A Simple S-Expression Parser

S-expressions are a plain ASCII form for representing complex data structures. They can be used to serialize data to send over a communication path, to persist into a database, or to otherwise use as a string representation of a hierarchical data element. When displayed in indented form, S-expressions are even suitable for human comprehension, providing a simple and intuitive nesting syntax, with parentheses used as the nesting operators. Here is a sample S-expression describing an authentication certificate:

(certificate
 (issuer
  (name
   (public-key
    rsa-with-md5
    (e |NFGq/E3wh9f4rJIQVXhS|)
    (n |d738/4ghP9rFZ0gAIYZ5q9y6iskDJwASi5rEQpEQq8ZyMZeIZzIAR2I5iGE=|))
   aid-committee))
 (subject
  (ref
   (public-key
    rsa-with-md5
    (e |NFGq/E3wh9f4rJIQVXhS|)
    (n |d738/4ghP9rFZ0gAIYZ5q9y6iskDJwASi5rEQpEQq8ZyMZeIZzIAR2I5iGE=|))
   tom
   mother))
(not-after "1998-01-01_09:00:00")
 (tag
(spend (account "12345678") (* numeric range "1" "1000"))))

The attraction of S-expressions is that they consist purely of lists of basic character or numeric strings, with structure represented using nested parentheses.

The languages Lisp and Scheme use S-expressions as their actual program syntax. Here is a factorial function written in Common Lisp:

(defun factorial (x)
   (if (zerop x) 1
       (* x (factorial (- x 1)))))

The online Wikipedia article (http://en.wikipedia.org/wiki/s-expression) has more background and additional links for further information on S-expressions.

In computer science classes, it is common ...

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