Regex square close bracket

Question

I can use Bash to match a set of characters

$ [[ a =~ [abc] ]]; echo $?
0

However if I want a close square bracket ] to be included in the set, it fails

$ [[ a =~ [abc\]] ]]; echo $?
1

$ [[ a =~ [abc\\]] ]]; echo $?
1

Answer 1

From the man page regex(7):

To include a literal ']' in the list, make it the first character (fol-
lowing a possible '^').  

Testing:

$ [[ "]" =~ []abc] ]]; echo $?
0