 are building an expression here to
      match image GIF or JPEG type image file. It must include the literal string <span class="QUOTE">"advert"</span>,
      then one or more digits, and a <span class="QUOTE">"."</span> (which is now a literal, and not a special
      character, since it is escaped with <span class="QUOTE">"\"</span>), and lastly either <span class=
      "QUOTE">"gif"</span>, or <span class="QUOTE">"jpeg"</span>, or <span class="QUOTE">"jpg"</span>. Some possible
      matches would inc