An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter.
Identifier:
IdentifierChars but not a Keyword or BooleanLiteral or NullLiteral
IdentifierChars:
JavaLetter {JavaLetterOrDigit}
JavaLetter:
any Unicode character that is a "Java letter"
JavaLetterOrDigit:
any Unicode character that is a "Java letter-or-digit"
A "Java letter" is a character for which the method Character.isJavaIdentifierStart(int) returns true.
A "Java letter-or-digit" is a character for which the method Character.isJavaIdentifierPart(int) returns true.
The "Java letters" include uppercase and lowercase ASCII Latin letters A-Z (|u0041-|u005a), and a-z (u0061-|u007a), and, for historical reasons, the ASCII underscore (_, or |u005f) and dollar sign ($, or |u0024). The $ sign should be used only in mechanically generated source code or, rarely, to access pre-existing names on legacy systems.
The "Java digits" include the ASCII digits 0-9 (|u0030-|u0039).
Letters and digits may be drawn from the entire Unicode character set, which supports most writing scripts in use in the world today, including the large sets for Chinese, Japanese, and Korean. This allows programmers to use identifiers in their programs that are written in their native languages.