How should a clone of the `grep` program highlight matches for Emacs?

[Raffaele Ricciardi <rfflrccrd@gmail.com>]

I am writing an Emacs script as a replacement for the `grep` program 
while executing the `rgrep` command.  Now, I can't make my Emacs script 
highlight the matches in a way that Emacs understands.

After having read the Emacs Lisp file that defines the `rgrep` command 
-- which I am using for my tests -- I guess that the script should 
surround matches with Select Graphic Rendition sequences; and it is 
doing so, like this:

     (princ "\x1b[31m") ; 31 means "red", as hard-coded in `grep.el`
     (princ (match-string-no-properties 0))
     (princ "\x1b[0m"))

These sequences do cause a terminal to highlight the matches, but not 
Emacs (of course, Emacs does highlight matches from `grep`).

What should the script do instead?  Thank you.