title: regex to search a letter
body: <p>I use regex in Objective-C, I want to search 'a' letter.
  After several tests, I see that if I want to search all 'a' letter, I just need to use simple regex:</p>
  
  <blockquote>
    <p>a</p>
  </blockquote>
  
  <p>But if I want to match the 'a' letter at the begin, I need to use regex:</p>
  
  <blockquote>
    <p>\Aa*</p>
  </blockquote>
  
  <p>but not</p>
  
  <blockquote>
    <p>\Aa</p>
  </blockquote>
  
  <p>What the different? Can you explain. Thank you!!!</p>
  
  <p>EDIT:</p>
  
  <p>My string is </p>
  
  <blockquote>
    <p>"ad db bd"</p>
  </blockquote>