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run code in 150+ php & hhvm versions
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<?php preg_match_all( '/^\[#(\d+)\]/', '[#1234] Subject [#89856] Subject', $matches); print_r($matches); // 1234
based on r9BrY
Output for 4.3.0 - 5.0.5, 5.1.1 - 5.6.28, hhvm-3.12.0, 7.0.0 - 7.1.0
Array ( [0] => Array ( [0] => [#1234] ) [1] => Array ( [0] => 1234 ) )
Output for hhvm-3.10.0

Process exited with code 153.
Output for 5.1.0
Fatal error: fatal flex scanner internal error--end of buffer missed in /in/NJqRe on line 3
Process exited with code 255.