What are the rules of creating identifier?

Identifiers refers to the names of variables, functions and array. These are user defined names and consist of a sequence of letters and digits, with a letters as a first character.Both uppercase and lowercase letters are permitted, although lowercase letters are commonly used. The underscore character is also permitted in identifiers. It is usually used as a link between two Words in long identifiers.

In C, identifiers may contain any alphanumeric characters (a-z, A-Z, 0-9) as well as underscores (_), but must not begin with a number.