Programming :: Using Sed To Search And Replace Backwards?

I'm trying to use sed to search and replace backwards. The problem is that I have a shell script that is required to put commas into big numbers. For example

9999999 as 9,999,999

I've tried a few things, but none seem to work:

Code:

$ echo 9999999 | sed -e 's/([0-9]{3})/,1/g'
,999,9999
$ echo 9999999 | sed -e 's/([0-9]{3})$/1,/g' -e 's/([0-9]{3})/1,/g'
999,999,9,
$ echo 9999999 | sed -e 's/([0-9]{3})$/1,/g' -e 's/([0-9]{3})/,1/g'

[ode]....

It would be much easier if I could search backwards! For example Bash parameter substitution style:

Code:
$ echo 9999999 | sed -e 's%([0-9]{3})%,1%g'