#!/usr/bin/perl -l
#
use strict;
use warnings;

my @strings = qw{
   a123ga123b
   a123bdfda123
   a123effa123
   a123
   d663h
   };

my $len;
my $rsLen = \$len;

my $rxCondMatch = qr
   {(?x)
       a
       (\d+)
       (?{ print q{digits at end of string} if pos() == $$rsLen })
       (??{ if ( pos() != $$rsLen ) { q{b} } })
   };

foreach my $string ( @strings )
{
    $len = length $string;
    print $string;
    print q{Match} if $string =~ $rxCondMatch;
    print q{-} x 20;
}

##</code><code>##

a123ga123b
Match
--------------------
a123bdfda123
Match
--------------------
a123effa123
digits at end of string
Match
--------------------
a123
digits at end of string
Match
--------------------
d663h
--------------------