I don't know where to post this question as it's somewhat off-topic. But seriously, I am trying to figure this out and AI wasn't able to help me. And I have tried to fool with this for hours to no use. So, I'm about to give up.

I saw a regex here on PerlMonks awhile back that took a string and inserts commas into numbers. It's pretty amazing how that works, and I am just now beginning to grasp why and how it works. But now I would like to port it to bash. Now, of course, bash doesn't have regex search and replace but sed does. So, when I plugged this into sed, it complains and says "sed: -e expression #1, char 39: Invalid preceding regular expression" (I'm using sed GNU v4.9 and bash 5.2.15 x64)

WHAT IS WRONG??? 

Original perl code:

#!/usr/bin/perl -w
use strict;
use warnings;
# I copied this regex from:
# www.PerlMonks.org/?node_id=157725
# Usage: STRING = Commify(STRING)
#
sub Commify
{
  defined $_[0] or return '';
  my $N = reverse $_[0];
  $N =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
  return scalar reverse $N;
}
print Commify('Testing 1234 testing... -123456789.01234567800 testing 
+test 4,500,000.00000');

[download]

And now the BASH script: 

#!/bin/bash
# This function inserts commas into numbers
# at every 3 digits and returns the result
# in a global variable called $STR.
#
function Commify
{
  # First, reverse the string
  STR=$(echo "$1" | rev)

  STR=$(echo "$STR" | sed -E 's/([0-9]{3})(?=[0-9])(?![0-9]*\.)/\1,/g'
+)

  # Now reverse it back:
  STR=$(echo "$STR" | rev)
}
Commify 'Testing 1234 testing... -123456789.01234567800 testing test 4
+,500,000.00000'
echo $STR

[download]

In reply to Commify function regex in Perl vs sed by harangzsolt33

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