list of valid characters in object names

Dear all,
Could someone please point me to the definitive list of valid
characters that are allowed in object names in R? I believe that the
following list covers them:
_.abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789

but I would like to make sure.

Thank you,
Liviu

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