Building upon Raystafarian's comment, you can use the Contains or Does Not Contain values if there are no characters in the string. If there's a chance of a character (e.g. 123c), then it will filter that value out.
To filter against the character v. number in only the first character, you'll need a helper column. Assuming your data in Column A:A, in an adjacent column (e.g. B:B), insert the following formula:
This evaluates the first character in your cell (LEFT(A1,1)) and returns it's ASCII code. If the code is between 48 (0) and 57 (9), it will return True, otherwise False. You can then filter off of this column. I agree that you should convert your data into a Table, Excel works and plays well with Table formatted data.