Unmatched ( before HERE mark in regex m/begin                # Start w
+ith 'begin',
          ( << HERE ?:                  # Followed by
             (?>[^be]+)        #    Not a b or e, many times,
            |b(?!egin)         #    Or a b not starting 'begin',
            |e(?!nd)           #    Or an e not starting 'end',
            |(??{ $re })       #    Or a balanced begin/ at fudge line
+ 6.
[download]

____________________________________________________
** The Third rule of perl club is a statement of fact: pod is sexy.

Whoops.

I added the comments after testing the code. The / in the comment messes things up.

Abigail

($re=$_)=~s/((begin)|(end)|.)/${['(','']}[!$2]\Q$1\E${[')','']}[!$3]/g
+s;
$re = join'|',map quotemeta,eval{/$re/};
$re = qr/$re/;
[download]

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

Yours is wrong.

#!/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
[download]

It matches strings that shouldn't be matched.

Abigail

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]