I am using `rx' as an alternative to the string-based regexp syntax.  `xr' in a very useful package to convert string regexp to `rx' notation.

But when I do `M-x package-list-packages', I do not see `xr' getting listed.

Furthermore, doing C-x C-e upon the expression below gives "Debugger entered--Lisp error: (void-function xr)".

(xr "\\<\\(co[glmnr]\\|coun\\|cum\\)")