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run code in 150+ php & hhvm versions
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<?php var_dump(preg_match($r = '{(?:\\{(?R)\\}|o{2,4})+}', 'foo{oo{{oo}ooo}}oo bar', $m), $m); echo "$r\n";
based on GATGp
Output for 4.3.0 - 7.1.0
int(1) array(1) { [0]=> string(17) "oo{oo{{oo}ooo}}oo" } {(?:\{(?R)\}|o{2,4})+}