I've been reading the wisdom of the monks for about 2 weeks now, and decided it was finally time to get an account and post a question that's been bothering me. Here's the problem: I am trying to set up a recursive pattern match for nested parentheses, and since my handy O'Reilly "Programming Perl" had an example of this exact regex, I thought I was going to do OK. Here's my code, slightly modified {in order to match \{ \} instead of \( \) } from its original format on p. 214 in O'Reilly: 

local $np; # I added this declaration myself
$np = qr{
          \{
          (?:
             (?> [^{}]+ )
             |
             (??{ $np })
           )*
           \}
        }x;

[download]
O'Reilly notes that you can't declare the variable in the same statement where it's used, which makes sense to me. So I added the declaration above. Problem is, when I run my code I get this error: 
Use of uninitialized value at shorty.pl line 19 (#1)
    
    (W) An undefined value was used as if it were already defined.  It
+ was
    interpreted as a "" or a 0, but maybe it was a mistake.  To suppre
+ss this
    warning assign an initial value to your variables.
    
Uncaught exception from user code:
        /
    {
    (?:
     (?> [^{}]+ )
     |
     (??{  })
     )*
         }
/: Sequence (??...) not recognized

[download]
The "uninitialized value" warning isn't surprising, since $np is of course uninitialized. It would seem to me that I am screwing up on correctly declaring this variable so that it can be used recursively. While trying to diagnose the problem, I set $np to various values and the error output changed to reflect this. For instance, if I declare $np = "foo"; the error becomes 
Uncaught exception from user code:
        /
    {
    (?:
     (?> [^{}]+ )
     |
     (??{ foo })
     )*
         }
/: Sequence (??...) not recognized

[download]
So it would seem to me that I am screwing up good code with a bad variable declaration. O'Reilly wouldn't lead me astray, right? :-) So after scouring this and other resources for past info on similar problems, I'm finally stumped enough to announce it publicly on this forum. I am using v. 5.005_02, is it possible that the (??{ }) is not present in this version? I was unable to find much technical documentation for match time pattern interpolation. Thanks to everyone for their time and considerations.

In reply to Problem with recursive pattern matching by tshabet

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