title: String replacement (to lowercase) in Bash 4.3.33 - bad substitution error
body: <p>I'm trying to change uppercase to lowercase using string replacement in bash, but i'm getting a bad substitution error. </p>
  
  <pre><code>&gt; a=HEY
  &gt; echo $a 
  HEY 
  &gt; echo ${a,,}
  -bash: ${a,,}: bad substitution
  # desired output is hey
  </code></pre>
  
  <p>I've seen similar questions to this, but in most cases it was down to using an earlier version of bash. I'm using GNU bash 4 and still having the same problems.</p>
  
  <pre><code>&gt; bash --version
  GNU bash, version 4.3.33(1)-release (x86_64-apple-darwin14.1.0)
  </code></pre>
  
  <p>This is a Mac thing maybe? Any help would be appreciated.</p>