PCRE docs say:
If the first character following (? is a circumflex, it causes all of the above options to be unset. > Thus, (?^) is equivalent to (?-imnsx).
There is "xx" option. So may be docs have a typo? - "all of the above options" -> "all of the above options but xx"- or "(?^) is equivalent to (?-imnsx)" -> "(?^) is equivalent to (?-imnsxxx)"
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