scroll through a list of known email providers and you will notice they registered tlds for the countries they operate in, very typical for the old ones like yahoo yandex Freemail Hotmail etcpp

web.de is an oldschool German provider, as is gmx.de (and .at, .fr, .com, ...)

the regex smells like an inexperienced developer trying to be clever

Or an pretty old regex from before all those new TLDs