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DTD Entities

DTD Entities

In XML, entities are variables used to define shorcuts to standard text or special characters. In XML an entity is made up of three parts - an ampersand (&), an entity name, and a semicolon (;). The general format of an entity is shown below:

<!ENTITY entity-name "entity-value">

Listing 2-8. Formatting entities.

entity-name refers to the name of the entitiy and entity-value is the value for the defined entity.

Instead of typing the same text over and over again, you can define an entity to contain the text and then you only need to use the entity where you want to insert the text. Because the entity is expanded by the parser, you can be assured that you'll get the same text in every location. The example below shows how to define and use an entity.

<?xml version="1.0"?>

XML File

    <collegename>&college;</collegename>

DTD File

<!ENTITY college "Middle Georgia State University">

Listing 2-9. Defining entities.

In the example, an XML contains the element <collegename>. The element includes the user-defined entity &college;. In the DTD file, the internal entity college is defined as "Middle Georgia State University". The user-defined entity can now be used anywhere in the XML file to define the college name. It is not necessary to re-type "Middle Georgia State University" each time. When the XML is viewed in the browser, the entity is parsed and displayed as <collegename>Middle Georgia State University</collegename>.

The following table contains a list of pre-defined entities that can be used in XML without being declared in the DTD.

Pre-defined Entities

Entity Name Replacement Text
lt The less than sign (<)
gt The greater than sign (>)
amp The ampersand (&)
apos The single quote or apostrophe (')
quot The double quote (")

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