Internal Entities

Internal General Entities

Internal general entities are the simplest among the five types of entities. They are defined in the DTD section of the XML document. First let's look at how to declare an internal general entity.

Declaring an internal general entity

The syntax for the declaration of an internal general entity is shown here:

<!ENTITY name "string_of_characters">

You can create your own general entities. General entities are useful for associating names with foreign language characters, such as � or �, or escape characters, such as <, >, and &. You can use Unicode character values in your XML documents as replacements for any character defined in the Unicode standard. These are called character references.

To use a Unicode representation in your XML document, you must precede the Unicode character value with &#. You can use either the Unicode characters' hex values or their decimal values. For example, in Unicode, � is represented as xFC and � is represented as xDF. These two characters' decimal values are 252 and 223. Thus, in your DTD you could create general entities for the preceding two characters as follows:

  <!ENTITY u_um "&#xFC">
  <!ENTITY s_sh "&#xDF">

The two entities could also be declared like this:

  <!ENTITY u_um "&#252">
  <!ENTITY s_sh "&#223">

Using internal general entities

To reference a general entity in the XML document, you must precede the entity with an ampersand (&) and follow it with a semicolon (;). For example, the following XML statement references the two general entities we declared in the previous section:

  <title>Gr&u_um;&s_sh;</title>

When the replacement text is inserted by the parser, it will look like this:

  <title>Gr��</title>

Internal general entities can be used in three places: in the XML document as content for an element, within the DTD in an attribute with a #FIXED data type declaration as the default value for the attribute, and within other general entities inside the DTD. We used the first location in the preceding example: (<title>Gr&u_um;&s_
sh;</title>).

The second place you can use an internal general entity is within the DTD in an attribute with a #FIXED data type declaration or as the default value for an attribute. For example, you can use the following general entities in your DTD declaration to create entities for several colors:

  <!ENTITY Cy "Cyan">
  <!ENTITY Lm "Lime">
  <!ENTITY Bk "Black">
  <!ENTITY Wh "White">
  <!ENTITY Ma "Maroon">

Then if you want the value of the bgcolor attribute for tr elements to be White for all XML documents that use the DTD, you could include the following line in the previous DTD declaration:

  <!ATTLIST tr align (Left | Right | Center) 'Center'
          valign (Top | Middle | Bottom) 'Middle'
          bgcolor CDATA #FIXED "&Wh;">

The internal general entities must be defined before they can be used in an attribute default value since the DTD is read through once from beginning to end. In this case, internal general entities for several colors have been created. The bgcolor attribute is declared with the keyword #FIXED, which means that its value cannot be changed by the user-the value will always be White. The color general entities could also be used as content for the elements in the body section of the XML document.

You can use the internal general entity as a default value-for example, bgcolor CDATA "&Wh;". In this case, if no value is given, &Wh; is substituted for bgcolor when the XML attribute is needed in the document body, and that reference will be converted to White.

The third place you can use internal general entities is within other general entities inside the DTD. For example, we could use the preceding special character entities as follows:

  <!ENTITY u_um "&#252>
  <!ENTITY s_sh "&#223">
  <!ENTITY greeting "Gr&u_um;&s_sh;">

At this point, it's not clear whether greeting will be replaced with Gr&u_um;&s_sh; in the XML document's body and then converted to Gr�� or whether greeting will be replaced directly with Gr�� when the entity is parsed. The order of replacement will be discussed in the section "Processing Order" later in this chapter.