Excerpt from Acad 2013 'AutoLISP Developer's Guide' (the last pdf version) pp. 10
Symbols and Variables
AutoLISP uses symbols to refer to data. Symbol names are not case sensitive
and may consist of any sequence of alphanumeric and notation characters,
except the following:
Characters restricted from symbol names
( (Open Parenthesis)
) (Close Parenthesis)
. (Period)
' (Apostrophe)
" (Quote Symbol)
; (Semicolon)
A symbol name cannot consist only of numeric characters.