title: add_rewrite_rule for custom sub pages
body: <p>For a plugin I need a <code>subpage</code> of a regular WordPress <code>page</code>. The normal <code>page</code> is located at</p>
  
  <pre><code>http://example.com/page
  </code></pre>
  
  <p>To get a <code>subpage</code> </p>
  
  <pre><code>http://example.com/page/subpage
  </code></pre>
  
  <p>I'm using the rewrite_rule:</p>
  
  <pre><code>add_rewrite_rule( '([^/]+)/(subpage|subpage2|subpage3|subpage4)', 'index.php?pagename=$matches[1]&amp;_mysubpage=$matches[2]', 'top');
  </code></pre>
  
  <p>This <strong>is working</strong> fine until I move the <code>page</code> to a <code>parent</code> page:</p>
  
  <pre><code>http://example.com/parentpage/page/subpage
  </code></pre>
  
  <p>or if I set the <code>page</code> as homepage</p>
  
  <pre><code>http://example.com/subpage
  </code></pre>
  
  <p>Which additional <code>add_rewrite_rule</code> do I need?</p>