title: Find all words with 3 letters with regex
body: <p>I'm trying to find all words with 3 letters in a string.<br>
  So in this list</p>
  
  <pre><code>cat monkey dog mouse
  </code></pre>
  
  <p>I only want</p>
  
  <pre><code>cat dog
  </code></pre>
  
  <p>This is my expression:</p>
  
  <pre><code>^[a-zA-Z]{3}$
  </code></pre>
  
  <p>I tested it with different online regex tester, but none of them matched my expression.</p>