How to check if a string "is" an int?

[Fredrik Lundh]

Paul Rubin wrote:

> That description is not quite right.  All characters in the empty
> string belong to the "digit" character class

    A: are there any blue cars on the street?
    B: no.  not a single one.
    A: you're wrong! all cars on the street are blue!
    B: no, the street is empty.
    A: yeah, so all the cars that are on the street are blue!
    B: oh, please.
    A: admit that you're wrong! admit that you're wrong! admit that you're wrong!
    *smack*
    B: (muttering) moron.

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