A short question about the "new" cucumber synthax

Where I found some documentation about the new syntax from cucumber/gherkin?

An Example:

The old one:

"^Some kind of problem ([A-Za-z]+) with (?:a|an) number (\d+)$"

The new one:

"Some kind of problem {string} with a number {int}"

My problem: How I get this (?:a|an) or this ^$ with the new syntax. And where did I find documentation to it?

Thx and best regards :)