[Form] fixed date type format pattern regex

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| License       | MIT

I don't understand what is the rationale behind the current regex, why is there mandatory in-between characters?

This regex passes with the format option set to ```dMyyyy``` while it doesn't with ```dMy```, on the linked icu documentation it is stated that ```y``` is equivalent to ```yyyy```.