title: How to get the contents of the subgroup matched by a regular expression
body: <p>like this:
  p.txt</p>
  <pre><code>abc-111-zzz-022-ffff
  abc-222-zzz-033-ffff
  abc-333-zzz-044-ffff
  </code></pre>
  <p>I can get the full match (<code>$0</code> in regex):</p>
  <pre class="lang-sh prettyprint-override"><code>[root@ser user]# grep -Po 'abc\-([0-9]+)\-zzz\-([0-9]+)' p.txt
  abc-111-zzz-022
  abc-222-zzz-033
  abc-333-zzz-044
  </code></pre>
  <p>How can I get only subgroup content <code>$1$2</code> like :</p>
  <pre><code>111022
  222033
  333044
  </code></pre>
  <p>thanks!</p>