Applescript to replace string using sed

I need and Applescript to replace some text as follow:

Original text:

 string  string  string  $  text

The required output is:

$ text

I can do it in the terminal with this command:

$ echo "string  string  string  $  text" | sed -r 's/^(.*)\$ ?(.) (.*)$/$ \3/g'
$ text

By the way, i'm using bash version 4.3.30 and sed 4.2.2, both from homebrew.

The issue here is that I need to do it from an applescript. This is my approach:

set commandString to "echo \"string  string  string  $  text\" | sed -r 's|^(.*)\\$ ?(.) (.*)$|$ \\3|g'" as string
set formattedCode to do shell script commandString

And I get the following error:

error "sed: illegal option -- r
usage: sed script [-Ealn] [-i extension] [file ...]
       sed [-Ealn] [-i extension] [-e script] ... [-f script_file] ... [file ...]" number 1

If I remove the -r option, I get a different error:

sed: 1: "s|^(.*)\$ ?(.) (.*)$|$  ...": \3 not defined in the RE

If I remove the \3, the output must be $ instead of $ text, but sed command do nothing and it outputs:

string  string  string  $  text

I supposed that this might be a problem with sed version, am I right? How can I solve it?