title: Why this regex does not work with grep?
body: <p>I have a text file this way</p>
  
  <pre><code>"an arbitrary string" = "this is the text one"
  "other arbitrary string" = "second text"
  "a third arbitrary string" = "the text number three"
  </code></pre>
  
  <p>I want to obtain only this</p>
  
  <pre><code>an arbitrary string
  other arbitrary string
  a third arbitrary string
  </code></pre>
  
  <p>That is, the text inside the first quotes, or between the first <code>"</code> and the <code>" =</code>. I used this regex</p>
  
  <p><code>(?!").*(?=(" =))</code></p>
  
  <p>This is working when I tried it in <a href="http://www.regexr.com/" rel="nofollow">RegExr</a> and in this <a href="http://www.online-utility.org/text/grep.jsp" rel="nofollow">online tool</a>. But in my OSX Terminal it does not work, the output is empty</p>
  
  <p><code>grep -o '(?!").*(?=(" =))' input.txt</code></p>
  
  <p>What is wrong here? Do I have to escape some characters? I try everyone and nothing changes.</p>
  
  <p>Thank you so much and please excuse my lack of knowledge about this topic.</p>