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 nil.
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,
Z' representing 35.
In base 10 (the default), the number can have a decimal part,
as well as an optional exponent part (see §2.1).
In other bases, only unsigned integers are accepted.