title: regex to match only one upper case letter
body: <p>I need to match (must has only one upper case letter at least) if more than won't match.</p>
  
  <pre><code>    Kitty   || Valid has one upper case letter  
      wiDivb  || Valid has one upper case letter
      ikBVu   || Invalid more than one upper case letter
      welcmw  || Invalid No upper case letter
      A       || Valid has one upper case letter  
      AA      || Invalid more than one upper case letter
  </code></pre>
  
  <p>I have tried :</p>
  
  <pre><code>(?=.*[a-z])(?=.*[A-Z]{1})
  </code></pre>
  
  <p>but it matches even it has more than one upper case letter and i want it only one so i've added <code>{1}</code> but it did not</p>