title: Negate a specific group in regular expressions
body: <p>How do you get a string not containing a specific group?</p>
  
  <pre><code>(?:[0-9-+*/()x]|abs|pow|ln|pi|e|a?(sin|cos|tan)h?)+
  </code></pre>
  
  <p>The above string is a regular expression for mathematical expressions. How do you get the string that is not a mathematical expression?</p>
  
  <p>Example input string: <code>WIDTH+LENGTH*abs(2)</code></p>
  
  <p>Expected output: <code>WIDTH</code> <code>LENGTH</code></p>