x <- c("2\tNO  PEMJNUM\t 2\tALTOGETHER HOW MANY JOBS\t216 - 217", 
 "1\tREFERENCE PERSON 2\tSPOUSE 3\tCHILD 4\tOTHER RELATIVE (PRIMARY FAMILY & UNREL) PRFAMTYP\t2\tFAMILY TYPE RECODE\t155 - 156", 
 "5\tUNABLE TO WORK  PUBUS1\t 2\tLAST WEEK DID YOU DO ANY\t184 - 185", 
 "2\tNO  PEIO1COW\t 2\tINDIVIDUAL CLASS OF WORKER CODE\t432 - 433")
sub("^(?:.*\\b)?(\\w+)\\s*\\b2\\b.*", "\\1", x, perl=TRUE)
# => [1] "PEMJNUM"  "PRFAMTYP" "PUBUS1"   "PEIO1COW"

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