Backtracking is something internal, and to mess with that, you ugly hacks. It would probably involve a new implementation of the regex engine to do what you want with backtracking.

To add the digits, you need to evaluate some code. With (??{}) it is almost impossible to evaluate and rematch, because when you try recursion, you can't easily use $1 over and over, because it'll be $2 (if you have a single set of parentheses) the second time. And you can't even use $1 within its own parentheses. Forget recursive regexes for now.

As I said, you need to evaluate some code, and you could try doing that with /e, and wrapping it in a loop. However, it's still nasty and there are better ways to do it.

Mathematics may be the answer to this problem. If you have a number, and add all digits and repeat until you have a single digits left, you're going through a lot of trouble to implement $digit = $number % 9 || 9 (figure out how that works yourself).

Illustation:

#!/usr/bin/perl -w

use strict;

sub sum {
    my $sum = 0;
    $sum += $_ for @_;
    return $sum;
}

for (1..10) {
    my $copy = my $number = int rand 1e4;

    print "=== Test: $_ - Number: $number ===\n";

    print "Brute force:\n";
    while ($copy > 9) {
        my @digits = split //, $copy;
        $copy = sum(@digits);
        print "\t", join(' + ', @digits), " = $copy\n";
    }

    print "Maths:\n";
    print "\t$number % 9 || 9 = ", $number % 9 || 9, "\n\n";
}
[download]

=== Test: 1 - Number: 5943 ===
Brute force:
        5 + 9 + 4 + 3 = 21
        2 + 1 = 3
Maths:
        5943 % 9 || 9 = 3

=== Test: 2 - Number: 3630 ===
Brute force:
        3 + 6 + 3 + 0 = 12
        1 + 2 = 3
Maths:
        3630 % 9 || 9 = 3

=== Test: 3 - Number: 3309 ===
Brute force:
        3 + 3 + 0 + 9 = 15
        1 + 5 = 6
Maths:
        3309 % 9 || 9 = 6

=== Test: 4 - Number: 8085 ===
Brute force:
        8 + 0 + 8 + 5 = 21
        2 + 1 = 3
Maths:
        8085 % 9 || 9 = 3

=== Test: 5 - Number: 117 ===
Brute force:
        1 + 1 + 7 = 9
Maths:
        117 % 9 || 9 = 9

=== Test: 6 - Number: 8474 ===
Brute force:
        8 + 4 + 7 + 4 = 23
        2 + 3 = 5
Maths:
        8474 % 9 || 9 = 5

=== Test: 7 - Number: 8132 ===
Brute force:
        8 + 1 + 3 + 2 = 14
        1 + 4 = 5
Maths:
        8132 % 9 || 9 = 5

=== Test: 8 - Number: 1518 ===
Brute force:
        1 + 5 + 1 + 8 = 15
        1 + 5 = 6
Maths:
        1518 % 9 || 9 = 6

=== Test: 9 - Number: 6491 ===
Brute force:
        6 + 4 + 9 + 1 = 20
        2 + 0 = 2
Maths:
        6491 % 9 || 9 = 2

=== Test: 10 - Number: 2170 ===
Brute force:
        2 + 1 + 7 + 0 = 10
        1 + 0 = 1
Maths:
        2170 % 9 || 9 = 1
[download]

There is a minor problem: 0 % 9 || 9 is of course 9, and you want 0. A simple && can fix it, I'll leave its placement to you.

I hope I've recognised the problem correctly. If this is not what you're trying to do (obfu? golf?), please forgive me :)

U28geW91IGNhbiBhbGwgcm90MTMgY
W5kIHBhY2soKS4gQnV0IGRvIHlvdS
ByZWNvZ25pc2UgQmFzZTY0IHdoZW4
geW91IHNlZSBpdD8gIC0tIEp1ZXJk

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