title: Grammar to recognize postfix formulas
body: <p>I'm trying to create a <em>postfix to infix</em> converter, and I'm unable to create the grammar for postfix formulas. I've been also looking for it for a while without success..</p>
  
  <p><strong>What grammar could I use to recognize valid postfix expressions?</strong></p>
  
  <p>The tokens I need are: <code>number</code>, <code>+</code>, <code>*</code> and <code>^</code> (<em>pow</em>).</p>