The only way of ensuring that a telephone number is in the US vs Canada or one of the Caribbean nations using the North American phone system is to maintain a list of area codes for the US vs anywhere else. Given that new area codes are being added all the time, any code would require a method of updating the list. And while "123-456-7890" may be invalid today, it could be valid in the future.

Information about American English usage here and here. Floating point issues? Please read this before posting. — emc

In reply to Re^3: Regular expression for Fax numbers by swampyankee
in thread Regular expression for Fax numbers by dxxd116

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