Quoting cppreference:

An identifier is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters, and Unicode characters specified using \u and \U escape notation(since C99), of class XID_Continue(since C23). A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character(since C99)(until C23), or Unicode character of class XID_Start)(since C23)). Identifiers are case-sensitive (lowercase and uppercase letters are distinct). Every identifier must conform to Normalization Form C.(since C23)

In practice depends on the compiler.