#!/usr/bin/perl

# https://perlmonks.org/?node_id=11106665

use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 0;

my $string_three = "a aa 21a a 123 a b9 b bb aa1 1aa aa11 11aa [b42] (12a)";

my @captures = $string_three =~ m{
	\b ([ab])? (\d+) ([ab])? \b
	(?(?{ 1 != grep defined, $1, $3 }) (*FAIL) )
	}xmsg;

print "RESULTS: ", ( Dumper \@captures ), "\n";