| Lesson 4 | Defining entities |
| Objective | Define entities in a DTD. |
Defining XML Entities
In HTML, you can include special characters through the use of pre-defined entities such as these:
<
"
©
These entities represent predefined characters that are not keyboard characters or that might be interpreted incorrectly by the browser.
For example, < represents the "less than" left-angle bracket character (<).
Entities are defined in HTML to help define certain characters that might otherwise be interpreted differently.
Built-in entities
In XML, there are five built-in entities that you need not define but can readily use. These are:
<
>
"
'
&
Declaring general entities
A general entity is defined in a DTD and referenced in the XML document. In a DTD, you can define your own entities in addition to five listed above.
You can also define multiple characters.
If you have ever performed a "mail merge," you are familiar with using replaceable fields to represent actual text.
Think of an entity as a replaceable field representing some other text.
The beauty of using entities is that you can define them once in a DTD and then use them throughout a range of documents.
Declare an entity using the following syntax:
< !ENTITY entityName "character string represented">
If in your DTD you declared the following entity:
< !ENTITY prodname "ACME Calendar">
you could use the following in your XML file: Thank you for choosing &prodname; as your primary scheduling program.
When rendered by a user agent, the actual text would then read as follows:
Thank you for choosing ACME Calendar as your primary scheduling program.
The next lesson shows you how to create parameter entities to use within a DTD.