Regex Pattern for Dates between 0001-01-01 and 9999-12-31, Accounting for All Leap Year Rules

gistfile1.txt

# Huge Pattern
`0(0(0([1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[48]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[2468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))))|[1235679](0([0-35679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[48]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[2468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))))|[48]([02468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))))|[13579]([01345789](0([0-35679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[48]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[2468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))))|[26]([02468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))))|[2468]([048]([02468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))))|[1235679](0([0-35679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[48]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[13579]([01345789]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[26]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))|[2468]([048]-(0(2-(0[1-9]|[12]\\d)|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d)))|[1235679]-(0(2-(0[1-9]|1\\d|2[0-8])|[13578]-(0[1-9]|3[01]|[12]\\d)|[469]-(0[1-9]|30|[12]\\d))|1(1-(0[1-9]|30|[12]\\d)|[02]-(0[1-9]|3[01]|[12]\\d))))))`

I verified the pattern using `interegular`, which generated the set of legal strings the pattern can produce. I verified it matches the set of date strings.

```
>>> fsm = interegular.parse_pattern(pattern).to_fsm()
>>> fsm_strings = set(["".join(s) for s in fsm.strings()])
>>> datetime_dates = set([(dt.date(1, 1, 1) + dt.timedelta(days=x)).isoformat() for x in range((dt.date(9999, 12, 31) - dt.date(1, 1, 1)).days + 1)])
>>> fsm_strings == datetime_dates
True
```