#!/usr/bin/perl

use strict;
use warnings;

my $match = qr[([ab]+)([ab]+)];
my $str = 'aba';

$str =~ /^ $match $ (?{  print "1: $1-$2\n" }) [c] /x;
$str =~ /^ $match $ (?{  print "2: $1-$2\n" }) (?!) /x;
$str =~ /^ $match $ (??{ print "3: $1-$2\n"; qr[(?!)] }) /x;

__DATA__
2: ab-a
2: a-ba
3: ab-a
3: a-ba