I would note that your formula is Volatile... I believe this modification to it would make it non-Volatile and yet still keep it working...That works as Instr not InstrRev though. For the latter you can also use this non-array-entered version:
=LOOKUP(1E+100,SEARCH(A1,MID(B1,ROW(INDIRECT("1:"&LEN(B1))),LEN(A1))),ROW(INDIRECT("1:"&LEN(B1))))