How to create a regex on bash to match a valid commit message?

I'm following this posts about git hooks and in one example there is a regular expresion to check the commit message.

The message should be match [XXX-123] My message or [hotfix] My message

REGEX="^\[(hotfix|\w+\-[0-9]+)\]( \w+)+$"

But running the examples "[hotfix] Fixing unit-tests" and "[PSBO-456] It's a valid one this time" I get an error response from the bash script.

This is the script

#!/usr/bin/env bash

echo "Checking commit-message format..."

## the first arg is the path to the git commit temporary file
TEMPORARY_FILE_PATH=$1

## get commit-message from the temporary file
COMMIT_MSG=`head -n1 $TEMPORARY_FILE_PATH`

## init regex to match commit-message format
REGEX="^\[(hotfix|\w+\-[0-9]+)\]( \w+)+$"

## checking commit-message format
if ! [[ $COMMIT_MSG =~ $REGEX ]];then
    echo -e "Your commit-message format is not valid:\n$COMMIT_MSG\n"
    echo "Valid format examples:"
    echo "[PSBO-123] My commit message"
    echo "[hotfix] My commit message"
    exit 1
else
    echo "Well done! Your commit-message is valid."
    exit 0
fi