Example 5     <<      >>       intro       contents     

If an element name in DTD is followed by the question mark [?], this element can occur zero or one times.

  DTD   HOME     

The root element XXX can contain one element AAA followed by precisely one element BBB. Element BBB must be always present.:

               <!ELEMENT XXX (AAA? , BBB)>
               <!ELEMENT AAA (#PCDATA)>
               <!ELEMENT BBB (#PCDATA)>
               
            


  Valid documents   HOME     

A valid document:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX> <AAA/> <BBB/> </XXX>
               
            

Element AAA is not mandatory:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX>  <BBB/> </XXX>
               
            

  Documents with errors   HOME     

Element BBB is missing:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX> ___ </XXX>
               
            

Maximaly one AAA element can occur inside the document:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX> <AAA/> <AAA/> <AAA/> <AAA/> <AAA/> <AAA/> <AAA/> <BBB/> </XXX>
               
            

Element BBB must follow element AAA:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX> <BBB/> <AAA/> </XXX>