Also, /begin.*end/ will allow a begin followed by an end, which will allow an item like beginbeginend to pass with no problems;

So does your regex:

$re = /begin
       (?:
          (?>[^be]*)
         |(??{ $re })
         | [be]
        )*
        end/x;
"begin begin end" =~ /$re/ and print "<$&>\n";
__END__
<begin begin end>
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Abigail