I think, I've found a bug in search algorithm at all latest releases of UE, including v17.30.0.1002.

Example.
I have a letter "Я" (Russian code page) and try to find "я" (Match Case check box is OFF), but UE cant find it.
The same is if I want to find lower case "я" by using UPPER case "Я" and with ANY Russian alphabet.
In short: Match Case option in "find/replace" doesn't work with Russian code page (may be other than Latin, I have not check other than Russian).

PS: Sorry for my broken English.

I see this problem also in 17.30.0.1002. I have a Unicode file with Russian letters and when I search for them, they are not found.

I can confirm this issue as well. The Russian character is not searched case insensitive within a Unicode or ANSI file using code page 1251 with UE v17.30.0.1002 on Windows XP SP3 x86. Word 2007 on same computer finds 'Я' and 'я' when searching not case sensitive for 'Я' or 'я'.

Interesting is what is displayed in the title bar of message box opened when the search does not find the character anymore until end of file (with Continue find at End of File unchecked). With a Unicode file the message box title is:

Search string '?' not found.

With an ANSI file using code page 1251 the message box title is:

Search string 'ß' not found.

The character 'Я' has in code page 1251 the hexadecimal value 0xDF like the character 'ß' in code page 1252 (Latin I). I have German set in Windows Regional and Language settings as default code page. It looks like the Windows default code page is always used instead of the code page of the file, at least for the string display in the title of the message box.

Please report this issue by email to IDM support as this is a user-to-user forum.