Use sed:

sed 's/[][]//g' file

• The sed command is s/pattern/replacement/modifiers, meaning substitute pattern with replacement. g modifier means globally, otherwise only the first match will be replaced.


• [...] is a character class, so any character inside the brackets match.


• ] closes the character class unless its at first place inside. As we want to include that in our character class, we place it first.


• Replacement is empty in our case.


• Use sed -i '...' to edit your file in place.

You can also use grep -P like I already suggested in your other question:

grep -Po '[K[^]]*' file