C character set and keywords

Identifiers

Identifiers are names that are given to various program elements, such as variable, functions and arrays.

Identifiers consist of letters and digits, in any order, except that the first character must be a letter.;

Both upper- and lowercase letters are permitted, though common usage favors the use of lowercase letters for most types of identifiers. Upper- and lowercase letters are not interchangeable.

The underscore (_) character can also be included, and is considered to be a letter. An underscore is often used in the middle of an identifier.

An identifier may also begin with an underscore, though this is rarely done in practice.

The following names are valid identifiers:

x y12 sum_1 tax_rate area _temperature