RE: rx.el sexp regexp syntax

[Drew Adams <drew.adams@oracle.com>]

It's not just about confusion/obscurity due to the "extra"
backslashes (for `(', `)', and `|').  It's also about the
fact that regexps themselves can be complicated.  For
example, `directory-listing-before-filename-regexp':

"\\([0-9][BkKMGTPEZY]? \\(\\([0-9][0-9][0-9][0-9]-\\)?[01][0-9]-[0-3][0-9][ T][ 0-2][0-9][:.][0-5][0-9]\\(:[0-6][0-9]\\([.,][0-9]+\\)?\\( ?[-+][0-2][0-9][0-5][0-9]\\)?\\)?\\|[0-9][0-9][0-9][0-9]-[01][0-9]-[0-3][0-9]\\)\\|.*[0-9][BkKMGTPEZY]? \\(\\(\\([A-Za-z']\\|[^\0-\x7f]\\)\\([A-Za-z']\\|[^\0-\x7f]\\)+\\.? +[ 0-3][0-9]\\|[ 0-3][0-9]\\.? \\([A-Za-z']\\|[^\0-\x7f]\\)\\([A-Za-z']\\|[^\0-\x7f]\\)+\\.?\\) +\\([ 0-2][0-9][:.][0-5][0-9]\\|[0-9][0-9][0-9][0-9]\\)\\|\\([A-Za-z']\\|[^\0-\x7f]\\)\\([A-Za-z']\\|[^\0-\x7f]\\)+\\.? +[ 0-3][0-9], +[0-9][0-9][0-9][0-9]\\|\\([ 0-1]?[0-9]\\([A-Za-z]\\|[^\0-\x7f]\\)? [ 0-3][0-9]\\([A-Za-z]\\|[^\0-\x7f]\\)? +\\|[ 0-3][0-9] [ 0-1]?[0-9] +\\)\\([ 0-2][0-9][:.][0-5][0-9]\\|[0-9][0-9][0-9][0-9]\\([A-Za-z]\\|[^\0-\x7f]\\)?\\)\\)\\) +"

Even after removing "extra" backslashes, it's still a bear:

"([0-9][BkKMGTPEZY]? (([0-9][0-9][0-9][0-9]-)?[01][0-9]-[0-3][0-9][ T][ 0-2][0-9][:.][0-5][0-9](:[0-6][0-9]([.,][0-9]+)?( ?[-+][0-2][0-9][0-5][0-9])?)?|[0-9][0-9][0-9][0-9]-[01][0-9]-[0-3][0-9])|.*[0-9][BkKMGTPEZY]? ((([A-Za-z']|[^\0-\x7f])([A-Za-z']|[^\0-\x7f])+\\.? +[ 0-3][0-9]|[ 0-3][0-9]\\.? ([A-Za-z']|[^\0-\x7f])([A-Za-z']|[^\0-\x7f])+\\.?) +([ 0-2][0-9][:.][0-5][0-9]|[0-9][0-9][0-9][0-9])|([A-Za-z']|[^\0-\x7f])([A-Za-z']|[^\0-\x7f])+\\.? +[ 0-3][0-9], +[0-9][0-9][0-9][0-9]|([ 0-1]?[0-9]([A-Za-z]|[^\0-\x7f])? [ 0-3][0-9]([A-Za-z]|[^\0-\x7f])? +|[ 0-3][0-9] [ 0-1]?[0-9] +)([ 0-2][0-9][:.][0-5][0-9]|[0-9][0-9][0-9][0-9]([A-Za-z]|[^\0-\x7f])?))) +"

As I said before:

 > Functions that transform a regexp string to an RX
 > sexp and vice versa would be very helpful.

(And yes, the latter exists.)

Ideally, we'd have the ability to put your cursor on
a regexp in some code and hit a key to:

 * see a corresponding `rx' sexp and
 * optionally replace the regexp with the `rx' sexp.

Just being able to see the `rx' sexp that corresponds
to a regexp in some code (even temporarily in a popup)
could help, I think.