"=~" 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.
[1] https://news.ycombinator.com/item?id=44359539
> As for idup... The first several search results for "dlang idup" are all useful.
Yes, I am sure it was, I am sure an LLM would have helped too, but I think that is besides the point here.