7.8. Marked Sections
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## 7.8. Marked Sections

XML provides a mechanism to indicate that particular pieces of the document should be processed in a special way. These are called “marked sections”.

Example 7.14. Structure of a Marked Section
<![`KEYWORD`[ Contents of marked section ]]>

As expected of an XML construct, a marked section starts with `<!`.

The first square bracket begins the marked section.

`KEYWORD` describes how this marked section is to be processed by the parser.

The second square bracket indicates the start of the marked section's content.

The marked section is finished by closing the two square brackets, and then returning to the document context from the XML context with `>`.

### 7.8.1. Marked Section Keywords

#### 7.8.1.1. `CDATA`

These keywords denote the marked sections content model, and allow you to change it from the default.

When an XML parser is processing a document, it keeps track of the “content model”.

The content model describes the content the parser is expecting to see and what it will do with that content.

The `CDATA` content model is one of the most useful.

`CDATA` is for “Character Data”. When the parser is in this content model, it expects to see only characters. In this model the `<` and `&` symbols lose their special status, and will be treated as ordinary characters.

### Note:

When using `CDATA` in examples of text marked up in XML, remember that the content of `CDATA` is not validated. The included text must be check with other means. For example, the content could be written in another document, validated, and then pasted into the `CDATA` section.

Example 7.15. Using a `CDATA` Marked Section
`<para>`Here is an example of how to include some text that contains many `<literal>`&lt;`</literal>` and `<literal>`&amp;`</literal>` symbols. The sample text is a fragment of `<acronym>`XHTML`</acronym>`. The surrounding text (`<para>` and `<programlisting>`) are from DocBook.`</para>` `<programlisting>`<![CDATA[`<p>`This is a sample that shows some of the elements within `<acronym>`XHTML`</acronym>`. Since the angle brackets are used so many times, it is simpler to say the whole example is a CDATA marked section than to use the entity names for the left and right angle brackets throughout.`</p>` `<ul>` `<li>`This is a listitem`</li>` `<li>`This is a second listitem`</li>` `<li>`This is a third listitem`</li>` `</ul>` `<p>`This is the end of the example.`</p>`]]>`</programlisting>`

#### 7.8.1.2. `INCLUDE` and `IGNORE`

When the keyword is `INCLUDE`, then the contents of the marked section will be processed. When the keyword is `IGNORE`, the marked section is ignored and will not be processed. It will not appear in the output.

Example 7.16. Using `INCLUDE` and `IGNORE` in Marked Sections
<![INCLUDE[ This text will be processed and included. ]]> <![IGNORE[ This text will not be processed or included. ]]>

By itself, this is not too useful. Text to be removed from the document could be cut out, or wrapped in comments.

It becomes more useful when controlled by parameter entities, yet this usage is limited to entity files.

For example, suppose that documentation was produced in a hard-copy version and an electronic version. Some extra text is desired in the electronic version content that was not to appear in the hard-copy.

Create an entity file that defines general entities to include each chapter and guard these definitions with a parameter entity that can be set to either `INCLUDE` or `IGNORE` to control whether the entity is defined. After these conditional general entity definitions, place one more definition for each general entity to set them to an empty value. This technique makes use of the fact that entity definitions cannot be overridden but the first definition always takes effect. So the inclusion of the chapter is controlled with the corresponding parameter entity. Set to `INCLUDE`, the first general entity definition will be read and the second one will be ignored. Set to `IGNORE`, the first definition will be ignored and the second one will take effect.

Example 7.17. Using a Parameter Entity to Control a Marked Section
<!ENTITY % electronic.copy "INCLUDE"> <![%electronic.copy;[ <!ENTITY chap.preface SYSTEM "preface.xml"> ]]> <!ENTITY chap.preface "">

When producing the hard-copy version, change the parameter entity's definition to:

<!ENTITY % electronic.copy "IGNORE">

### 7.8.2. To Do…

1. Modify `entities.ent` to contain the following:

   <!ENTITY version "1.1"> <!ENTITY % conditional.text "IGNORE"> <![%conditional.text;[ <!ENTITY para1 SYSTEM "para1.xml"> ]]> <!ENTITY para1 ""> <!ENTITY para2 SYSTEM "para2.xml"> <!ENTITY para3 SYSTEM "para3.xml">

2. Normalize `example.xml` and notice that the conditional text is not present in the output document. Set the parameter entity guard to `INCLUDE` and regenerate the normalized document and the text will appear again. This method makes sense if there are more conditional chunks depending on the same condition. For example, to control generating printed or online text.

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