Regex.IsMatch - Apply On Byte Or String Level?

I would like to know if the result of RegEx.IsMatch is affected by encoding. I am checking if a string is contained within another one by using a regular expression pattern. I am 99.9 % sure the pattern is correct, so my question is... The matching test with the Regex.IsMatch, is applied on "byte level" or "string level"?

This one is the Output... TEΣT
This one is the Word to match... ΤΕΣΤ
And here is the pattern...
If Regex.IsMatch(Output, "(?<=^||s)" & Regex.Escape(Word) & "(?=s||$)") Then
'dooooo
end if