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]

That just means your test isn't good enough. You are testing whether "begin end end begin" *contains* a matched begin/end pair. My test however anchor the regex to the beginning and end, and hence correctly flag "begin end begin end" as to *not* be a nested begin/end construct.

It's the same that /\d/ is *not* a correct regex to test if a string is a number. It's a test to see if a string contains a number. But if all you want to know is whether a string contains a begin/end delimited substring, all you need is /begin.*end/. No recursion required.

Abigail

I again disagree. Delimited text is generally part of a larger document that needs to be processed. I can see your point that anchoring the regex is the best way to fully verify the text item's syntax. However, why would this be needed? The item must have followed some pattern to be extracted in the first place.

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