Block Style Variations: Simplify block style variation selector regex

[aaronrobertshaw]

What?

Backports Gutenberg PR WordPress/gutenberg#79924 to Core, tidying up the regex used to add a block style variation's class (e.g. .is-style-custom) to a block's selector in WP_Theme_JSON.

Trac ticket: https://core.trac.wordpress.org/ticket/65588

Why?

The regex is hard to follow and its first capture group is effectively dead. It only matches a literal :(...), which isn't valid selector syntax (functional pseudo-classes like :is() and :where() always have a name between the colon and the parenthesis), so $matches[1] is always empty. The second group does all the work, so dropping the dead group gives identical output and is much easier to follow.

How?

• Simplifies the pattern from /((?::\([^)]+\))?\s*)([^\s:]+)/ to /[^\s:]+/ and adds a comment with worked examples.
• Keeps the replacement callback rather than switching to backreferences, so the variation name is always treated as a literal (no chance of $/\ being interpreted).

The :is selector-list test case from the Gutenberg change already exists in Core's suite (tests/phpunit/tests/theme/wpThemeJson.php), so no test change was needed here.

Testing Instructions

This is a behaviour-preserving change, so the output should be identical to trunk.

1. Confirm the test_get_block_style_variation_selector PHP tests pass

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[andrewserong]

This matches the GB PR and is already covered by tests, so this LGTM and I don't think it needs a ticket annotation since it's effectively a no-op. Good idea adding the extra comment to clarify the expected matches.

LGTM

[ramonjd]

Matches WordPress/gutenberg#79924, which I tested. Test pass.

[t-hamano]

I would appreciate it if you could commit this PR in time for the 7.1 Beta1 release on July 15.

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Props aaronrobertshaw, andrewserong, ramonopoly, wildworks.

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[ramonjd]

I would appreciate it if you could commit this PR in time for the 7.1 Beta1 release on July 15.

I'll get it in 👍🏻 Thanks @t-hamano

[ramonjd]

Committed in r62700 / 3f456ac