How do I negate two logic tests with regular expressions in a bash script?

In bash, we would like to do the following logic: if the variable "$SLACK" is TRUE and ${OUTPUT} is not equal to "mysql: [Warning] Using a password on the command line interface can be insecure", call the slack_it function.

I have tried more than a dozen solutions provided from the posts here How do I negate a test with regular expressions in a bash script?, Check if a string matches a regex in Bash script, but none of them work. Could any guru enlighten?

#!/bin/bash

if [ "$SLACK" == "TRUE" ]
   then
        if ! [[ ${OUTPUT} =~ "*mysql: [Warning] Using a password on the command line interface can be insecure*" ]]
            then
                slack_it
        fi
  fi