My general advice is that you are doing way more in one regex than you need to do. Perl is extremely good with regex both performance and feature wise. But not every feature needs to be used for every problem.

In Perl, I would use a module like Config::Tiny to parse the .ini file. This would handle different [Sections] in the .ini file. However it appears that you only have the "root", section (no name) to deal with.

I would consider a sequence of regular expressions instead of one complex regex. Something perhaps like below. In Perl sometimes it is actually faster execution wise to run a couple of regex'es on the same variable rather one complex one. In this case, as I understand it, your INI file which will only be parsed once at the start of your program (meaning that performance really doesn't matter that much for this task). I recommend forgetting this lookahead stuff, it is not needed here. Go with simple multiple statements.

#!/usr/bin/perl
use strict;
use warnings;

my @example = (
            'RepeatingGroup = Waiver, Flatten, out',
            'RepeatingGroup = Waiver, out',
            'RepeatingGroup = Waiver, out, Flatten',
            'RepeatingGroup = Waiver , in',
            'RepeatingGroup = Waiver , Flatten', 
            );            

foreach my $example (@example)
{
   $example =~ s/^RepeatingGroup =\s*//;
   print "This is what .ini parser says: $example\n";
   
   my ($name,$direction,$flatten) = parseRepeatingGroup($example);
   print "  name      = $name\n".
         "  direction = $direction\n",
         "  flatten   = $flatten\n";
}

sub parseRepeatingGroup
{
   my $value_text = shift;
   
   my ($name,$rest) = $value_text =~ /^\s*(\S+)(.*)+/;
   my $direction = ($value_text =~ /\bin\b/i) ? 'in' : 'out';
   my $flatten   = ($value_text =~ /\bFLATTEN\b/i)? 1: 0;

   return ($name,$direction,$flatten);
}

__END__
This is what .ini parser says: Waiver, Flatten, out
  name      = Waiver,
  direction = out
  flatten   = 1
This is what .ini parser says: Waiver, out
  name      = Waiver,
  direction = out
  flatten   = 0
This is what .ini parser says: Waiver, out, Flatten
  name      = Waiver,
  direction = out
  flatten   = 1
This is what .ini parser says: Waiver , in
  name      = Waiver
  direction = in
  flatten   = 0
This is what .ini parser says: Waiver , Flatten
  name      = Waiver
  direction = out
  flatten   = 1
[download]

Also in Perl, I recommend that you learn about the "//=" operator which can assign a default value to a variable which is undefined. $v //='default';

In C, I would use a very different approach. I don't see the need for regex in the C code. There are other ways of doing this, but this is not a C or C++ forum.