c:\@Work\Perl\monks>perl -e
"use warnings;
 use strict;
 use feature qw{ say };
 ;;
 for my $string (qw(ab1b2b3b4d  ab5b6b7d  ab8b9d  abxd  ad)) {
   my @two;
   my @matches = $string =~ /(a)
                             (b.(?{ push @two, $2 if defined $2 }))+
                             (d)/x;
   say for @matches, '---', @two, '======';
   }
"
a
b4
d
---
b1
b2
b3
======
a
b7
d
---
======
a
b9
d
---
======
a
bx
d
---
======
---
======