tonumber(e [, base])
Tries to convert its argument to a number. If the argument is already a number or a
string convertible to a number, then tonumber returns this number; otherwise, it
returns null.
An optional argument specifies the base to interpret the numeral. The base may be any integer
between 2 and 36, inclusive. In bases above 10, the letter 'A' (in either upper or
lower case) represents 10, 'B' represents 11, and so forth, with
'Z' representing 35. In base 10 (the default), the number can have a decimal part,
as well as an optional exponent part (see Values and
Types). In other bases, only unsigned integers are accepted.