Description:
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Failed regardless of OPCache being enabled or disabled and if it was TS or NTS. 

Test script:
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setlocale(LC_ALL, 'pt_PT', 'pt', 'pt_PT.ISO8859-1', 'portuguese');
var_dump(preg_match('/^\w{6}$/', 'aאבחיט'));


Expected result:
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int(1)

Actual result:
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int(0)