"=~" is the operator testing a regular expression match. It should be obvious, because on the right side you see regex, and you ought to know what "=" or "==" does.

FWIW, I knew this as a kid, too, despite knowing absolutely nothing about the language at the time.

Anyways, you should read https://news.ycombinator.com/item?id=44463391 if you care about why I dislike the way D does it.