title: Processing instruction validation in XML schema
body: <p>Reasonably simple one I suppose, I just can't find anything definitive (<em>besides the fact that there is a lack of anything definitive</em>)</p>
  
  <p>Is there any way to incorporate into a schema, the requirement of a processing instruction node at a place in the document, also perhaps validating the processing instruction target?</p>
  
  <p>For instance:</p>
  
  <pre><code>&lt;node&gt;
      &lt;?my-instruction data ?&gt;
  &lt;/node&gt;
  </code></pre>
  
  <p>Would validate, whereas:</p>
  
  <pre><code>&lt;node&gt;&lt;/node&gt;
  </code></pre>
  
  <p>Would not?</p>
  
  <p>And, better yet:</p>
  
  <pre><code>&lt;node&gt;
      &lt;?my-other-instruction data ?&gt;
  &lt;/node&gt;
  </code></pre>
  
  <p>Would not either (<em>given I could supply a required target of <code>my-instruction</code></em>)</p>
  
  <p>As mentioned, I can't find anything definitive (<em>or at all for that matter</em>) on the topic, so I'm left to assume this is something not supported. Others have mentioned that PIs are intended for "out-of-band" data and processing instructions, so it appears sensible that their inclusion would not be required in validation. Would be nice though.</p>