#!usr/bin/perl
use warnings;
$_ = "start b2 end start b2 b2 end start b2 end";

@Matches = /        # list context to catch all
  (?<= \b start \b )# 'start' must precede
  ( .*? )           # catch fewest possible anythings
  (?=  \b end   \b )# 'end' must follow
/gx                 # global and expressive

and print join("\n", @Matches);

__DATA__
 b2 
 b2 b2 
 b2 

##</code><code>##

@Matches = /
  \b start \b
  ( .*? )
  \b end   \b
/gx