The reader page at http://clojure.org/reader states that symbols (and keywords) cannot start with a number and the regex used in LispReader (and EdnReader) also has this intention. addressed this by fixing the broken reader regex to match the spec. However, that broke some existing code so we rolled back the change. There is still a disconnect here and this ticket serves to decide what to do instead.
I presume that we are effectively deciding that keywords like :5 are ok to read. If so, we should alter the regex to more accurately capture that intent - right now it allows these purely by accident due to backtracking. A secondary question is whether the Clojure and EDN reader spec should also explicitly allow these as valid. My preference would be to have the reader and the spec match, so I would lobby to loosen the reader spec.