c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $string_three = 'foo bar [21a] plus (b23) baz bax';
 my @string_three_results = $string_three =~ m{\b ( [ab] \d+ | \d+ [ab] ) \b }gx;
 dd \@string_three_results;
"
["21a", "b23"]