\$ # Our '$' prefix
(?:
    # Optionally *don't* find the opening paren.
  |
    (\() # Optionally find the opening paren
)
(\w+) # The middle part is captured in $2
(?(1)\)) # Require a closing paren only if $1 matched.

##</code><code>##

\$ # Our '$' prefix
( # Capture into $1
    (?:
        \w+ # Plain word.
      |
        \( \w+ \) # A word with parentheses around it.
    )
)