title: How to remove two specific chacters
body: <p>I have the following text:</p>
  
  <pre><code>rule="Aa&amp;~a-&gt;c"
  </code></pre>
  
  <p>and I need to remove the letter a that is alone with the ~ sign
  so far I tried this regex expression:</p>
  
  <pre><code>auxRegla=re.sub(r"(?&lt;!~)\ba\b",r"",regla)
  </code></pre>
  
  <p>the <code>\ba\b</code> are to set the boundaries and that works just fine and the <code>(?&lt;!~)</code> has the intention to remove the ~ sign, but those two togethere aren't doing anything.</p>
  
  <p>I need the result to be:</p>
  
  <pre><code>Aa&amp;-&gt;c
  </code></pre>