#!/bin/bash
s='4539 DECK AAA                      
OO-99999999-99999999-99999999-99999999 -99999999-99999999   259800  
259800   259800-99999999-99999999 4539 DECK ABC                      
OO-99999999-99999999-99999999-99999999 -99999999-99999999   259800  
259800   259800-99999999-99999999 4539 DECK ABA                      
OO-99999999-99999999-99999999-99999999 -99999999-99999999   259800  
259800   259800-99999999-99999999 4539 DECK ABD                      
OO-99999999-99999999-99999999-99999999 -99999999-99999999   259800  
259800   259800-99999999-99999999'
sed -n '/.*DECK[[:blank:]]*\([[:upper:]]*\).*/s//\1/p' <<< "$s"