That's a special case of the
[[ "\\" =~ ["."] ]]
returning true (because bash called regcomp("[\\.]") instead of
regcomp("[.]") I had reported some time ago and was then fixed.
Here. It's similar:
$ bash -c '[[ "\\" =~ [^]"."] ]]' || echo unexpected
unexpected
$ ltrace -e regcomp bash -c '[[ "\\" =~ [^]"."] ]]'
bash->regcomp(0x7ffc146e78c0, "[^]\\.]", 1) = 0
I suspect bash thinks the first "]" closes the bracket
expression and thus assumes the "." is outside them so needs to
be escaped for the RE engine as it's quoted in the bash RE.
--
Stephane
