Hi K, I took your comment that this is difficult with regex as a challenge. As I do not speak any Javascript, I have no idea whether my little code can be translated, so it will most likely not fit your purpose. I found it more difficult than expected, especially when trying to add variables for the separators. So here is my piece, not to be taken as a competitor to yours:

use strict;
use warnings;

sub fmthd { # number, decimal separator, thousands separator
  my ( $dec, $thd ) = ( $_[1] // ".", $_[2] // "," );
  my ( $n, $d ) = split /\Q$dec\E/, shift;
  my $s = $n<0?"-":"";
  $n = reverse $n;
  return $s.reverse( $n = join $thd, $n =~ /(\d{1,3})/g ).($d?"$dec$d"
+:"${dec}00");
}

print fmthd( 1234567.14 ), "\n";
print fmthd( 1234567 ), "\n";
print fmthd( 12.14 ), "\n";
print fmthd( 123456.14343 ), "\n";
print fmthd( 1234567.14 ), "\n";
print fmthd( 8881234567.14 ), "\n";
print fmthd( -8881234567.14 ), "\n";
print fmthd( "-8881234567,14", ",", "." ), "\n";
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One comment though: your Perl code seems to lose the minus sign for negative numbers.