Bug when replacing regular expressions?

With the help of this forum (and some useful online regex resources) I've been trying to automate my renaming of song titles. What I have is:

Field:
TITLE

Regular expression:
(.+)\s\((.+)\)

Replace matches with:
$1 [$lower($2)]

I'm basically trying to replace any parentheses with square brackets, and also change the case of what's inside these parens, so that:
Outlaw Blues (Take 1, Complete)
becomes
Outlaw Blues [take 1, complete]

Unfortunately, MP3tag renames it: Outlaw Blues [take 1]. I can't see any reason why it ignores the stuff after the comma, my expression seems sound (checked with RegExr). I'm thinking this is a bug, or am I missing something?

Thanks in advance for any replies!
Bluenote

There seems to be a bug in the action "Replace with regular expression", ...
when a string function is applied in the regex's replace string.

Action "Format value" Field: TEST Formatstring: 'Outlaw Blues [Take 1, Complete]' ==> TEST = Outlaw Blues [Take 1, Complete] Action "Format value" Field: TEST Formatstring: $regexp(%TEST%,'(.+)\s\((.+)\)','$1 [\L$2]') ==> TEST = Outlaw Blues [take 1, complete] ------------------------------------------------ Action "Format value" Field: TEST Formatstring: 'Outlaw Blues (Take 1, Complete)' ==> TEST = Outlaw Blues (Take 1, Complete) Action "Replace with Regular Expression" Field: TEST Regular expression: (.+)\s\((.+)\) Replace matches with: $1 [\L$2] ==> TEST = Outlaw Blues [take 1, complete] ------------------------------------------------ Action "Format value" Field: TEST Formatstring: 'Outlaw Blues (Take 1, Complete)' ==> TEST = Outlaw Blues (Take 1, Complete) Action "Replace with Regular Expression" Field: TEST Regular expression: (.+)\s\((.+)\) Replace matches with: $1 [$lower($2)] ==> TEST = Outlaw Blues [take 1] <== Failure ------------------------------------------------

DD.20160620.1650.CEST

Thank you!
I didn't realize I could replace $lower with \L. That worked!

By the way, how did you generate the output from your tests? I wasn't able to find this option in MP3tag.

Thanks again.

How does that work when \L is supposed to represent a non-lower case character?

The bug in the function seems to be the order of evaluation of commas. It seems to be backward and the text following the comma is interpreted as a bad parameter.

About "Replacement Text Case Conversion" learn from there ...
http://www.regular-expressions.info/replacecase.html

DD.20160620.2044.CEST

It's the best reference I have found also. I never had a need for that section. Thanks!