title: Validate hexadecimal string using regular expression
body: <p>I am validating a string whether it is hexadecimal or not using regular expression. </p>
  
  <p>The expression I used is <code>^[A-Fa-f0-9]$</code>. When I using this, the string <code>AABB10</code> is recognized as a valid hexadecimal, but the string <code>10AABB</code> is recognized as invalid.</p>
  
  <p>How can I solve the problem?</p>