ANSI body and ANSI Addendum provide the following syntax.
qualified-id ⊃ module-name period simple-numeric-variable (18.2 (26))
From these, the following syntax can be deduced.
simple-numeric-variable = numeric-identifier (5.2.2 (2))
numeric-identifier ⊃ module-name period letter identifier-character* (18.2 (27))
qualified-id ⊃ module-name period module-name period letter identifier-character*