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run code in 150+ php & hhvm versions
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<?php var_dump(preg_match('{(?:\\{(?R)\\}|o{2,4})+}', 'foo{oo{{oo}ooo}}oo bar', $m), $m);
based on AIINi
Output for 4.3.0 - 5.6.28, hhvm-3.12.0, 7.0.0 - 7.1.0
int(1) array(1) { [0]=> string(17) "oo{oo{{oo}ooo}}oo" }
Output for hhvm-3.10.0

Process exited with code 153.