Variables in Variables for RegexAnnotator

Hello,

We are successfully using the RegexAnnotator, but would like to use variables in variables (see example below), is it currently possible?
If not, does anyone has a workaround or code?
If not, we will provide a patch.

Thanks, Renaud

<conceptSet>
  <variables>
    <variable name="volume_units" value="(?:k|d|m|M|µ)?(?:l|L|m3)" />
    <variable name="mass_units" value="(?:k|d|m|M|µ)?(?:g)" />
    <variable name="units" value="\v{mass_units}/\v{volume_units}" />