JSON is not extensible and syntactically more verbose. However, the syntax corresponds exactly to many modern dynamic languages and is now nearly ubiquitous.

> Of course JSON (http://www.ietf.org/rfc/rfc4627.txt) does this as well but using character code points.

I hope Rich will define "alphanumeric" as Unicode letter and number classes. Clojure itself is ambiguous on valid symbol identifiers. http://clojure.org/reader has the same "alphanumeric plus some punctuation" rule, but the actual reader implementation [1] is extremely permissive:

    static Pattern symbolPat = Pattern.compile("[:]?([\\D&&[^/]].*/)?([\\D&&[^/]][^/]*)");

[1]: https://github.com/clojure/clojure/blob/master/src/jvm/cloju...

> In perl there is YaML ...

YAML is not just limited for use in Perl, nor is it as closely related to Perl as JSON is to Javascript. One of its creators, Ingy döt Net, was (and still is) an active member of the Perl community, so Perl had excellent support for YAML from the very beginning.

From http://www.yaml.org/about.html:

The founding members of YAML are Ingy döt Net (author of the Perl module Data::Denter), Clark Evans, and Oren Ben-Kiki. YAML emerged from the union of two efforts. The first was Ingy döt Net's need for a serialization format for Inline, this resulted in his Data::Denter module. The second, was the joint work of Oren Ben-Kiki Clark Evans on simplifying XML within the sml-dev group. YAML was first publicized with a <?xmlhack?> article on 12 May 2001. Oren and Clark's vision for YAML was very similar to Ingy's Data::Denter, and vice versa, thus a few days later they teamed up and YAML was born.

Your link says "external", not "extensible", and in fact seems to make no mention of extensibility.

Absolutely correct, brain fart. The description language was designed to represent any data structure you could represent in C, in XDR but more importantly insure that moving those structures across a network between disparate architectures would return them the natively 'correct' format when received.