my ($start, $end);
    my $res = $str =~ m{
        ^.{$pos}
        \d
        (
            (?<=
                (?= (?: \D | ^ ) (?{$start = pos}) | (?1))
                .
            )
        )
        (\d*)(?{$end = pos})
    }x ? substr($str, $start, $end - $start) : undef ;