title: Sed capture group not working
body: <p>This seems like one of the sipmlest possible examples of a <code>sed</code> capture group, and it doesn't work. I've tested the regex in online regex testers, and does what I want. At the linux command line it does not.</p>
  
  <pre><code>$ echo "a 10 b 12" | sed -E -n 's/a ([0-9]+)/\1/p'
  $
  </code></pre>
  
  <p>and</p>
  
  <pre><code>$ echo "a 10 b 12" | sed -E -n 's/a ([0-9]+)/\1/p'
  10 b 12
  </code></pre>
  
  <p><a href="https://regex101.com/r/WS3lG9/1" rel="nofollow noreferrer">https://regex101.com/r/WS3lG9/1</a></p>
  
  <p>I would expect the "10" to be captured.</p>