CS2121 lab exercise 1 - hint

To find a character that is not a decimal digit i.e.
find a character other than "0" or "1" or "2" or "3" or "4" or "5" or "6" or "7" or "8" or "9"

You could try to write a list containing everything else e.g.

but how do you know all the characters to include, like spaces, and punctuation etc.?

Instead, you can use "^" inside the "[ ]" to invert its normal meaning: "recognise any one character except those listed here" e.g.

will recognise any one character except a digit.
If you want to use "[", "]", "-" or "^" in your list, look here

If you want to recognise any character (except newline), look here.