Rules for naming convention of a variable :

1. First letter of a variable name cannot be a digit.

Example: 

5id  ✖

id5  ✔

i5d  ✔

2. Special characters are not allowed within the variable name except an underscore _

Example:

rollno.   ✖    //full stop is not allowed

roll no   ✖   //space is not allowed

_rollno  ✔

3. Keywords are not allowed within the variable name

Example: 

void  ✖