Filter operations may be case sensitive.

Filtering files and tags (Searching):

QUOTE (Mp3tag online help)
All operations are non-case sensitive.

Workaround: you can use the case sensitive function $strstr().

Example: find files with " i " (wich must be written in uppercase in English) in title tag.
Filter expression: "$if($grtr($strstr(%title%,' i '),0),1,0)" IS 1

Slower, but more exact way for this special case - it finds "i" at start or end, too:
Filter expression:
"$if($grtr($strstr($regexp(%title%,\b\x{69}\b,lowercase_I),lowercase_I),0),1,0)" IS 1

Not a big problem, but the second expression, sorry, matches "Ku-u-i-po" too.
I tried:
"$if($grtr($strstr($regexp(%title%,[^.-]\b\x{69}\b[^.-],lowercase_I),lowercase_I),0),1,0)" IS 1
...but it didn't work.
It's very interesting because this works:
%title% MATCHES [^.-]\bi\b[^.-]
(I mean, it don't match "Ku-u-i-po" - but, of course, case insensitive.)

Easier with regex switch:

title MATCHES (?-i)i

It doesn't work at me. :frowning: Matches all songs with i in title (about 50 % of all files).

I see you want the i as word or surrounded by spaces..

title MATCHES "(?-i) i "
title MATCHES "(?-i)\bi\b"

Perfect! Thank you, dano.