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:
$len($regexp($char(9)$char(10)$char(11)$char(12)$char(13)$char(32)$char(133)$char(160),'\s',))
==> 0
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
Feature request:
RegEx to remove Square Brackets
https://docs.mp3tag.de/scripting (Characters with special functionality).
DD.20171026.0551.CEST