[[ "\\" =~ ["."] ]] && echo bad 1

[[ "\\" =~ "[.]" ]] && echo bad 2

[[ "\\" =~ [\. ]] && echo bad 3

[[ "\\" =~ [\.] ]] && echo bad 4

[[ "\\" =~ [\\] ]] || echo bad 5

[[ dog =~ [[=d=]].. ]] && echo ok 1
[[ d.. =~ [[=d=]]\.\. ]] && echo ok 2
[[ dog =~ ['[=d=]'].. ]] && echo ok 3	# dubious

[[ dog =~ [[=d=]].[[=G=][=g=]] ]] && echo ok 4
[[ dog =~ [[=d=]].[\[=G=][=g=]] ]] && echo ok 4a

[[ dog =~ "d.." ]] || echo ok 5

[[ dog =~ [[.d.][.D.]]o. ]] && echo ok 6

[[ dog =~ ([[.d.][.D.]])o(.) ]] && echo ok 7 -- ${BASH_REMATCH[1]}
[[ dog =~ d([[.o.][.O.]])(.) ]] && echo ok 8 -- ${BASH_REMATCH[1]}
[[ dog =~ .([[.o.][.O.]])(.) ]] && echo ok 9

[[ dog =~ 'd''o''g' ]] && echo ok 10

[[ ']' =~ [']'] ]] && echo ok 11

[[ a =~ ['a]'] ]] || echo ok 12