To say that using "\z" is "the correct regex solution" (and imply that using "$" is "incorrect") is a bit silly, esp. in the absence of more detailed info regarding the application.

If, as the OP requested, the variable must contain an integer between 1 and 5 inclusive, "1\n" seems like an invalid option. Unfortunately, /^\d$/ gives a false positive for that value.

But in many common situations, they behave the same...

I can only think of one case, and that's where the string does not have a trailing newline.