Understanding complex regular expressions in Emacs Lisp code (for
example, the ones in compile.el) is sometimes difficult with all the
backslashes, so I've written a program that, using the ELPA package
xr[1], enters a minor mode that overlays the rx form of any regular
expression string.  This idea came from macroexpand and how it helps
with understanding Lisp macros.

This program autoloads a new function: regexp-expand.  When the point is
in a program string and regexp-expand is invoked, Emacs enters a minor
mode that puts the buffer temporarily in read-only mode and shows the
regular expression in rx form as an overlay.  You can ask for help about
the rx notation by pressing e.  You can exit the minor mode and return
to the original text by pressing q.

Is there general interest in this kind of functionality as a free
program?  I don't know if this could be a new feature of the xr package
(most probably), or if it could be its own separate package.

A first version of this program is attached to this email, along with
some tests that demonstrate how it works.

Thanks.

[1] https://elpa.gnu.org/packages/xr.html