[5.x] Fix case insensitive Comb search for UTF-8 characters

[heidkaemper]

This pull request fixes an error in the Comb search.

The search normally operates case insensitive. To ensure that this also works with UTF-8 characters (e.g. german umlauts), the regex patterns require the "u" modifier:
https://www.php.net/manual/en/reference.pcre.pattern.modifiers.php

We had this problem with a german website. Lets say there is the word “Ökoland” indexed. A search for “Öko” returns the correct result. But a search for “öko” (lowercase) does not. This has now been resolved with the modifier.