I have some titles that have "feat." followed by the featured artist. All of them are contained in a parenthesis (Ex: The one (feat. U2)). I wanted to remove the featuring artist protion and append that to the artist. I figured the best way to do this was to move the featured artist material temporarily to another field and then append it to the artist.
I tried to use Guess Values with the Source Format of %title% and a Guessing Pattern of %title% (%composersort%) but that did absolutely nothing. I tried a Source Format of %title% and a Guessing Pattern of %title% (%dummy%) but that did not even remove the featured artist information. I cannot figure our what I am doing wrong.
I even used an old action that I saved, which has a Source Format of %title% and a Guessing Pattern of %dummy% (%album%) which should remove the title and add the featured artist info into the ALBUM field, but that did nothing as well. 
I am on 3.18 as I downgraded trying to solve another issue.
Either the example you supplied with
The one (feat. U2)
or the guessing pattern
%title% (%composersort%)
does not match that what you typed in.
I tried it and I ended up with
The one
in TITLE
and
feat U2
in COMPOSERSORT
Are you sure that you used the correct type of action and not accidentally e.g. "Format value"?
And if you cannot find the problem yourself ... screendumps might help us to analyze.
Thanks ohrenkino!
Here you go. A little different example, as I was playing with different versions, but the concept is the same.
This is very weird. As you can see it deposited "Saw Mommy Kissing Santa Claus (feat. Barry Biggs)" into %composersort% but kept the complete title. It is almost like Mp3Tag is using the space as a delimiter rather than the open parenthesis.
TBH: I see a cramped tag panel - but as you know: this does not say anything about the fields behind the tag panel elements.
The best dialogue for analysis is the extended tags dialogue Alt+T.
That would also reveal duplicate fields etc.
What you could also try: copy the parenthesis from the title and copy it to the action and see whether it is actually a normal bracket or something else.
This is exactly what I was just thinking. Either the brackets, or even the space character(s)? Using the suggested copy/paste will confirm this.
You liking a clean panel? I would have never guessed! 
Genius! There must be something there though I could not detect it, but when I just copied the "I Saw Mommy Kissing Santa Claus (feat. Barry Biggs)" and replaced I Saw Mommy Kissing Santa Claus with %title% and feat. Barry Biggs with %composersort% everything worked fine.
So I was not crazy, it should have worked, there was some weird character in there as you suspected. Thank you, Mp3Tag Crew, for your help. It is very much appreciated!! 
I tried this in Excel using the formula to grab Unicode (in Hex) and all of the spaces here were U+0020 (the correct code for the standard space). Did you copy/paste the quote above from your actual file, or did you type it in separately here? I was curious to see what character was actually being used.
FYI the formula to get this info in Excel is =DEC2HEX(UNICODE(xx)), where xx is the cell you want to look at, and the returned value is from the first character in that cell. If you delete one character at a time, you will see the Unicode value and can find out what was being used.
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You can do that easily online, just copy & paste a string into tools like
https://www.babelstone.co.uk/Unicode/whatisit.html
or
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You can the character code also in MP3tag in Convert>tag-tag with
Format string: $ord()
where you insert the character in question in the brackets
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There are several ways to find the "offending" character. With spaces in particular, I'm always curious what was used, and why.
Thanks for this babelstone tip, this is a great resource and shows all characters in a string with their Unicode reference and description.
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You're welcome. I use it for foreign characters like "η½ζ₯εΊζ²ηζηοΌιηοΌ" too. Just to be sure...
(For example: I did not even know that something like FULLWIDTH RIGHT PARENTHESIS exists )
The second URL repeats every single character. That makes it easy to spot such special characters:
I believe I just copied and pasted. Thanks everyone for the tips.
