The reason that :100 parses as a keyword is that [
D&[^/]] matches a colon character, too. So the first : of :100 is not matching the [:]? at the beginning of the regular expression, but by a later part of it. Regexps can be tricky.

Whether this is intentional or not, my guess is probably not. Given that the docs at http://clojure.org/reader also say "A symbol can contain one or more non-repeating ':'s", writing a regexp that matches only what is documented there looks fairly complex.

Clojure has a history of allowing things, without throwing exceptions or giving any other errors, even if they are documented as not legal. This may be one of those cases. Do not consider this last paragraph as any kind of authoritative answer. It is only my observation.