Lowercase parentheses words

How do I change words like "live" or "demo" when they are in parentheses with RegEx?

For example: Bargain (Live) --> Bargain (live)

You could use an Actions to replace dedicated words.
Or you enter further characters in an action of the type "Case conversion" that indicate where a new word starts.
In this case add the opening bracket.

I tried using this:

Field: _ALL
RegEx: (?<!-)\s(live|demo)
Replace matches with: $lower($0)

But that only works when the words are not in parentheses. Sorry, I don't know much about this, so I don't know what part I'm doing wrong.

I was talking about an action of the type "Case conversion"
In your example, I take it that the search words are already in lower case - so there will be no match as the regular expression is usually case sensitive.

What do I have to write in "Words begin from/after any of:"? I'm trying all sort of things, but it keeps changing every first letter of each word to lowercase.

you could try an action of the type "Format value" for TITLE
Format string: $regexp('Guitar (Live),'(.*) \((.*)','$1 (\l$2')
(please replace 'Guitar (Live)' with %title%)

This worked! Thank you very much!