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From manual page: https://php.net/regexp.reference.escape
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From the manual: "The precise effect of "\cx" is as follows: if "x" is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B."

Doesn't hex(40) refer to the 7th bit of a binary string?