Characters in names

[Graham Samuel]

I can't find a list in the RR docs of legal characters for variable  
names. Obviously letters of the alphabet (at least the ascii version)  
and the numerals are OK, plus hyphen and underscore - but what else?  
Clearly no character used as an operator, like \ or +, can be legal,  
but maybe some others are: for example is period (full stop to me)  
permitted? If I've missed a complete list, can someone please point me  
at it?

TIA

Graham
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Graham Samuel / The Living Fossil Co. / UK and France