Warning found in post-thumbnail-template.php on PHP 8.0.14

[SierraTR]

Warning thrown when displaying post category page that includes thumbnail images.
WP 5.9 RC2 PHP 8.0.14, not browser-specific, not theme-specific

Warning: preg_match(): No ending delimiter '/' found in ..../wp-includes/post-thumbnail-template.php on line 198

[audrasjb]

Assigning to milestone 5.9 for further investigation.

[costdev]

The warning occurs in other PHP versions as well. See ​preg_match.

---

The regex in question:

'/(^|&)loading='

It's simply missing a trailing /.

---

"Why are we only finding this out now?"

No tests hit the condition on Line 198. See ​the report (this is subject to change, of course). As a result, the faulty preg_match() is never triggered and so the warning is not thrown.

This is why tests should at least achieve line coverage for all new code.

---

To trigger the warning (+hint for unit tests):

1. Switch to Twenty Twenty-One.
2. Create a page and add a Featured Image.
3. Add this to page.php:

<?php

if ( have_posts() ) : while ( have_posts() ) : the_post();
        get_the_post_thumbnail(
                get_the_ID(),
                'post-thumbnail',
                'does_not_contain_loading=true'
        );
endwhile; endif;

4. Load the page.
5. You should see this warning:

Warning: preg_match(): No ending delimiter '/' found in /wp-includes/post-thumbnail-template.php on line 198

[costdev]

Add missing / to regex.

[audrasjb]

You're right, I'm seeing this now and this is a high priority, thanks @costdev.

The issue was introduced in [52065].
The proposed patch fixes the issue.

Self-assigning for commit asap in 5.9.

Trac ticket: https://core.trac.wordpress.org/ticket/54815

[audrasjb]

In 52574:

Media: Add a missing / in post thumbnail lazy loading regex.

This change avoids a warning thrown by a missing slash in a post thumbnail lazyload related regular expression.

Follow-up to [52065].

Props SierraTR, audrasjb, costdev.
Fixes #54815.

[audrasjb]

Reopening for backport.

We'll need a second committer sign-off before backporting this change to 5.9.

Pinging @adamsilverstein and/or @flixos90 for second sign-off as they worked on [52065].

Committed in https://core.trac.wordpress.org/changeset/52574

[flixos90]

The change in [52574] looks good to me - thank you @SierraTR for catching it and @costdev for the fix!

@audrasjb Would you like me to backport or are you going to do that?

[audrasjb]

Thanks! I’ll backport but we needed a second committer sign off :)

[audrasjb]

In 52579:

Media: Add a missing / in post thumbnail lazy loading regex.

This change avoids a warning thrown by a missing slash in a post thumbnail lazyload related regular expression.

Follow-up to [52065].

Props SierraTR, audrasjb, costdev, flixos90.
Merges [52563] to the 5.9 branch.
Fixes #54815.