title: How to insert hyphen after third character
body: <p>I want to remove the first three characters of the string and insert a hyphen between 3rd and 4th of the remaining 6 characters.
  For example
  Input:</p>
  
  <pre><code>123456789
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>456-789
  </code></pre>
  
  <p>My regular expression removes the first three characters, but I'm not sure how to insert hyphen.</p>
  
  <pre><code>(^.{3})(\w+) $2
  </code></pre>