Types of XML Entities

Internal

Definied locally within the DTD.

Syntax:

<!ENTITY name "entity_value">

Example:

<!ENTITY person "<name>John</name>">

External

They are used when referenced data is not definied internally.

There are two types of External Entities:

Private

SYSTEM keyword indicates that it is private external entity.

Syntax:

<!ENTITY name SYSTEM "URI">

Example:

<!ENTITY url SYSTEM "http://hacker-studio.pl">

Public

The PUBLIC keyword indicates that this is a public external entity

Syntax:

<!ENTITY name PUBLIC "public_id" "URI">

Example:

<!ENTITY information PUBLIC "//publicly_known_data" "https://hacker-studio.pl/publicly_known_data.xml">

Parameter

Includes % prefix.

Syntax:

<!ENTITY % name SYSTEM "URI">

Example:

<!ENTITY % input 'created variable'>
<!ENTITY text 'This is text from %input;' >

Unparsed External Entities

We can access binary content with unparsed entities.

Preventing XML parser from processing the referenced data.

<!ENTITY name SYSTEM "URI" NDATA TYPE>
<!ENTITY name PUBLIC "id" "URI" NDATA TYPE>