Alphabetic, Lowercase, Uppercase, WhiteSpace,
NoncharacterCodePoint, DefaultIgnorableCodePoint, Any,
ASCII, Assigned), but also bidirectional types, blocks, etc.
- (see L</"Unicode Character Properties">)
+ (see "Unicode Character Properties")
[4] \d \D \s \S \w \W \X [:prop:] [:^prop:]
[5] can use regular expression look-ahead [a] or
user-defined character properties [b] to emulate set operations