extensibility

Occurrence Indicators

Occurrence indicators are used within element content models to specify how many times an element may appear at a given location. The indicators available to schema developers are listed below:

Occurrence Indicator Meaning XML Authority Qlicker
none The element must appear once and only once. Neither optional nor repeatable.
? The element (or group of elements) may appear zero or one times. The element is optional, but is only allowed to appear once. Optional but not repeatable.
+ The element (or group of elements) must appear one or more times. The element is required to appear at least once, but multiple consecutive occurrences may be present. Repeatable but not optional.
* The element (or group of elements) may appear zero or more times. The element can appear as many times consecutively as needed, or even zero times. Repeatable and optional.
{} The element (or group of elements) must appear within the specified range. (For XSDL and SOX only.) Set occurrence range.

In combination with sequence indicators, these choices make it possible to describe complex structures. For example, a memo might allow multiple entries in its To: and From: fields, multiple (or zero) entries in its Cc: field, a single entry for the subject, required content for the body, and an optional set of initials at the bottom for the typist. A MEMO element might therefore have the following content model:

(To+, From+, Cc*, Subject, Body, Typist?)

This declaration requires the MEMO element to contain, in sequence, one or more To elements, one or more From elements, zero or more Cc elements, a single Subject element, a single Body element, and zero or one Typist elements.

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