title: what does "$&" mean in Ruby
body: <p>I notice one line code in spree library:</p>
  
  <pre><code>label_with_first_letters_capitalized = t(options[:label]).gsub(/\b\w/)#{$&amp;.upcase}
  </code></pre>
  
  <p>could someone tell me what does "$&amp;" mean ? thanks!</p>