Optional Children
May 3, 1999
Using the "?" character specifies that the element named is
optional. Thus, in the following code snippet, we specify
that every CONTACT must have a NAME and either a PHONE or
EMAIL and may have an optional ADDRESS child.
<!ELEMENT CONTACT (NAME, (PHONE | EMAIL), ADDRESS?)>
<!ELEMENT NAME (#PCDATA)>
<!ELEMENT PHONE (#PCDATA)>
<!ELEMENT EMAIL (#PCDATA)>
<!ELEMENT ADDRESS (STREET+, CITY, STATE, ZIP, COUNTRY?)
<!ELEMENT STREET (#PCDATA)>
<!ELEMENT CITY (#PCDATA)>
<!ELEMENT STATE (#PCDATA)>
<!ELEMENT ZIP (#PCDATA)>
<!ELEMENT COUNTRY (#PCDATA)>
Either/Or
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