Bash function for testing an IPv4 address

is_ipv4.sh

is_ipv4() {
    local -r regex='^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$'

    [[ $1 =~ $regex ]]
    return $?
}

is_ipv4_example.sh

is_ipv4 $ip

if [ "$?" -ne 0 ]
then
    echo "[x] $ip is not an IPv4"
    exit 1
fi