Requirement: selector.identifier.name

An identifier is an unlimited-length character sequence that must begin with a Java identifier start character; all following characters must be Java identifier part characters. An identifier start character is any character for which the method Character.isJavaIdentifierStart returns true. This includes '_' and '$'. An identifier part character is any character for which the method Character.isJavaIdentifierPart returns true.

See:

• Section 3.8.1.1, Message Selector Syntax