See also ... https://perldoc.perl.org/perlrecharclass.html
See also ... https://regular-expressions.mobi/charclass.html
There is a list of backslash sequences, which are character classes.
Probably these character classes also work within Mp3tag's "Boost" regexp implementation.
Simplified said ...
\s ... matches any "whitespace character", ... the 5 characters [\t\n\f\r ]
\w ... matches any "word character", ... the 63 characters [a-zA-Z0-9_]
... and perhaps other unicode characters as set by modifiers within the regexp machine.
If someone has the need to define and apply a character class for only the space character, ...
then it is written as this: [ ]
Check this formatstring within Mp3tag:
My hint in a post above ...
"Have a look to the square brackets ... these are "characters with special functionality"
points to a special function within Mp3tag, which leads sometimes to confusion, when creating a regular expression:
within Mp3tag scripting language the "square bracket clamb" is an inline function or an inline operator.
Therefore within a regular expression a square bracket clamp has to be enclosed (devalued) within single apostrophes (... or just simply the entire regular expression).
See also ...
Preventing empty output lines in export
RegEx to remove Square Brackets
http://help.mp3tag.de/main_scripting.html (Characters with special functionality).