And as for using \x followed by 2 hex chars in the replacement, according to the code that should work, see:
https://github.com/Sigil-Ebook/Sigil...extBuilder.cpp
So any byte sequence can be made by a string of \xHH where H is a hex character (0-9,a-f,A-F).
If need be we can add support for longer hex specified chars to that code by supporting braces { and } so that would work as well.
Is support of using \x{abcd} format in replacements desired or can people live with just \xab\xcd which should be supported now according to the ReplacementBuilder code.
Update: \xab\xcd actually does *not* work, instead of combining the two consecutive byte values into an unsigned int16 they promote them both separately to two different u16 byte values. I will add support for replacement using \x{hhhhhhhh} to a sequence of uint16 values.