      <result>The example worked but he didn't understand why since he didn't know about E4X.</result>
    </EPISODE>

    <EPISODE>
      <subgoal>He then decided to learn e4X first.
</subgoal>
      <ATTEMPT>
	<action>
	Reading 2-3 tutorials and creating a simple example only took 2-3 hours.
        </action>
      </ATTEMPT>
      <result>
      He now understood how to write e4X code in Flex.

</result>
    </EPISODE>
  </THREADS>

  <moral>Divide a problem into subproblems and you will get there ...</moral>
  <INFOS>
    <a xlink:href="http://edutechwiki.unige.ch/en/ECMAscript_for_XML"
       xlink:type="simple">ECMAscript for XML</a>
  </INFOS>
</STORY>

Story grammar is text centric DTD. There it can be easily styled with CSS. You can look at the file story-grammar.xml and also consult story-grammar.css.

A simple family DTD

Simple 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>

RSS

RSS is a news syndication format. There are several RSS variants. 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 DTD element definitions

We will come back to this when we will learn how to write our own DTDs in the DTD tutorial (don’t worry too much about unexplained details ....)

syntax element	short explanation	Example
,	order of elements	<!ELEMENT Name (First, Middle, Last)>
?	optional element	MiddleName?
+	at least one element	movie+
*	zero or more elements	item*
\|	pick one (or operator)	economics\|law
()	grouping construct	(A,B,C)

Understanding DTD entities

Most professional DTDs use entities. Entities are just symbols that contain some information which substitutes when the symbol is used ...

There exist tow kinds of entities: XML entities and DTD entities

DTD entities

Some more complex DTD use the same structures all over. Instead of typing these several times one can use a 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 computer will then simply translate these into:

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

... think of these entities as shortcuts.

Choosing and using an XML Editor

Requirements

There a lots of XML editors and there is no easy choice ! Depending on your needs you may choose a different editor:

To edit strongly structured data (i.e. data-centric XML) a sort of "tree" or "boxed" view is practical
To edit text-centric data (e.g. an article) you either want a text-processor like tool are a structure editor.
Really good XML editors cost a lot ...

Here is my own little advice with respect to XML editors (also read the XML editor article)

Minimal things your XML editor should be able to do

Check for XML well-formedness
Check for validity against several kinds of XML grammars (DTD, Relax NG, XML Schema)
Highlight errors (of all sorts)
Suggest available XML tags (in a given context). Also clearly show which ones are mandatory and which ones are optional, and display them in the right order.
Allow the user to move/split/join elements in a more or less ergonomic way (although it is admitted that these operations need some training)
Include support for XSLT and XQuery (However, if you have installation skills you can easily compensate lack of support by installing a processor like Saxon

We then suggest some additional criteria depending on the kind of XML

For data-centric XML:

Allow viewing and editing of XML documents in a tree view or boxed view (or both together)
Provide a context-dependent choice of XML tags and attributes (DTD/XSD awareness)

For text-centric XML:

Allow editing of XML documents in a structure view
Allow editing of XML documents in somewhat WYSIWYG view. Such a view can be based on an associated CSS (most common solution) or XSLFO (I am dreaming here) or use some proprietary format (which is not very practical for casual users!). Also allow users to switch on/off tags or element boundary markers.
Provide a context-dependent choice of XML tags and attributes (DTD/XSD awareness). The user should be able to right-click within the XML text and not in some distant tree representation.
Automatically insert all mandatory sub-elements when an element is created.
Automatically complete XML Tags when working without a DTD or other schema.
Indent properly (and assist users to indent single lines as well as the whole document)

Suggested free editors

Any XML editor is difficult to learn (because XML editing is not so easy). Please, make an effort to learn the interface, e.g. read the help !

(1) Exchanger XML Lite V3.3

If you are looking for a general purpose editor that is both DTD and Schema aware and that offers XSLT support, I suggest to try this editor first. Try others you are unhappy with it or if you plan to focus on a single kind of editing, e.g. just edit "data-centric" XML documents.

Download: [Download at Google] (multi-platform, needs java to be installed first).

Hints for editing with Exchanger

To insert an element or attribute:

In the contents window press Ctrl-T to insert an element.
Pressing "<" in the editing window gives more options and you can do it in any place.
To insert an attribute, position the cursor after the element name and press the space bar
Alternatively (and better if you don't know your DTD): Select the Helper pane to the left. Then (in the editing window) click on the element tag you wish to edit or put your cursor in a location between child elements. The helper pane will then display the structure of the current parent element and list available elements on which you can click to insert.

Links

About XML

XML principles
XML (Wikipedia)
XML Tutorial (W3Schools)

About DTDs

Document Type Definition (Wikipedia)
DTD tutorial (W3Schools)

Related subjects

XML Schema (W3C) (Wikipedia)
RELAX NG (Wikipedia)
XML Schema languages (Wikipedia)
Schematron (Wikipedia)