title: sed got error if no single quotes around the regular expression
body: <p>I tried to do the following command in bash:</p>
  
  <pre><code>ls -1 | sed s/\(.*\)/"\1"/
  </code></pre>
  
  <p>which is add double quotes around each output of <code>ls</code>, but the result shows
  <code>sed: 1: "s/(.*)/\1/": \1 not defined in the RE</code></p>
  
  <p>after I add single quotes around the regular expression, I got the right result. the right one is:</p>
  
  <p><code>ls -1 | sed 's/\(.*\)/"\1"/'</code></p>
  
  <p>theocratically I do not need the outer quotes right? any one has the same experience?</p>