title: Mask out part first 12 characters of string with *?
body: <p>How can I take the value <code>123456789012345</code> or <code>1234567890123456</code> and turn it into:</p>
  
  <p><code>************2345</code> and <code>************3456</code></p>
  
  <p>The difference between the strings above is that one contains 15 digits and the other contains 16.</p>
  
  <p>I have tried the following, but it does not keep the last 4 digits of the 15 digit number and now matter what the length of the string, be it 13, 14, 15, or 16, I want to mask all beginning digits with a *, but keep the last 4.  Here is what I have tried:</p>
  
  <pre><code>String.Format("{0}{1}", "************", str.Substring(11, str.Length - 12))
  </code></pre>