title: Regex+Javascript calculator?
body: <p>I'm currently practicing with Regex and I'm was thinking about making something like a calculator -> When I type "104+33" it should replace the text with "104 + 33 = 137". I already did a basic setup, so it's now just the regex stuff:</p>
  
  <h2>What I tried</h2>
  
  <pre><code>function regexStuff(string){
      return string.replace(/([\d+])\+([\d+])/g, '$1 + $2 = '+eval('$1+$2'));
  }
  </code></pre>
  
  <p><strong>ERROR</strong>: <code>Uncaught ReferenceError: $1 is not defined</code></p>
  
  <h2>Important</h2>
  
  <p>As I already mentioned: I am new to this regex stuff. It would be fantastic if you explain in in one or two sentences. Thanks!   </p>