[a] You can mimic class subtraction using lookahead.
For example, what UTS#18 might write as
- [{Greek}-[{UNASSIGNED}]]
+ [{Block=Greek}-[{UNASSIGNED}]]
in Perl can be written as:
- (?!\p{Unassigned})\p{InGreekAndCoptic}
- (?=\p{Assigned})\p{InGreekAndCoptic}
+ (?!\p{Unassigned})\p{Block=Greek}
+ (?=\p{Assigned})\p{Block=Greek}
But in this particular example, you probably really want
- \p{GreekAndCoptic}
+ \p{Greek}
which will match assigned characters known to be part of the Greek script.