| Constrained Child Nodes |
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<!ELEMENT name (child_node_regexp)[? | * | +]>
XML provides a simple
regular-expression syntax that can be
used to limit the order and number of child elements within a parent
element. This language includes the following operators:
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Operator
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Meaning
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Name
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Matches an element of the given name
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( )
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Groups expressions for processing as sets of sequences (using the
comma as a separator) or choices (using | as a separator)
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?
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Indicates that the preceding name or expression can occur zero or one
times at this point in the document
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*
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Indicates that the preceding name or expression can occur zero or
more times at this point in the document
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+
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Indicates that the preceding name or expression must occur one or
more times at this point in the document
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