Constrained Child Nodes |
    <!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:

Operator | Meaning
Name | Matches an element of the given name
( ) | Groups expressions for processing as sets of sequences (using the comma as a separator) or choices (using | as a separator)
? | Indicates that the preceding name or expression can occur zero or one times at this point in the document
* | Indicates that the preceding name or expression can occur zero or more times at this point in the document
+ | Indicates that the preceding name or expression must occur one or more times at this point in the document