I want to write a regular expression that only allows a user to enter an input which contains alphanumeric characters, underscores, periods, dashes, and plus signs.

In a bash script, I have:

VALID="^[0-9a-zA-Z_\.\-\+]+$"
THE_INPUT=$1
if [[ ! $THE_INPUT =~ $VALID ]]; then
    echo "ERROR"
    exit 1  
fi

Why does this not work when the user enters an input which has a - in it?

Example user inputs:

1. great-food.png

   i. produces error

2. great_+food.png

   i. doesn't produce error