Please let us copy & paste your existing regular expression and don't let us type it character by character.
Your screenshot is fine, but please add the regex formatted as code like this:
(?i)(\s)f(?:ea)?t(?:(?:\.?|\s)|uring)(?=\s)
BTW:
If you use a code snippet from this forum please read the exact use case.
This one is for some derivates of "featuring" if it is has a leading space.
Your example has an opening bracket (not a space) in front of "Featuring".
And your replacement text has also a space in front of ␣feat.
This would make "20 Broken ( feat. Amy Lee).mp3" if it would match at all.