title: Using regex to check if a string contains only one digit
body: <p>I'm writing an algorithm and I need to check if a string contains <strong>only one</strong> digit (no more than one). Currently I have:</p>
  
  <pre><code>if(current_Operation.matches("\\d")){
  ...
  }
  </code></pre>
  
  <p>Is there a better way to go about doing this? Thanks.</p>