=IFERROR(IF(IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(SEARCH("Banner",AC5),SEARCH("EBL2",AC5)),SEARCH("Movie Art",AC5)),SEARCH("Use as is",AC5)),SEARCH("TTT",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1)<IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(SEARCH("Banner",AC5),SEARCH("EBL2",AC5)),SEARCH("Movie Art",AC5)),SEARCH("Use as is",AC5)),SEARCH("TTT",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1),MID(AC5,IFERROR(IFERROR(IFERROR(SEARCH("Customs",AC5),SEARCH("Custom",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1),IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(SEARCH("Banner",AC5),SEARCH("EBL2",AC5)),SEARCH("Movie Art",AC5)),SEARCH("Use as is",AC5)),SEARCH("TTT",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1)-IFERROR(IFERROR(IFERROR(SEARCH("Customs",AC5),SEARCH("Custom",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1)),MID(AC5,IFERROR(IFERROR(IFERROR(SEARCH("Customs",AC5),SEARCH("Custom",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1),IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(IFERROR(SEARCH("Banner",AC5),SEARCH("EBL2",AC5)),SEARCH("Movie Art",AC5)),SEARCH("Use as is",AC5)),SEARCH("TTT",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1)-IFERROR(IFERROR(IFERROR(SEARCH("Customs",AC5),SEARCH("Custom",AC5)),SEARCH("Generic",AC5)),LEN(AC5)+1))),"")
(source: https://www.quora.com/What-is-the-longest-excel-formula-you-... )