Problem with $regexp in an Action

I'm trying to extract %GUEST ARTIST% from the following %title%:

Keep On Movin' (feat. Inga Lühning)

[#0]
T=5
F=GUEST ARTIST
1=$regexp(%title%,(^.*)feat\\\\. (.*?)\\\\),$2)

In Notepad++, the regexp (^.)feat. (.?)) returns (via $2): Inga Lühning

However, under mp3tag's format value action for %GUEST ARTIST%, $regexp(%title%,(^.)feat. (.?)),$2) it returns: ,$2)

It is the same when the regexp is encased in quotation marks.

The problem with Guess Value is that the term "feat." can be variable like "ft.", "featuring", etc. I can use "|" in regexp to enable lookup of such variations of "feat." To my understanding, this is not possible with guess value.

I think that this expression works:

$regexp(%title%,'^.feat. (.))',$1)

Yes, this works. I guess I didn't bracket my regexp with apostrophe's earlier. Thanks!

Anyways, in the meantime, I came up with an action that does a little bit of everything in taking GUEST ARTIST information from %title% and placing it in its own field while also formatting %title% to make it look uniform. I'm posting this here just for reference in case someone comes across this thread seeking similar answers:

[#0]
T=5
F=TITLE
1=$regexp(%title%,(^.*)(Ft\\\\.|ft\\\\.|feat\\\\.|featuring|Featuring|Feat\\\\. )(.*$),$1\\\\'('feat\\\\. $3\\\\')')

[#1]
T=2
F=TITLE
1=((
2=(
3=0|0

[#2]
T=2
F=TITLE
1=))
2=)
3=0|0

[#3]
T=2
F=TITLE
1=  
2= 
3=0|0

[#4]
T=7
F=%title%
1=%dummy% (feat. %GUEST ARTIST%)

[#5]
T=7
F=%title%
1=%dummy% (feat. %GUEST ARTIST%) (%dummy%)

[#6]
T=2
F=GUEST ARTIST
1=,
2=;
3=0|0

[#7]
T=2
F=GUEST ARTIST
1= & 
2=; 
3=0|0

[#8]
T=2
F=GUEST ARTIST
1=; Jr.
2=, Jr.
3=0|0

[#9]
T=16
F=GUEST ARTIST
1=;