DTD Tutorial

With character [ | ] you can select one from several elements.

The root element XXX must contain one element AAA followed by one element BBB. Element AAA must contain one element CCC folowed by element DDD. Element BBB must contain either one element CCC or one element DDD:

               <!ELEMENT XXX (AAA , BBB)>
               <!ELEMENT AAA (CCC , DDD)>
               <!ELEMENT BBB (CCC | DDD)>
               <!ELEMENT CCC (#PCDATA)>
               <!ELEMENT DDD (#PCDATA)>

A valid document:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX>
                    <AAA>
                         <CCC/> <DDD/>
                    </AAA>
                    <BBB>
                         <CCC/>
                    </BBB>
               </XXX>

Another valid document:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX>
                    <AAA>
                         <CCC/> <DDD/>
                    </AAA>
                    <BBB>
                         <DDD/>
                    </BBB>
               </XXX>

The element BBB can contain either element CCC or DDD but not both:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX>
                    <AAA>
                         <CCC/> <DDD/>
                    </AAA>
                    <BBB>
                         <CCC/> <DDD/>
                    </BBB>
               </XXX>

The element BBB can contain either element CCC or DDD but not both:

               <!DOCTYPE tutorial SYSTEM "tutorial.dtd">

               <XXX>
                    <AAA>
                         <CCC/> <DDD/>
                    </AAA>
                    <BBB>
                         <DDD/>  <CCC/>
                    </BBB>
               </XXX>