PHP Warning: Uninitialized string offset 0

[dregad]

I'm seeing many occurrences of the following errors in my web server's logs:

PHP message: PHP Warning:  Uninitialized string offset 0 in .../dokuwiki/lib/plugins/tag/helper.php on line 457; 

They are also reported in Dokuwiki's error log:

2025-06-01 04:39:09     .../dokuwiki/lib/plugins/tag/helper.php(479)
       E_WARNING: Uninitialized string offset 0
  #0 .../dokuwiki/lib/plugins/tag/helper.php(479): dokuwiki\ErrorHandler::errorHandler(2, 'Uninitialized s...', '/home/clients/3...', 479)
  #1 .../dokuwiki/lib/plugins/tag/helper.php(252): helper_plugin_tag->getIndexedPagesMatchingTagQuery(Array)
  #2 .../dokuwiki/lib/plugins/tag/action.php(84): helper_plugin_tag->getTopic('', '', ']')
  #3 .../dokuwiki/inc/Extension/EventHandler.php(80): action_plugin_tag->performTagAction(Object(dokuwiki\Extension\Event), Array)
  #4 .../dokuwiki/inc/Extension/Event.php(75): dokuwiki\Extension\EventHandler->process_event(Object(dokuwiki\Extension\Event), 'BEFORE')
  #5 .../dokuwiki/inc/Action/Plugin.php(31): dokuwiki\Extension\Event->advise_before()
  #6 .../dokuwiki/inc/template.php(120): dokuwiki\Action\Plugin->tplContent()
  #7 [internal function]: tpl_content_core('showtag')
  #8 .../dokuwiki/inc/Extension/Event.php(134): call_user_func_array('tpl_content_cor...', Array)
  #9 .../dokuwiki/inc/Extension/Event.php(200): dokuwiki\Extension\Event->trigger('tpl_content_cor...', true)
  #10 .../dokuwiki/inc/template.php(102): dokuwiki\Extension\Event::createAndTrigger('TPL_ACT_RENDER', 'showtag', 'tpl_content_cor...')
  #11 .../dokuwiki/lib/tpl/dokuwiki/main.php(59): tpl_content()
  #12 .../dokuwiki/inc/actions.php(30): include('/home/clients/3...')
  #13 .../dokuwiki/doku.php(131): act_dispatch()
  #14 {main}

This is with plugin version 2023-10-17, on Dokuwiki 2025-05-14a "Librarian" but the errors were already present before the upgrade (2024-02-06b "Kaos").

[dregad]

Cross-checking with the apache access logs, it appears that some bots are crawling our site and following all links. Here is an (edited) extract at the time of the error mentioned in the opening post:

[01/Jun/2025:04:39:08 +0200] "GET /dokuwiki/doku.php?id=tag&do=showtag&tag=[ HTTP/1.1" 200 3377 "-" "Mozilla/5.0 (compatible; MJ12bot/v1.4.8; http://mj12bot.com/)"
[01/Jun/2025:04:39:09 +0200] "GET /dokuwiki/doku.php?id=tag&do=showtag&tag=] HTTP/1.1" 200 3377 "-" "Mozilla/5.0 (compatible; MJ12bot/v1.4.8; http://mj12bot.com/)"

So the bot is trying to show tags named [ and ].

Searching the site, I noticed that on some pages tags were defined wrapped in square brackets like this {{tag>[ active tier2 ]}}, which renders as shown on the screenshot:

As a workaround, I have updated the offending pages to remove the square brackets, which should prevent the problem from happening again on my site.

But the problem's root cause is due to filtering of special characters in tag names by the plugin's parseTagList() method, which relies on cleanID(). This may result in an empty tag name being included in the tag list, leading to the warning when getIndexedPagesMatchingTagQuery() tries to access $tag[0] on an empty string.

I will provide a PR to fix this.