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

...
test "aabbbbcccc";
test "abc";
test "aabbcc";

matched
not matched
...
not matched
matched
matched