title: Regular Expression in Linux command sed
body: <p>I have a shell variable:</p>
  
  <pre><code>all_apk_file="a 1 2.apk x.apk y m.apk"
  </code></pre>
  
  <p>I want to replace the <code>a 1 2.apk</code> with <code>TEST</code>, using the command:</p>
  
  <pre><code>echo $all_apk_file | sed 's/(.*apk ){1}/TEST/g'
  </code></pre>
  
  <p>The <code>.*apk</code> means end with <code>apk</code>, <code>{1}</code> means only match one time, but it doesn't work; I only got the original variable as output: <code>a 1 2.apk x.apk y m.apk</code></p>
  
  <p>Can anyone tell me why?</p>