Ignored characters treat as word boundaries

This is causing near constant warnings:

Warning: mb_eregi_replace(): mbregex compile err: end pattern with unmatched parenthesis in Drupal\pathauto\AliasCleaner->cleanString() (line 260 of modules/contrib/pathauto/src/AliasCleaner.php).
Drupal\pathauto\AliasCleaner->cleanString('Publisher') (Line: 318)
Drupal\nasdaq_datalayer\NasdaqMetatags->getMetatags() (Line: 65)
nasdaq_datalayer_page_attachments(Array) (Line: 297)
Drupal\Core\Render\MainContent\HtmlRenderer->invokePageAttachmentHooks(Array) (Line: 273)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 117)

Fixed constant warnings.

Hope this patch fixes the issue.

Problem

When punctuation characters are set to "Do nothing" (kept as-is in aliases), the ignored words regex used \b as word boundary, which treats those punctuation characters as boundaries.
This caused ignored words adjacent to kept punctuation to be incorrectly stripped (e.g., "a.b" with "a" ignored and . kept would become ".b").

Fix applied to AliasCleaner::cleanString():

1. Track kept punctuation characters in $kept_punctuation using preg_quote()
2. Build a custom word boundary ($wb) using lookaround assertions that treat kept punctuation as part of words, wrapped in a non-capturing group to avoid conflicts with the | alternation in the ignored words regex
3. Force preg_replace instead of mb_eregi_replace when kept punctuation exists, since POSIX ERE doesn't support lookarounds

Code review of MR #127

The commit fixes the issue where punctuation characters configured to "Do nothing" (kept in aliases) were incorrectly treated as word boundaries by the ignored words regex. For example, "a.b" with "a" as an ignored word and "." kept would become ".b" because \b treats punctuation as a word boundary.

Changes

AliasCleaner::cleanString() — The fix tracks which punctuation characters are kept as-is using preg_quote(). When kept punctuation exists, the standard \b word boundary is replaced with a custom lookaround pattern that treats kept punctuation as part of words rather than boundaries: (?:(?<![\w...])(?=[\w...])|(?<=[\w...])(?![\w...])). Since POSIX ERE does not support lookarounds, the fix also forces preg_replace instead of mb_eregi_replace when custom boundaries are needed, and adds the /u (unicode) modifier.

Kernel test — testIgnoredWordsWithKeptPunctuation covers the key scenarios:

• "a.b" stays "a.b" when period is kept (the core bug).
• Standalone ignored words are still removed ("this thing with that thing" → "thing-thing").
• Ignored words not adjacent to kept punctuation are still stripped ("a test thing" → "test-thing").
• Multiple kept punctuation characters work correctly.

The fix is correct and well-targeted. The custom word boundary pattern only activates when there are kept punctuation characters, so the default behavior with \b and mb_eregi_replace is preserved when no punctuation is kept.

This looks closely related or even overlapping with #3311669: Punctuation processed before replacing strings

It's related but quite different in fact. AFAIUI at least ;)