in reply to Complex regex question

Yet another way. Pre-5.10 compatible; avoids explicit captures. Unfortunately, rather heavy on the look-arounds. 

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = qq{a aa 21a a 123 a b9 b bb aa1 1aa @ARGV};
 dd $s;
 ;;
 my @captures = $s =~ m{
   \b \d+ (?= (?: a|b) \b)
   |
   (?<= \b (?: a|b)) \d+ \b
   |
   \b (?: a|b) (?= \d)
   |
   (?<= \d) (?: a|b) \b
   }xmsg;
 dd \@captures;
"  "aa11 11aa [b42] (12a)"
"a aa 21a a 123 a b9 b bb aa1 1aa aa11 11aa [b42] (12a)"

[21, "a", "b", 9, "b", 42, 12, "a"]

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Give a man a fish:  <%-{-{-{-<

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Re^2: Complex regex question
by rsFalse (Chaplain) on Sep 25, 2019 at 18:52 UTC
    One more variant: 
    #!/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";

    [download]
    RESULTS: $VAR1 = [undef,'21','a','b','9',undef,'b','42',undef,undef,'1
+2','a'];

    [download]
    It generates triplets with one undefined value ($1 or $3), but you can grep defined values as AnomalousMonk mensioned.

    EDIT:
    Sorry for mistake. I changed condition "2 == grep defined, $1, $3" to "1 != grep defined, $1, $3", because one of $1 or $3 must be defined.

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

    [download]
    Similar approach using conditional. It captures in same way.

    ADDED-2:
    Also:
    my @captures = $string_three =~ m{
    \b ([ab])? (\d+) (?(1) (*ACCEPT) ) ([ab]) \b
    }xmsg;

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