title: sed insert before specific character
body: <p>Im trying to sed insert:</p>
  
  <pre><code>echo "usr_web88_2" | sed 's/^\(usr\_\)'
  </code></pre>
  
  <p>but I can't find a solution</p>
  
  <p>I have this string:</p>
  
  <pre><code>usr_web88_2
  </code></pre>
  
  <p>and want to add backslashes before the underscores like this:</p>
  
  <pre><code>usr\_web88\_2
  </code></pre>
  
  <p>Can anybody help me? thank you..</p>