Regex substitution in Python to add period to sentence that doesn't have one but only if it is the only line in string

I'm looking to add periods to a single sentence in a string that doesn't have one, such as

string = "This is a sentence"

But while avoiding adding periods to things like

string = "Item 1\nItem 2\nItem 3"

So is there a way to add periods to a sentence if it is the only line in a string? Thanks.

Actually you can do it without regex. Take a look if these conditions are enough in your case:

s = "Some string"
s += "." if s[0].isupper() and not s.endswith(".") and "\n" not in s else ""