U+00AD is 0xAD, so "Cf cannot be addressed by \xNN" was false for the
first example in its own list. The real boundary is the category: one
\xNN form covers Cc exactly, and escaping the single addressable Cf
member would leave a rule that is neither category- nor
addressability-shaped.
A lone surrogate is the NUL case rather than the invisible-character
case -- Python source must be UTF-8-encodable, and compile() raises
UnicodeEncodeError for one in a string literal or a # comment alike
(measured on 3.9). JSON.parse on a wire "\ud800" escape produces them,
so escape them as \uNNNN; the regex's u flag keeps well-formed astral
pairs intact.
Pin the whitespace-plus-surviving-control boundary, which also pins
trim-after-escape.
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