Excel lastindex of a substring

I think I get what you mean. Let's say for example you want the right-most in the following string (which is stored in cell A1):

Drive:FolderSubFolderFilename.ext

To get the position of the last \, you would use this formula:

=FIND("@",SUBSTITUTE(A1,"","@",(LEN(A1)-LEN(SUBSTITUTE(A1,"","")))/LEN("")))

That tells us the right-most is at character 24. It does this by looking for "@" and substituting the very last "" with an "@". It determines the last one by using

(len(string)-len(substitute(string, substring, "")))len(substring)

In this scenario, the substring is simply "" which has a length of 1, so you could leave off the division at the end and just use:

=FIND("@",SUBSTITUTE(A1,"","@",LEN(A1)-LEN(SUBSTITUTE(A1,"",""))))

Now we can use that to get the folder path:

=LEFT(A1,FIND("@",SUBSTITUTE(A1,"","@",LEN(A1)-LEN(SUBSTITUTE(A1,"","")))))

Here's the folder path without the trailing

=LEFT(A1,FIND("@",SUBSTITUTE(A1,"","@",LEN(A1)-LEN(SUBSTITUTE(A1,"",""))))-1)

And to get just the filename:

=MID(A1,FIND("@",SUBSTITUTE(A1,"","@",LEN(A1)-LEN(SUBSTITUTE(A1,"",""))))+1,LEN(A1))

However, here is an alternate version of getting everything to the right of the last instance of a specific character. So using our same example, this would also return the file name:

=TRIM(RIGHT(SUBSTITUTE(A1,"",REPT(" ",LEN(A1))),LEN(A1)))