title: What flavor of regex does git use
body: <p>I'm trying to use the git diff --word-diff-regex= command and it seems to reject any types of lookaheads and lookbehinds. I'm having trouble pinning down what flavor of regex git uses. For example </p>
  
  <pre><code>git diff --word-diff-regex='([.\w]+)(?!&gt;)'
  </code></pre>
  
  <p>Comes back as an invalid regular expression. </p>
  
  <p>I am trying to get all the words that are not HTML tags. So the resulting matches of the regex should be 'Hello' 'World' 'Foo' 'Bar' for the below string</p>
  
  <pre><code>&lt;p&gt; Hello World &lt;/p&gt;&lt;p&gt; Foo Bar &lt;/p&gt;
  </code></pre>