Hi, Kavy.
Looks to me like both functions return the index of the "search" string from the beginning of the "within" string. The difference is that InstrRev starts looking at the end of the "within" string. So, Instr("This String has back in it near the front and near the back as well","back") gives 17, whilst InstrRev("This String has back in it near the front and near the back as well","back") gives 56.
I suppose you could subtract the result of the InstrRev function from the length of the "within" string and add 1 to get the length from the end of the string.
eta: (I didn't find the VBA Help for InstrRev helpful at all, did you? No example, no "see also" link. It must have been one of the last things added.)
Any help at all?