XML in a Nutshell, 3rd Edition

The Number of Children

As the previous examples indicate, not all instances of a given element necessarily have exactly the same children. You can affix one of three suffixes to an element name in a content specification to indicate how many of that element are expected at that position. These suffixes are:

? Zero or one of the element is allowed.

* Zero or more of the element is allowed.

+ One or more of the element is required.

For example, this declaration says that a name element must contain exactly one first_name, may or may not contain a middle_name, and may or may not contain a last_name:

<!ELEMENT name (first_name, middle_name?, last_name?)>

Given this declaration, all these name elements are valid:

<name>
  <first_name>Madonna</first_name>
  <last_name>Cicconne</last_name>
</name>
<name>
  <first_name>Madonna</first_name>
  <middle_name>Louise</middle_name>
  <last_name>Cicconne</last_name>
</name>
<name>
  <first_name>Madonna</first_name>
</name>

However, these are not valid:

<name>
  <first_name>George</first_name>
  <!-- only one middle name is allowed -->
  <middle_name>Herbert</middle_name>
  <middle_name>Walker</middle_name>
  <last_name>Bush</last_name>
</name>
<name>
  <!-- first name must precede last name -->
  <last_name>Cicconne</last_name>
  <first_name>Madonna</first_name>
</name>

You can allow for multiple middle names by placing an asterisk after the middle_name:

<!ELEMENT name (first_name, middle_name*, last_name?)>

If you wanted to require a middle_name to be included, but still allow for ...