title: RegEx multiple optional characters in group
body: <p>How would I define optional characters in a group?</p>
  
  <p>I am trying to match the following...</p>
  
  <pre><code>kg
  kilo
  kilos
  kilogram
  kilograms
  g
  gram
  grams
  </code></pre>
  
  <p>I know I can put them individually in a group, but was wondering if I could do something fancy like this...</p>
  
  <pre><code>(kg|kilo?g?ram?s?)
  </code></pre>
  
  <p>Problem is it could match only the s? or none of the second alternation so it would match zero length.</p>