What stevieb said. Handing someone a bunch of code and saying "This doesn't work. Please figure out how it should work and fix it" is not likely to be productive unless you also hand over a bunch of money.

That said, one note: In the regex
    my @urls=$response_body =~ m{(http://b.thumbs.redditmedia.com/ ... }gi;
the sub-pattern  b.thumbs.redditmedia.com has embedded  . (dot) metacharacters that match anything (except a newline, unless the  /s switch is asserted, which it isn't). Here's the effect:

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $str (qw(aXbXc  a.b.c)) {
   printf qq{for '$str' };
   print $str =~ m{ a.b.c }xms ? 'match' : 'NO match';
   }
"
for 'aXbXc' match
for 'a.b.c' match
[download]

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $str (qw(aXbXc  a.b.c)) {
   printf qq{for '$str' };
   print $str =~ m{ \Qa.b.c\E }xms ? 'match' : 'NO match';
   }
"
for 'aXbXc' NO match
for 'a.b.c' match
[download]

I was just wondering how to parse out the jpgs links from reddit wallpaper page following a commandline fu bash script,just for fun. The main problem is how to reject a match which matches beginning and end but by using .+? matches falsely in the middle.I know you can reject matching letters with [^ahi] type expression.But I dont know how to reject a string in the middle of a large amount of data. The following shows the problem match.

http://b.thumbs.redditmedia.com/HUX1reWBCHSIQunAgKXYkb8nXEXY6cw0cTizkTcEw4U.png

random html etc alot

http://b.thumbs.redditmedia.com/bqYiA dIiTp01k7ca6UIjpWSJqOjHGeTv7JPwko4WrEQ.jpg

Rejecting the png but matching random file names in a mess of data is what Im struggling to do. Thanks for any help.This is just for fun so no sweat.

Have solved problem but I dont understand why one lookahead works and another does not.

use strict;
use warnings;
use WWW::Curl::Easy;
my $curl = WWW::Curl::Easy->new;

$curl->setopt(CURLOPT_HEADER,1);
$curl->setopt(CURLOPT_URL, 'http://www.reddit.com/r/wallpapers.rss');
my $response_body;
$curl->setopt(CURLOPT_WRITEDATA,\$response_body);
# Starts the actual request
my $retcode = $curl->perform;
# Looking at the results...
if ($retcode == 0){
  print("Transfer went ok\n\n");
  my $response_code = $curl->getinfo(CURLINFO_HTTP_CODE);
   my @urls=$response_body =~ m{(http://b.thumbs.redditmedia\.com/(?:(
+?!png).)*?\.jpg)}gi;
  $" ="\n\n";
  print "@urls\n";
} else {
  # Error code, type of error, error message
  print("An error happened: $retcode ".$curl->strerror($retcode)." ".$
+curl->errbuf."\n");
}
[download]

Thank you all that helped.

Hey grasshopper!!!,

Could you please provide us with some sample links, and then some expected output (including both urls that should match, and a couple that should not)?

If indeed you are looking for .jpg files under http://b.thumbs.redditmedia.com/ that do not have png in their names, it might be easiest to just get rid of your lookaheads and lookbehinds

my @urls=$response_body =~ m{(http://b.thumbs.redditmedia.com/.+?\.jpg)}gi;

and then add another line to get rid of the lines in the just created @urls that have png in them. Writing regexes that must match one thing but cannot match another are prone to caveats or are not blazingly intuitive.