title: what is [^\\d] ? how replaceAll("[^\\d]", ""); is working?
body: <p>I want to remove alphabets from my string, I used replace function as follows</p>
  
  <pre><code>String hello = "hello007";
  hello = hello.replaceAll("[^\\d]", "");
  </code></pre>
  
  <p>It worked, but I do know how it’s working? What is “^” and “//d”? 
  I get to know ^ is “xor”. But unable to understand usage?</p>
  
  <p>Why are we covering it under []? Please guide me?  </p>