title: Regex to check string contains only Hex characters
body: <p>I have never done regex before, and I have seen they are very useful for working with strings. I saw a few <a href="http://www.vogella.de/articles/JavaRegularExpressions/article.html" rel="noreferrer">tutorials</a> (for example) but I still cannot understand how to make a simple Java regex check for hexadecimal characters in a string.</p>
  
  <p>The user will input in the text box something like: <code>0123456789ABCDEF</code> and I would like to know that the input was correct otherwise if something like <code>XTYSPG456789ABCDEF</code> when return false.</p>
  
  <p>Is it possible to do that with a regex or did I misunderstand how they work?</p>