ISO 8601 dates

Write a function from8601 that uses regular expressions to parse an ISO 8601 date. We’re interested only in the date part, which can come in one of two formats:

• YYYY-MM-DD

• YYYYMMDD

Your code should return a triple of integers holding the year, month, and day—or raise a ValueError if things don’t work out. Use doctest to check your code! We won’t be marking it automatically for you.