use strict;
use warnings;
use Data::Dumper;

$Data::Dumper::Indent = 0;

my $string_three = 'foo bar [21a] plus (b23) baz bax';
my @string_three_results = $string_three =~ / (?| (\d+) (a|b) | (a|b) (\d+) ) /gx;

print 'RESULTS: ', Dumper(\@string_three_results), $/;