Symbols are groups of characters, selected from the:

Any lowercase alphabetic character in a symbol is translated to uppercase (that is, lowercase a-z to uppercase A-Z) before use.

These are valid symbols:

Fred
Albert.Hall
WHERE?

If a symbol does not begin with a digit or a period, you can use it as a variable and can assign it a value. If you have not assigned a value to it, its value is the characters of the symbol itself, translated to uppercase (that is, lowercase a-z to uppercase A-Z). Symbols that begin with a number or a period are constant symbols and cannot directly be assigned a value. (See Section 1.13.6, “Environment Symbols”.)

One other form of symbol is allowed to support the representation of numbers in exponential format. The symbol starts with a digit (0-9) or a period, and it can end with the sequence E or e, followed immediately by an optional sign (- or +), followed immediately by one or more digits (which cannot be followed by any other symbol characters). The character sequence to the left of the "E" or "e" must be a valid simple number, consisting only of digits or '.'. There must be at least one digit and at most one '.'. The sign in this context is part of the symbol and is not an operator.

These are valid numbers in exponential notation:

17.3E-12
.03e+9

These are not valid numbers in exponential notation, but rather multiple tokens with an operator between:

.E-12     -- no digits
3ae+6     -- non-digit character
3..0e+9   -- more than one '.'