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run code in 200+ php & hhvm versions
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<?php $a = "ab"; echo preg_replace("/(a)|(b)/", "(+${2}+)(-$1-)", $a);
based on 7eGY0
Output for 5.6.0 - 5.6.30, hhvm-3.15.4, 7.0.0 - 7.2.6
Notice: Undefined variable: 2 in /in/9g8lg on line 5 (++)(-a-)(++)(--)