bash regex quotes?

the following code

number=1
if [[ $number =~ [0-9] ]]
then
  relate matched
fi

works. i try quotes regex, however, stops:

number=1
if [[ $number =~ "[0-9]" ]]
then
  relate matched
fi

i attempted "\[0-9\]", too. am i missing?

funnily enough, suggests should work.

bash chronicle 3.2.39.