$r = qr/^(a+b)(?{ $a = -1+length $^N; })(??{ $b=$a-1;"(b{$b})" })(??{ "(c{$a})" })/x;

sub test {
        print
        $_[0] =~ $r
        ? "matched\n"
        : "not matched\n";
}

test "aaabbbccc";
test "aaaabbbccc";
test "aaaabbbbccc";
test "aaaabbbbcccc";
test "aabbbbcccc";
test "abc";
test "aabbcc";