I actually did use that pattern in a few limited other places in the code, but as far as I see, it can only be practically used after you've parsed out a parameter and want to know what things that specific parameter matches.

>replace officer sci with Spock

if ($command =~ /^(?:replace\s+)?officer\s+([a-z]+)\s+with ([a-z]+)/i)
{
  $crewStation = $1;
  $newOfficerName = $2;
  $officer->{science} = $newOfficerName if ('science' =~ /$crewStation
+/);
  ...
}
[download]

I don't see how that trick could be used like a subexpression to help out the first command recognition regex. (Being usable as a subexpression is critical)

However, I do very much like the idea of grepping over a list of keywords in order to make synonyms trivial to add.