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 : '';

##</code><code>##

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

##</code><code>##

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