IIRC, identifiers should consist only alphanumerics (i.e. "A".."Z", "a".."z", "0".."9") and underscore ("_"), and can only start with an alphabetic ("A".."Z", "a".."z") or underscore ("_") character (i.e. they can't start with a digit). Identifiers are case-insensitive (i.e. doesn't matter if it's upper or lower case).