Re: backward-sexp and insert-parentheses are lacking for Common Lisp

[Richard Stallman]

Implementing support for #-constructs that involve
a letter or digit, such as #c or #2, is not easy.
It would have to be done in syntax.c.

If you're interested in working on it, it is ok to try.

On the Lisp Machine I implemented a form of Lisp parsing
that always worked by parsing forward, even when the job
was moving backward.  Using text properties, that might
be feasible now in Emacs.  It would make this sort of thing
much easier.  But it is a substantial job.