Introduction to XML | Defining XML Structures with DTD | Specifying Element Occurrence Counts

Tags:

DTD lets you specify how many times an element may appear. Without a symbol, an element must appear exactly once. For example, this model allows only one product:

<!ELEMENT inventory (product)>

To allow one or more products, append +:

<!ELEMENT inventory (product+)>

Occurrence symbols

Symbol	Allowed occurrences
`+`	One or more
`?`	Zero or one
`*`	Zero or more
None	Exactly one

Write the symbol after an element model:

<!ELEMENT element-name (child1-symbol)>
<!ELEMENT element-name (child1-symbol, child2-symbol, ...)>
<!ELEMENT element-name (child1-symbol, child2-symbol, ...)symbol>

The following DTD allows one or more products, optional details, and zero or more colors:

<!DOCTYPE inventory[
    <!ELEMENT inventory (product+)>
    <!ELEMENT product (name, quantity, details?)>
    <!ELEMENT name (#PCDATA)>
    <!ELEMENT quantity (#PCDATA)>
    <!ELEMENT details (weight, color*)>
    <!ELEMENT weight (#PCDATA)>
    <!ELEMENT color (#PCDATA)>
]>

Remember that optional elements need ? or *. Without a symbol, the element must appear once.