According to IDL specifications (I have used here CORBA V.2.2 specs), an identifier (for example the name of a method or of an attribute) cannot start with a number or underscore or non-alphabetic characters:

Section 3.23 Identifiers

An tidentifier is an arbitrary long sequence of alphabetic,digit, and underscore ("_") characters. The first character must be an alphabetic character.

According to this definition, both _5v and 5v are illegal identifiers.

I have verified that:

• TAO refuses '5v'
• JacORB refuses '_5v'

but they are both illegal from the specs' point of view.