About Modifiers in Element Definitions in DTDs

When you define an element, you specify one or more modifiers. A modifier specifies a rule about the structure or occurrence of the element being defined. An element can have only one top-level modifier. However, you can add one or more modifiers to the top-level modifier. A modifier can aggregate elements or other modifiers.

This section discusses the following topics:

Description of Element Modifiers in DTDs

Simple Example of Aggregating Modifiers in DTDs

More Complex Example of Aggregating Modifiers in DTDs

Aggregating Modifiers to Allow Any Order and Any Number in DTDs

Description of Element Modifiers in DTDs

Table 54 describes the available modifiers:

Table 54. Element Modifiers
Modifier	Description	Indicator in DTD
Optional	This element can appear once or not at all. (0 or 1)	Question mark ( `?`)
Zero or more	This element is optional and repeatable. (0, 1, or more)	Asterisk ( `*`)
One or more	This element is required and repeatable. (1 or more)	Plus sign ( `+`)
Choice	Exactly one of the specified subelements must appear.	Vertical bar ( `\|`)
Sequence	If no other modifiers are defined on the `Sequence` modifier, each subelement in this element must appear exactly once. In other words, it is required. Also, the subelements must appear in the order in which they are specified in the referencing element. You can define other modifiers on the `Sequence` modifier. In this way, you can specify that some subelements are optional, some appear zero or more times, and some appear one or more times.	Comma ( `,`)

Simple Example of Aggregating Modifiers in DTDs

Suppose you want a book element to always contain exactly one title element and any number of author elements. The title and author elements contain only raw data. To accomplish this, you would perform steps that generate the following tree representation:


book Sequence title One or More author title Zero or More #PCDATA author Zero or More #PCDATA

In the book element definition, Sequence modifies book and One or More modifies Sequence. Because the title element immediately follows the Sequence modifier, the default occurrence rule is assumed. That is, the title element must appear exactly once. In the Text view of the DTD, the definition for the book element is as follows:

<!ELEMENT book (title, (author)+)>

More Complex Example of Aggregating Modifiers in DTDs

Following is a more complicated example. Suppose you want book elements to include

Exactly one title

Either an author or an editor, but it is okay if neither appear

Zero or more paragraphs

To accomplish this, you would perform steps that generate the following tree representation:


book Sequence title Optional Choice author editor Zero or More paragraph

In the Text view of the DTD, the definition for the book element is as follows:


<!ELEMENT book (title, (author\|editor)?, paragraph*)>

Aggregating Modifiers to Allow Any Order and Any Number in DTDs

The Choice modifier specifies that only one of the specified elements can appear in an instance document. However, if you specify the Zero or More modifier and then the Choice modifier, the result is that the specified elements can appear in any order and each element can appear any number of times.

The text for such an element definition is as follows: