function to search for a string in a comma delimiter string

My strings can be one of the following as a comma delimiter string

AA,AU

AA, AU

aa,au

aa, au

And my search text is let's say AU or au, I want to search to see if it exists in my strings or not, case insensitive.

I tried B7900007 - CheckforStringMatch but no luck.

now if my string is AAAU and my search text is AU, it should not return true that it found the string because AU is not the same as AAAU. So it has to somehow takes the delimiter into consideration.

Do you know of a business function that I can use?