The word censor provides the option for restricting censor matches to whole words, or simply match in place.
When matching whole words, this relies on using the \w marker (or is it \W, I forget, either way it's a PCRE control character), which is fine -- until you're in UTF-8 mode.
As covered by http://www.simplemachines.org/community/index.php?topic=363219.msg2612779#msg2612779 and specifically I'll requote the paragraph from the PHP manual on the subject:
QuoteMatching characters by Unicode property is not fast, because PCRE has to search a structure that contains data for over fifteen thousand characters. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE.
Since PCRE does not support \w in UTF-8 mode, the option is actually pointless (since it doesn't work) so the option should be removed when in UTF-8 mode.
Moved to github.
https://github.com/SimpleMachines/SMF2.1/issues/1189