Hello Cody Fendant,

Your regex is actually working correctly: in the alternation m{ (\d+) (a|b) | (a|b) (\d+) }gx, the 4 captures are assigned to $1, $2, $3, and $4, even though only 2 of the 4 matches are possible in any particular instance (hence, you will always get 2 undef results). To avoid this behaviour, you can use the Extended Pattern (?|pattern) (from Perl 5.10.0 on):

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), $/;
[download]

Output:

16:38 >perl 2020_SoPW.pl
RESULTS: $VAR1 = ['21','a','b','23'];

16:38 >
[download]

Hope that helps,