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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";
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.24 - 5.5.35, 5.6.7 - 5.6.21, 7.0.0 - 7.0.20, 7.1.0 - 7.1.25, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
int(1) array(1) { [0]=> string(17) "oo{oo{{oo}ooo}}oo" } {(?:\{(?R)\}|o{2,4})+}

preferences:
212.88 ms | 406 KiB | 327 Q