my $match = $str =~ m{^.{$pos}
        (?{local $pre = ''})
        \d
        (
            (?<=
                (?= \D | ^ | (?1))
                (?: \D | (\d))
                (?{$pre .= $2 if defined $2})
            )
            (?{$prefix = $pre})
        )
        (\d*)}x;