Rules for naming a variable
A variable name can have only letters both uppercase and lowercase letters, digits and underscore.
The first letter of a variable should be either a letter or an underscore.
There is no rule on how long a variable name (identifier) can be. However, you may run into problems in some compilers if the variable name is longer than 31 characters.
Note: You should always try to give meaningful names to variables. For example: firstName is a better variable name than fn.