=over

=item $<I<digits>> ($1, $2, ...)
X<$1> X<$2> X<$3> X<$I<digits>>

Contains the subpattern from the corresponding set of capturing
parentheses from the last successful pattern match in the current
dynamic scope. (See L</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, C<(x?)> and
C<(x)?> The latter may be undef, the former not.

These variables are read-only.

Mnemonic: like \digits.

=back