It is not.

We simply test differently; I tested with something like this (using your input):

my$re =
qr/
    begin
    (?:
         (?> [^be]* )
        |(??{ $re })
        | [be]
    )*
    end
/x;

foreach (<DATA>)
{
    chomp;
    my @matches = $_ =~ /($re)/g;
    print qq(For "$_":\n\t);
    print (@matches ? join("*",@matches) : "no matches", "\n");
}
__DATA__
begin end
begin en
begin nd
begin begin end end
beginend
beginbeginbeginendendend
begin begin end begin begin end begin end end end
begin begin end begin  egin end begin end end end
begin end begin end
[download]

Which prints:

For "begin end":
    begin end
For "begin en":
    no matches
For "begin nd":
    no matches
For "begin begin end end":
    begin begin end end
For "beginend":
    beginend
For "beginbeginbeginendendend":
    beginbeginbeginendend
For "begin begin end begin begin end begin end end end":
    begin begin end begin begin end begin end end
For "begin begin end begin  egin end begin end end end":
    begin begin end begin  egin end begin end end
For "begin end begin end":
    begin end*begin end
[download]

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