How to check if a string is a valid date

Question

I have a string: \"31-02-2010\" and want to check whether or not it is a valid date. What is the best way to do it?

I need a method which which returns

Answer 1

A stricter solution

It's easier to verify the correctness of a date if you specify the date format you expect. However, even then, Ruby is a bit too tolerant for my use case:

Date.parse("Tue, 2017-01-17", "%a, %Y-%m-%d") # works
Date.parse("Wed, 2017-01-17", "%a, %Y-%m-%d") # works - !?

Clearly, at least one of these strings specifies the wrong weekday, but Ruby happily ignores that.

Here's a method that doesn't; it validates that date.strftime(format) converts back to the same input string that it parsed with Date.strptime according to format.

module StrictDateParsing
  # If given "Tue, 2017-01-17" and "%a, %Y-%m-%d", will return the parsed date.
  # If given "Wed, 2017-01-17" and "%a, %Y-%m-%d", will error because that's not
  # a Wednesday.
  def self.parse(input_string, format)
    date = Date.strptime(input_string, format)
    confirmation = date.strftime(format)
    if confirmation == input_string
      date
    else
      fail InvalidDate.new(
        "'#{input_string}' parsed as '#{format}' is inconsistent (eg, weekday doesn't match date)"
      )
    end
  end

  InvalidDate = Class.new(RuntimeError)
end