You could try to write a list containing everything else e.g.
[A-Za-z]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.
[^0-9]will recognise any one character except a digit.
If you want to recognise any character (except newline), look here.