from the JLS 3.8 identifiers
3.8 Identifiers
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
IdentifierChars JavaLetterOrDigit
JavaLetter:
any Unicode character that is a Java letter (see below)
JavaLetterOrDigit:
any Unicode character that is a Java letter-or-digit (see below)
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.