#!/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";

##</code><code>##

RESULTS: $VAR1 = [undef,'21','a','b','9',undef,'b','42',undef,undef,'12','a'];

##</code><code>##

my @captures = $string_three =~ m{
	\b ([ab])? (\d+)
	(?(1) | ([ab]) ) \b
	}xmsg;

##</code><code>##

my @captures = $string_three =~ m{
	\b ([ab])? (\d+) (?(1) (*ACCEPT) ) ([ab]) \b
	}xmsg;