title: Match only A or B case insensitive
body: <p>I want a regular expression that validates a string of length 1 which has either the characters <code>A</code> or <code>B</code> (case insensitive) at the Begin</p>
  
  <p>So this should return true</p>
  
  <pre><code>  A
    a
    B
    b
  </code></pre>
  
  <p>Any word would not match, so the following would return false:</p>
  
  <pre><code> "America"
   "Bi"
   "a door"
  </code></pre>