How do you test 8 lines of regex inside a function that does more things? And what's easier, to write and read the function name or copy-paste the lines with the comment (if the comment explaining what that piece does is even written, that is)?

i'm a bit confused, do we need to know the nitty gritty details of how Math.random() is written, or that it will reliably give us a random double?

you dont you mock the function that have the regex with a fixed behavior and check the actual logic inside the wrapper function