This section provides a formal syntax for Scheme written in an extended BNF.
All spaces in the grammar are for legibility. Case is insignificant; for example, #x1A and #X1a are equivalent. <empty> stands for the empty string.
The following extensions to BNF are used to make the description more concise: <thing>* means zero or more occurrences of <thing>; and <thing>+ means at least one <thing>.