title: Print only parts that match regex
body: <p><code>echo "a b _c d _e f" | sed 's/[ ]*_[a-z]\+//g'</code></p>
  
  <p>The result will be <code>a b d f</code>.</p>
  
  <p>Now, how can I turn it around, and only print <code>_c _e</code>, while assuming nothing about the rest of the line?</p>