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I thought maybe he meant Lisps whose compiler isn't another Lisp, or maybe macros at a stretch ..

Virtually every lisp uses some non s-expression syntax which must be dealt with at the reader level. For example, using ' for quotation.

(3 4 +)

(2 + 3)

(- 3 4)

All are valid s-expressions. Only one is a valid Lisp expression.

Or, in ANSI CL terms, only one is a valid form (S-expression suitable for or subject to evaluation).

A form is already 'read' - it's an object.

When we parse an expression, we usually parse a textual representation into an object. If the object is meant / subject to evaluation -> then we call this a form.

'(a b c) is considered an S-expression in Lisp dialects which support the 'X <-> (quote X) shorthand. It's just a shorthand, and it is read-print consistent, like other such shorthands. If '(a b c) isn't a S-exp, then we have backed into uncomfortable corners in which #(a b c) isn't an S-exp.

Every character in a S-expression must be "dealt with at the reader level" (and produced at the printer level).

The term "S-expression" was originally in contrast with "M-expression", but that's a historic usage.

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