Declaring Zero or One Occurrences of the Same Element

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Examples 

<!ELEMENT element-name (child-name?)>
example
<!ELEMENT note (message?)>

example

<!ELEMENT note (to+,from,header,message*,#PCDATA)>
  • The ? sign in the example above declares that the child element message can occur zero or one times inside the note element.
  • The + sign declares at least one "to" child element.
  • With no sign declaration, exactly one "from" child element.
  • With no sign declaration, exactly one "header" child element.
  • The * sign declares zero or more "message" child elements,
  • The #PCDATA declaration, indicates some other parsed character data.