Possible? Or not? I use correct capitalisation action, and have separate rules to capitalise known capitalisation errors (Dj, Mc, etc...)
Any way to have strings of capitals that are already capitals, missed out or skipped over by the correct capitalisation action?
See the scripting functions in the help
with special attention to $caps2()
Can this be applied to _FILENAME?
a) have you tried it?
b) why do you doubt it?
c) in which context do you want to use the function?
d) why do you have to apply it to the filename when the filename can be put together from the tag fields? Aren't the tag fields already in the correct shape?
a) yes
b) I have tried using Actions within a group, and Actions (Quick), and can't get it to work... 
c) I want to apply it in an existing Action Group, where Case Conversion already happens (using Case Conversion "_FILENAME": Mixed Case, which works, but destroys already capitalised strings)
d) My Filename is where the tags are created from - so I format my filename first, then convert filename > Tags
The you have to show us what you did.
As I said: the normal way is to manipulate the structured tag fields instead of the amorphous filename.
Using the filenames as master for tag fields also has the drawback that not all characters that appear in tag fields are also valid for filenames. Like that you lose information.
You then probably have to change the order of actions in the action group.
I will remove the existing Case Conversion action from my action group, when I can get $caps2(string) working.
I Will keep trying to get the syntax correct using Actions (Quick) and when I find the right syntax, I will create that action within an existing Action Group.
Looks like I got it!
Action (Quick) > Format Value > Field: _FILENAME Format String: $caps2(%_FILENAME%)
Which action do you intend to use to
I would suggest "Format value" for _FILENAME
Format string: $caps2(%_filename%)
What is the difference between _filename and _FILENAME?
Except the case there is none.
_FILENAME addresses the name of a field.
%_filename% addresses the contents of the named field.
I wish I’d known about $caps2() before now!
It is a super useful little action!