# expansions involving patterns
var='[hello'
echo "${var//[/}"

red='\[\e[0;31m\]'
printf "%s\n" "${red//\\[\\e/}"

foo="${red//\\[\\e/}"

# foo == [0;31m\]
printf "%s\n" "${foo//[0;31m\\/}"

echo "${var//[]/}"
echo "${red//[]/}"

v=hello
foo='[:alpha:]'

echo ${v//[[:alpha:]]/}
echo ${v//[[:alpha]]/}
echo ${v//[[:alpha]/}