#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11106665
use warnings;

my $string_three = 'foo bar [21a] plus (b23) baz bax';

my @string_three_results = $string_three =~
  m{ ( [ab] | \d+ ) ( (??{$1 lt 'a' ? '[ab]' : '\\d+' }) ) }gx;

use Data::Dump 'dd'; dd \@string_three_results;

##</code><code>##

[21, "a", "b", 23]