title: How do I create a regex in Emacs for exactly 3 digits?
body: <p>I want to create a regexp in Emacs that matches exactly 3 digits.   For example, I want to match the following:</p>
  
  <pre><code>123
  345
  789
  </code></pre>
  
  <p>But not</p>
  
  <pre><code>1234
  12
  12 23
  </code></pre>
  
  <p>If I use <code>[0-9]+</code> I match any single string of digits.   I thought <code>[0-9]{3}</code> would work, but when tested in re-builder it doesn't match anything.</p>