Function: rgrep

Recursively grep for REGEXP in FILES in directory tree rooted at DIR.
The search is limited to file names matching shell pattern FILES.
FILES may use abbreviations defined in `grep-files-aliases', e.g.
entering `ch' is equivalent to `*.[ch]'.

With C-u prefix, you can edit the constructed shell command line
before it is executed.
With two C-u prefixes, directly edit and run `grep-find-command'.

Collect output in a buffer. While the recursive grep is running,
you can use C-x ` (M-x next-error), or
Uses keymap `grep-mode-map', which is not currently defined.
M-x compile-goto-error in the grep output buffer,
to visit the lines where matches were found. To kill the job
before it finishes, type M-x kill-compilation.

This command shares argument histories with M-x lgrep and M-x grep-find.

When called programmatically and FILES is nil, REGEXP is expected to specify a command to run. (fn REGEXP &optional FILES DIR CONFIRM)