title: Insert text between html tag using regex
body: <p>I want to insert <code>&amp;nbsp</code> between empty paragraph tags. </p>
  
  <p><strong>Input:</strong></p>
  
  <pre><code>1. &lt;div&gt;&lt;p&gt;&lt;/p&gt;&lt;/div&gt;
  2. &lt;div&gt;&lt;p style="margin: 0 0 0 0"&gt;&lt;/p&gt;&lt;/div&gt;
  </code></pre>
  
  <p><strong>Expected output:</strong></p>
  
  <pre><code>1. &lt;div&gt;&lt;p&gt;&amp;nbsp;&lt;/p&gt;&lt;/div&gt;
  2. &lt;div&gt;&lt;p style="margin: 0 0 0 0"&gt;&amp;nbsp;&lt;/p&gt;&lt;/div&gt;
  </code></pre>
  
  <p>I tried to use look behind and forward but apparently I can't use quantifier here:  <code>(?&lt;=&lt; p[^&gt;]*&gt;)(?=&lt;\/p&gt;)</code></p>
  
  <p>Any ideas? My available tools are regex and c#</p>