Hi Monks,
I've run into some strange logic, which I'm suspecting is a bug within the regex engine of later Perl versions.
Using libdevel-leak-perl with this simplified code sample:
use strict;
use warnings;

use Devel::Leak;

my $string = "TESTING STRING";

my $count = Devel::Leak::NoteSV (my $handle);

print ($string =~ s/\ STRING//);

undef $string;

Devel::Leak::CheckSV ($handle);

[download]
Testing this using Perl 26 / 30 the code snippet leaks showing: 
new 0x55fd7b6db1e0 : SV = PV(0x55fd7b6dbe60) at 0x55fd7b6db1e0
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x55fd7b713c00 "TESTING STRING"\0
  CUR = 14
  LEN = 16
  COW_REFCNT = 1

[download]
Testing with Perl 16, this does not leak.
It looks like a copy of the input string is being made and stored in a SV but not being cleared up after the regex completes its replace?
My questions are, is this actually a bug within Perl? Am I doing something fundamentally wrong? If it is a fundamental bug are there any work arounds that could avoid leaking?

Striker.

Edit: Adding Perl version information.

Tested and confirmed leaking on:

Ubuntu 16.04
This is perl 5, version 22, subversion 1 (v5.22.1) built for x86_64-linux-gnu-thread-multi

Ubuntu 18.04
This is perl 5, version 26, subversion 1 (v5.26.1) built for x86_64-linux-gnu-thread-multi

Ubuntu 20.04
This is perl 5, version 30, subversion 0 (v5.30.0) built for x86_64-linux-gnu-thread-multi

Tested and not leaking on:

Centos 7
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi

OEL 7
This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi

Additional Edit: Adding full instructions for reproduction on WSL.

Seems like this hasn't been so easy to reproduce elsewhere, although I've seen it on multiple systems. After getting home I spun up a brand new Ubuntu 18.04 Windows Subsystem for Linux version 2, I was able to reproduce it with these steps:

Run an sudo apt update

Install sudo apt install libdevel-leak-perl

Create the file regex_test.pl with the contents: 

use strict;
use warnings;

use Devel::Leak;

my $string = "TESTING STRING";

my $count = Devel::Leak::NoteSV (my $handle);

print ($string =~ s/\ STRING//);

undef $string;

Devel::Leak::CheckSV ($handle);

[download]

Run the test with perl regex_test.pl

Which results in:

new 0x561fa5f971e0 : SV = PV(0x561fa5f97e60) at 0x561fa5f971e0
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x561fa5fc6320 "TESTING STRING"\0
  CUR = 14
  LEN = 16
  COW_REFCNT = 1

[download]

I'm not sure why it seems to be a bit flaky to reproduce, perhaps slight library / distribution differences that cause this to manifest. But thanks to everyone that's had a look at this so far! I'll investigate some more myself and if I do find a solution I'll make sure to add it to the end of this post.

Another, Another Update:

After doing some more testing I saw the posts indicating that the CPAN version behaved differently to the APT version. I believe this is the case although they seem to have the same version number?

If I use the CPAN version I can see the behaviour that the others are seeing where It simply prints:
1
new 000000000072a728 :

[download]
To me this indicates that the CPAN module is "broken" it indicates a leak as noted by the "new 000000000072a728 :" section, but does not print the "debug" information associated with the SV that is being kept. If I switch from the CPAN module back to the APT version I get the full trace of information associated with the SV as previously noted. Thoughts?

In reply to Regex replace leaking memory. by Striker1440

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