var digits = "0123456789";

// U.S. phone numbers have 10 digits.
// They are formatted as 123 456 7890 or (123) 456-7890.
var digitsInUSPhoneNumber = 10;
var phoneNumberDelimiters = "()- ";
var validUSPhoneChars = digits + phoneNumberDelimiters;

function stripCharsInBag (s, bag)
{   
    var returnString = "";
    for (var i = 0; i < s.length; i++)
    {   
        var c = s.charAt(i);
        if (bag.indexOf(c) == -1) returnString += c;
    }
    return returnString;
}

function validUSPhone(phoneNumber)
{   
	var normalizedPhone = stripCharsInBag(phoneNumber, phoneNumberDelimiters)
	return (isInteger(normalizedPhone) && normalizedPhone.length == digitsInUSPhoneNumber)
}

function isInteger(s)
{    
    for (var i = 0; i < s.length; i++)
    {   
        if (!isDigit(s.charAt(i))) return false;
    }
    return true;
}

function isDigit (c)
{   return ((c >= "0") && (c <= "9"))
}

function validFax(faxNumber)
{   return validUSPhone(faxNumber);
}