The first character must be a letter (a-z, A-Z)
The following characters can be
a letter (a-z, A-Z)
a digit (0-9)
a pound sign (#)
an underscore ( _ )
An internal name can have any number of characters. Only the first 15 characters are actually used. Characters after this limit are ignored.
The name must not be a reserved word (see the section on reserved words).