You could use + if you want. I only do it if not doing will produce incorrect output for my given input, which happens occasionally but is rare.
Bear in mind in this case there is also _A.csv or _B.csv you need to account for as well which your version doesn't. Mine will still pick it up because I'm not specifying the end, as I'm assuming I've done the necessary preprocessing steps to get good data so it will produce the expected output.
I'm not usually fussed on being that strict when I write regex as I tend to do various preprocessing steps like grep -v ^$ to filter out any blank lines if I don't want them in the stream etc.
Bear in mind in this case there is also _A.csv or _B.csv you need to account for as well which your version doesn't. Mine will still pick it up because I'm not specifying the end, as I'm assuming I've done the necessary preprocessing steps to get good data so it will produce the expected output.
I'm not usually fussed on being that strict when I write regex as I tend to do various preprocessing steps like grep -v ^$ to filter out any blank lines if I don't want them in the stream etc.