DTD Regular Expression Operations

• For example,

  <!ELEMENT section (title, (paragraph | (image, title?))+)>

  defines an element section whose children are:
  1. A title element
  2. A sequence of one or more of the following:
     • paragraph elements
     • image elements followed by optional title elements
        
• Thus, he following is not valid

  <section>
     <paragraph/>
     <paragraph/>
     <title/>
  </section>

  because there is no starting title, and the title at the end doesn't follow an image