fixed date type format pattern regex
merged branch bamarni/date-format-regex (PR #8198)

This PR was submitted for the 2.1 branch but it was merged into the 2.2 branch instead (closes #8198).

Discussion
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[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?

The current 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```.

So when setting this format option to dMy, fields are rendered in a wrong order because of the fallback (year, month, day).

Commits
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6a91bbb [Form] fixed date type format pattern regex