Model Group Operators

• ( open bracket for grouping
• ) close bracket
• , follows
• | or
• ? maybe
• * repeated 0 or more times
• + repeated once or more times

What to watch out for:

• Content model ambiguity:

  <!ELEMENT a   - - ((b, c)|(b, d))>
  

• Mixed content

  carriage return = record end (RE)

  In the DTD:

  <!ELEMENT listitem  - - (#PCDATA, para+)>
  <!ELEMENT para     - - (#PCDATA)>
  

  In the document:

  <listitem>
  Oranges
  <para>Oranges are a lovely fruit.</para>
  <para>They grow in warm climates.</para>
  </listitem>
  

Some further twists:

• A more complex poem type:
  

  <!ELEMENT poem - O 
            (title?,
            ( (line+)
            | (refrain?, (stanza, refrain?)+ ) ))>
  

• Model Groups:
  <!ELEMENT (note | variant) - - (#PCDATA)>
• EMPTY content:
  <!ELEMENT poemref - O EMPTY >
• ANY content:
  <!ELEMENT memo - - ANY>
• Inclusion / Exclusion Exceptions:
  

  <!ELEMENT poem - O 
            (title?, (stanza+ | couplet+ | line+))
            +(note | variant) >
  
  <!ELEMENT title - O 
            (#PCDATA)  
            -(note | variant) >
  
  <!ELEMENT (note | variant) - - 
            (#PCDATA)  
            -(note | variant) >