title: Regex matching groups number
body: <p>I am learning regex and ran into a problem.
  I have the following:</p>
  <p><code>href=&quot;http://google.com/topic/713725-dogs-are-great-3&quot;&gt; href=&quot;http://google.com/topic/213225-humans-are-great&quot;&gt; href=&quot;http://google.com/topic/342315-cats-are-great-but-small&quot;&gt;</code></p>
  <p>with this code
  <code>href=&quot;(?:[^&quot;]*)topic/([^&lt;]*)&quot;&gt;</code></p>
  <p>i can select</p>
  <p><code>713725-dogs-are-great-3 213225-humans-are-great 342315-cats-are-great-but-small</code></p>
  <p>but i only want to match the numbers</p>
  <p>342315
  213225
  713725</p>
  <p>any ideas?</p>