title: Check if string is valid representation of hex number
body: <p>I am total noob regarding <code>regex</code>.</p>
  <p>My goal is to check whether a string is a valid representation of a hex number.</p>
  <p>Currently my implementation (which I find really inefficient) is having a List with all hex digits (0-9, A-F) and checking whether my string contains characters not contained in given List.</p>
  <p>I bet this can be easily done using regular expressions but I have no idea how to implement it.</p>
  <pre><code>private bool ISValidHEX(string s)
  {
      List&lt;string&gt; ToCheck = new List&lt;string&gt;();
      for (int i = 0; i &lt; 10; i++)
      {
          ToCheck.Add(i.ToString());
      }
      ToCheck.Add(&quot;A&quot;);
      ToCheck.Add(&quot;B&quot;);
      ToCheck.Add(&quot;C&quot;);
      ToCheck.Add(&quot;D&quot;);
      ToCheck.Add(&quot;E&quot;);
      ToCheck.Add(&quot;F&quot;);
      for (int i = 0; i &lt; s.Length; i++)
      {
          if( !ToCheck.Contains(s.Substring(i,1)))
          {
              return false;
          }
      }
      return true;
  }
  </code></pre>