#!/bin/bash

#
# Building a regular expression to match a simple-format ISO8601 date
#

re='^[0-9]{4}(-[0-9]{2}){2}$'

#
# The date is expected as the only argument
#
if [[ $# -ne 1 ]]; then
    echo "Usage: $0 ISO8601_date"
    exit 1
fi

#
# Validate against the regex
#
if [[ $1 =~ $re ]]; then
    echo "$1 is a valid date"
else
    echo "$1 is not a valid date"
fi