title: using negative conditions within regular expressions
body: <p>Is it possible to use negative matches within gsub expressions?
  I want to replace strings starting by <code>hello</code> <strong>except</strong> those starting by <code>hello Peter</code></p>
  
  <pre><code>my-string.gsub(/^hello@/i, '')
  </code></pre>
  
  <p>What should I put instead of the <code>@</code>?</p>