title: java replace regex a-z0-9 only
body: <p>I would like to show only <code>a-z0-9</code> in a string, and the other characters should be replaced by null string</p>
  
  <pre><code>String s=this.saveFileName.replaceAll("/[a-z0-9. ]/", "");
  </code></pre>
  
  <p>This those not work, any ideas why?</p>