Reuse Patterns Using Capture Groups: why ^ and $

I don’t understand why the ^ and $ are necessary in this problem. *From other problems, I’m getting that if I do the following

let re = /abc/
let teststr = 'abcdef'

then the match made would be ‘abc’, and not the entire string, ‘abcdef.’

So then why in this problem, if we don’t have the ^ and $, for strings such as “test test test test”, i.e. which contain the regex plus more, does the entire thing match?

*Maybe this is wrong? I have to say I found FCC doesn’t adequately explain Regex’s at all. It’s impossible from going through their examples to actually know how they work. I’ve been able to understand them better from other sources but this I still don’t get.

Link to the challenge:

Because of this test:

Your regex should not match "42 42 42 42".

You can only make sure of it if you capture the whole string with the delimiters.