Contains the subpattern from the corresponding set of capturing parentheses from the last successful pattern match in the current dynamic scope. (See "Scoping Rules of Regex Variables".)
Note there is a distinction between a capture buffer which matches the empty string a capture buffer which is optional. Eg, (x?)
and (x)?
The latter may be undef, the former not.
These variables are read-only.
Mnemonic: like \digits.