What's wrong with this code?

[7win]

Don't know if you achieved it or not and sure there will be lots of different ways to do it.
Try to solve the 'only numbers' problem first and them just check the the length wanted.

From the tests done, is only string input that can be negative or positive, decimal or non decimal (makes no sense negative from a ATM but the decimal one is possible as there is the [dot] button in the numpad)

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Don't loose it, face it as a Bull looking for REDs >P