Just for grins, here's another approach, although I would still recommend the two-step approach outlined above:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 print qq{Perl version: $]};
 ;;
 my $ra = qr{ a  }xms;
 my $rb = qr{ b. }xms;
 my $rc = qr{ c  }xms;
 for my $string (qw(ab1b2b3b4c  ab5b6b7c  ab8b9c  abxc  ac  b0)) {
   my @all = $string =~ m{
     \G (?: $ra (?= $rb)
             |
            $rb (?= $rb | $rc)
             |
            $rc (?= \z)
        )
     }xmsg;
   print qq{'$string' -> }, pp \@all;
   }
"
Perl version: 5.008009
'ab1b2b3b4c' -> ["a", "b1", "b2", "b3", "b4", "c"]
'ab5b6b7c' -> ["a", "b5", "b6", "b7", "c"]
'ab8b9c' -> ["a", "b8", "b9", "c"]
'abxc' -> ["a", "bx", "c"]
'ac' -> []
'b0' -> []
[download]

Note that this runs under 5.8.9.