A variable name in the programming language C must satisfy the

following:

$$ It must include only lower case letters $($a$-$z$),$ upper case letters $($A$-$Z$),$ digits $(0-9),$ or the underscore character $().$

$$ It cannot be any of the $32$ reserved words.

$$ It cannot start with a digit.

$$ It must be at most $63$ characters long.