Symbol tokens

In Ion text, symbols are represented in three ways:

• Quoted symbol: a sequence of zero or more characters between single-quotes, e.g., 'hello', 'a symbol', '123', ''. This representation can denote any symbol text.
• Identifier: an unquoted sequence of one or more ASCII letters, digits, or the characters $ (dollar sign) or _ (underscore), not starting with a digit and not including the keywords null, nan, true, and false.
• Operator: an unquoted sequence of one or more of the following nineteen ASCII characters: !#%&*+-./;<=>?@^`|~ Operators can only be used as (direct) elements of an S-expression. In any other context those characters require single-quotes.

A subset of identifiers have special meaning:

• Symbol Identifier: an identifier that starts with $ (dollar sign) followed by one or more digits. These identifiers directly represent the symbol's integer symbol ID, not the symbol's text. This form is not typically visible to users, but they should be aware of the reserved notation so they don't attempt to use it for other purposes.