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

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