title: Replace all characters not in range (Java String)
body: <p>How do you replace all of the characters in a string that do not fit a criteria.  I'm having trouble specifically with the NOT operator.  </p>
  
  <p>Specifically, I'm trying to remove all characters that are not a digit, I've tried this so far:</p>
  
  <pre><code>String number = "703-463-9281";
  String number2 = number.replaceAll("[0-9]!", ""); // produces: "703-463-9281" (no change)
  String number3 = number.replaceAll("[0-9]", "");  // produces: "--" 
  String number4 = number.replaceAll("![0-9]", ""); // produces: "703-463-9281" (no change)
  String number6 = number.replaceAll("^[0-9]", ""); // produces: "03-463-9281"
  </code></pre>