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run code in 150+ php & hhvm versions
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<?php $userId = '12'; echo preg_match('/\A[0-9]+\z/', $userId) ? 'true' : 'false';
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
true