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<?php $r1 = '/(?!<\\\\)\\\\n/'; $r2 = '((?:\\\\(*SKIP)(*FAIL))\\\\n)'; var_dump(preg_match($r1, "\\\\\\n")); var_dump(preg_match($r2, "\\\\\\n"));

preferences:
31.87 ms | 402 KiB | 5 Q