Negative Look Ahead issue

prasadbabu has asked for the wisdom of the Perl Monks concerning the following question:

Hi Fellow Monks,

Good Day.
A few minutes back while coding I faced the below problem.

I was trying to match all the string till it matches two consecutive new line character. Usually I use negative look ahead format for matching such kind of string. But today I was not able to match the string and I got the below error message since I used 'warnings'.

May I know the reason why I am getting the error, can't I use newline characters for these kind of matching?

The below code and data is sample for testing, not real code and data.

use strict;
use warnings;

my $s = do {local $/, <DATA>};

$s =~ s/\w+\s+\w+\s*\d+\s*\,(?:(?!(?:\n\n).)*)\n\n/'test'/egsi; #getti
+ng error 
#$s =~ s/\w+\s+\w+\s*\d+\s*\,.*?\n\n/'test'/egsi;  #working fine

print $s;

output:
-------
(?!(?:\n\n).)* matches null string many times in regex; marked by <-- 
+HERE in m/
\w+\s+\w+\s*\d+\s*,(?:(?!(?:\n\n).)* <-- HERE )\n\n/ at pract.pl line 
+6.
LMTRILOG     db 5 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 
+, 0 , 0 ,
0;
             db 25 , 1 , 1 , 1 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0
+ , 0 , 0 ,
 0;




__DATA__
LMTRILOG     db 5 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 
+, 0 , 0 , 0;
             db 25 , 1 , 1 , 1 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0
+ , 0 , 0 , 0;
[download]

updated: added __DATA__

Thanks in advance

Prasad

Comment on Negative Look Ahead issue Download Code

Replies are listed 'Best First'.
Re: Negative Look Ahead issue by moritz (Cardinal) on Jul 28, 2008 at 12:38 UTC
`(?!(?:\n\n).)` matches the empty string, and doing that many times (``) doesn't make sense. You probably want `(?:(?!\n\n).)` instead. But it's hard to tell without seeing the expected result. `my $s = do {local $/, <DATA>};` That's pretty useless since you haven't defined any `__DATA__` section.	[reply] [d/l] [select]
Re: Negative Look Ahead issue by olus (Curate) on Jul 28, 2008 at 12:44 UTC
This prints `test`. Changed the lookahead to `((?:(?!\n\n).))` `use strict; use warnings; my $s = do {local $/, <DATA>}; $s =~ s/\w+\s+\w+\s\d+\s\,((?:(?!\n\n).))\n\n/'test'/egsi; print $s; __DATA__ LMTRILOG db 5 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 +, 0 , 0 , 0; db 25 , 1 , 1 , 1 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 + , 0 , 0 , 0;` [download]	[reply] [d/l] [select]

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