A symbol is one or more characters chosen from the set of all letters (both upper and lower case), digits and the three characters ‘_.$’. On most machines, you can also use $ in symbol names; exceptions are noted in Machine Dependent Features.
Symbols are delimited by characters not in that set, or by the beginning of a file (since the source program must end with a newline, the end of a file is not a possible symbol delimiter).
See the following documentation for more discussion.
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