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#!/usr/bin/perl

...
    }xmsg;

print "RESULTS: ", ( Dumper \@captures ), "\n";

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

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

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