=encoding utf8

=head1 NAME

perldelta - what is new for perl v5.45.4

=head1 DESCRIPTION

This document describes differences between the 5.45.3 release and the 5.45.4
release.

If you are upgrading from an earlier release such as 5.45.2, first read
L<perl5453delta>, which describes differences between 5.45.2 and 5.45.3.

=head1 Security

=head2 CVE-2026-107401 Regex engine incorrectly skipped UTF8 characters

A regular expression performing a greedy match of a UTF8 character could
have incorrectly failed if the first attempt at a match was unsuccessful.
Instead of skipping over all consecutive instances of that I<character>
associated with the first attempt, all consecutive characters that had
the same first I<byte> were skipped.

[L<GH #24916|https://github.com/Perl/perl5/issues/24916>]

=head1 Performance Enhancements

=over 4

=item *

Changes have been made to the "super-linear cache", which is used to avoid
excessive backtracking in some regexes which include quantifiers on
variable-length sub-patterns, such as C<(a|bc)*> and C<(ab+){2,}>. This
change may reduce the amount of backtracking that takes place, and may
reduce the amount of temporary memory used for the cache.

=item *

C<substr()> now updates the UTF-8 position cache correctly, avoiding
repeated scans when accessing the same part of a string. [L<GH #24531|https://github.com/Perl/perl5/issues/24531>]

=back

=head1 Modules and Pragmata

=head2 Updated Modules and Pragmata

=over 4

=item *

L<Archive::Tar> has been upgraded from version 3.12 to 3.14.

=item *

L<Compress::Raw::Bzip2> has been upgraded from version 2.224 to 2.225.

=item *

L<Compress::Raw::Zlib> has been upgraded from version 2.224 to 2.225.

=item *

L<CPAN> has been upgraded from version 2.38 to 2.41.

=item *

L<Devel::PPPort> has been upgraded from version 3.74 to 3.75.

=item *

L<ExtUtils::PL2Bat> has been upgraded from version 0.005 to 0.006.

=item *

L<IO::Compress> has been upgraded from version 2.224 to 2.225.

=item *

L<Module::CoreList> has been upgraded from version 5.20260924 to 5.20261022.

=item *

L<POSIX> has been upgraded from version 2.26 to 2.27.

=back

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages. For the complete list of
diagnostic messages, see L<perldiag>.

=head2 New Diagnostics

=head3 New Warnings

Two warnings in C<malloc.c> which were not previously documented have been
added to L<perldiag>.

=over 4

=item L<Unaligned "next" pointer in the free chain 0x% at 0x%|perldiag/"Unaligned "next" pointer in the free chain 0x% at 0x%">

=item L<Unaligned pointer in the free chain 0x%|perldiag/"Unaligned pointer in the free chain 0x%">

=back

=head1 Platform Support

=head2 Platform-Specific Notes

=over 4

=item VMS

The C<Perl_seed> function will now use the native entropy source
C<sys$get_entropy> when available (OpenVMS 9.2 and later).

=back

=head1 Internal Changes

=over 4

=item *

Locale thread-safety is now enabled by default for the next few v5.45
development releases.

This feature, introduced with default-off in v5.45.3, is never turned on
for builds that don't have threads, and on platforms where perl believes
that the locale handling is natively thread-safe.

Otherwise, the feature is on by default during a portion of the
remaining 5.45 development cycle, so as to test it in the field, but the
intention is to make it opt-in for the official 5.46 release.  To turn
it off, pass this to F<Configure>:

 -Accflags=-DNO_EMULATE_THREAD_SAFE_LOCALES

See L<perllocale/Multi-threaded operation>.


=item *

The inline function C<Perl_newSV_type> now only handles the SV types most
commonly seen at runtime. C<Perl_newSV_type_generic> has been added as
a non-inline function that can handle all SV types. This change reduces
binary bloating and increases the chances of C<Perl_newSV_type> being
successfully inlined.

=item *

When a shifted array is unshifted, C<Perl_av_extend_guts> used to perform
a C<(key > *maxp - 10)> check and always reallocate. That check dates from
Perl 3.0 and the rationale for it seems to have been lost to time. (It may
have been to encourage array growth, but the general levers for doing so
have changed significantly over time.)

Always reallocating when it is not strictly necessary could cause Perl
applications that repeatedly perform a mixture of C<shift>/C<push> operations
(or similar) to suffer from unnecessary allocator thrashing.
C<Perl_av_extend_guts> will now only reallocate if the array really is out
of space and must be extended.

=back

=head1 Selected Bug Fixes

=over 4

=item *

The regular expression optimizer no longer rejects valid matches when a
positive lookahead can match an empty string. For example,
C</\A(?=a*)[0-9]a\z/> now matches C<"0a">.

[L<GH #24943|https://github.com/Perl/perl5/issues/24943>]

=item *

General regular expression repeats clear captures in their bodies when
matching zero repetitions, as optimized repeats do. Adding a no-op
C<(?{})> code block no longer changes whether a backreference to a skipped
optional group matches.

[L<GH #8267|https://github.com/Perl/perl5/issues/8267>] [L<GH #18865|https://github.com/Perl/perl5/issues/18865>]

=item *

Negative lookahead and lookbehind assertions restore capture buffers after
evaluating their bodies. Captures from failed assertion attempts no longer
appear in a successful match or affect later backreferences and conditions.

[L<GH #14387|https://github.com/Perl/perl5/issues/14387>] [L<GH #24946|https://github.com/Perl/perl5/issues/24946>]

=item *

The regular expression optimizer no longer uses the fixed-width repeat
optimization for a body containing captures inside nested quantifiers.
This prevents failed or backtracked iterations from leaving incorrect
capture values. For example, C<"aa" =~ /(?:(aa){2})*/> now leaves C<$1>
undefined.

[L<GH #24945|https://github.com/Perl/perl5/issues/24945>]

=item *

Line number information is stored rather than discarded when an anonymous
subroutine is encountered during compilation. This should improve the
accuracy of line numbers emitted by diagnostic messages and `caller()`
for code following an anonymous subroutine declaration.

Note that (i) this change could break any tests that hardcode the
less-accurate line numbers (ii) further improvements to line number
accuracy may follow within this development cycle.

[L<GH #24396|https://github.com/Perl/perl5/issues/24396>]

=item *

Regular expression compilation could have mistaken the width of a
quantified subpattern (e.g. C<b{2}>) when a potentially infinite
qualifier (e.g. C<+>) occurred earlier in the pattern. This could lead to
viable match candidates at the start of the target string being missed.

[L<GH #24915|https://github.com/Perl/perl5/issues/24915>]

=item *

Following an unsuccessful earlier non-greedy match, a regular expression
intended to perform a separate I<greedy> match could have instead
performed a I<non-greedy> match because of skipped flag clearance.

[L<GH #24926|https://github.com/Perl/perl5/issues/24926>]

=item *

An anchored regular expressions starting with a repeating character could
have incorrectly skipped later instances of that character if the match
contained a C<(*SKIP)> and was initially unsuccessful.

=item *

A anchored regular expression needing zero (or more) characters for a
successful match could have failed because the search space was
incorrectly truncated.

[L<GH #24944|https://github.com/Perl/perl5/issues/24944>]

=item *

A regular expression performing a case insensitive match with Unicode
rules could fail to match correctly when the ability of folding
characters to both decrease I<minlen> and increase I<maxlen> at the
same time was not taken into account.

=item *

The offset of a \K is no longer narrowed on rollback at an offset >2GiB.

=item *

In some cases warnings that occurred while using unpack on an invalid
UTF-8 marked string would result in an invalid free.  [L<GH #24913|https://github.com/Perl/perl5/issues/24913>]

=item *

Builds with C<PERL_NO_COW> could crash when modifying or freeing boolean
values. This has been fixed. [L<GH #19987|https://github.com/Perl/perl5/issues/19987>]

=item *

C<goto> within a defer block should now work more consistently. [L<GH #19240|https://github.com/Perl/perl5/issues/19240>]

=back

=head1 Acknowledgements

Perl 5.45.4 represents approximately 4 weeks of development since Perl
5.45.3 and contains approximately 12,000 lines of changes across 210 files
from 27 authors.

Excluding auto-generated files, documentation and release tools, there were
approximately 5,600 lines of changes to 120 .pm, .t, .c and .h files.

Perl continues to flourish into its fourth decade thanks to a vibrant
community of users and developers. The following people are known to have
contributed the improvements that became Perl 5.45.4:

Alin Iacob, Andreas König, Andrew Hewus Fresh, Brad Smith, Branislav
Zahradník, Chris 'BinGOs' Williams, Craig A. Berry, Cyncrovee, Dan Book,
Darafei Praliaskouski, David Mitchell, Eric Herman, Frank,
GlobalTechStrategics, James E Keenan, Karl Williamson, Leon Timmermans,
Lukas Mai, Paul Evans, Paul Marquess, Philippe Bruhat (BooK), Richard Leach,
Sevan Janiyan, TAKAI Kousuke, Tigranuhy Grigoryan, Tony Cook, Yves Orton.

The list above is almost certainly incomplete as it is automatically
generated from version control history. In particular, it does not include
the names of the (very much appreciated) contributors who reported issues to
the Perl bug tracker.

Many of the changes included in this version originated in the CPAN modules
included in Perl's core. We're grateful to the entire CPAN community for
helping Perl to flourish.

For a more complete list of all of Perl's historical contributors, please
see the F<AUTHORS> file in the Perl source distribution.

=head1 Reporting Bugs

If you find what you think is a bug, you might check the perl bug database
at L<https://github.com/Perl/perl5/issues>. There may also be information at
L<https://www.perl.org/>, the Perl Home Page.

If you believe you have an unreported bug, please open an issue at
L<https://github.com/Perl/perl5/issues>. Be sure to trim your bug down to a
tiny but sufficient test case.

If the bug you are reporting has security implications which make it
inappropriate to send to a public issue tracker, then see
L<perlsec/SECURITY VULNERABILITY CONTACT INFORMATION>
for details of how to report the issue.

=head1 Give Thanks

If you wish to thank the Perl 5 Porters for the work we had done in Perl 5,
you can do so by running the C<perlthanks> program:

    perlthanks

This will send an email to the Perl 5 Porters list with your show of thanks.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view exhaustive details on
what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=cut