our $pre;
    my $prefix;
    my $match = $str =~ m{^.{$pos}
        (?{local $pre = ''})
        (
            (?<=
                (?= \D | ^ | (?1))
                (.)
                (?{my $m2 = $2; $pre .= $m2 if $m2 =~ m{\d}})
            )
            (?{$prefix = $pre})
        )
        (\d+)}x;
    my $suffix = $3;
    say $match ? $prefix . $suffix : '';