#!/usr/bin/perl
             
use strict;
use warnings 'all';
            
use vars qw /$re/;
          
$re = qr /begin
          (?:
             (?>[^be])*
            |(??{ $re })
            |   [be]
          )*
          end/x;

sub pass {local $_ = shift; print   /^$re$/ ? "ok\n" : "not ok: $_\n"}
sub fail {local $_ = shift; print ! /^$re$/ ? "ok\n" : "not ok: $_\n"}
          
pass 'begin end';
fail 'begin en';
fail 'begin nd';
pass 'begin begin end end';
pass 'beginend';
pass 'beginbeginbeginendendend';
pass 'begin begin end begin begin end begin end end end';
fail 'begin begin end begin  egin end begin end end end';
fail 'begin end begin end';
__END__
ok
ok
ok
ok
ok
ok
ok
not ok: begin begin end begin  egin end begin end end end
not ok: begin end begin end