Check for valid dates for all steps of the date

Question

I want to check if a given string is a date of the form YYYY-mm-dd, including all the steps in it, i.e. YYYY-mm-dd, YYYY-mm and YYYY.

For example, all of the following dates are valid:

2014

2014-07

2014-07-03

This is my current regex:

^\d{4}(:?-\d{1,2}(:?-\d{1,2})?)?$

Is it OK?

Answer 1

Is it OK?

Yes.

In the sense that it is "correct". It matches your stated specification, as amended to include both single- and double-digit months and days.

Is it maintainable?

No, in a setting where multiple engineers who come and go will work on the code, it is less maintainable than this regex:

^\d{4}(-\d{1,2}){0,2}$

which in turn is less maintainable than this one:

^\d{4}(-\d{1,2})?(-\d{1,2})?$

Programs must be written for people to read, and only incidentally for machines to execute.

-- Abelson & Sussman, SICP

The last regex immediately conveys the notion of y-m-d to a newly hired engineer at a glance, in a way that the other formulations do not.