Hi. I tried out what you sed above. It worked. thanks.. Now what i need to do further is count the occurrences of each of these lines. As you can see in <DATA>, the string "c_2_1_2- + a_123_1_1-" has occurred 2 times and the reverse of it "a_123_1_1- + c_2_1_2-" has occurred once. Now i need to get a cumulative count for this pair (irrespective of the order in which it occurs i.e. as "a_123_1_1- + c_2_1_2-" or as "c_2_1_2- + a_123_1_1-", so that the total count of this entry is =3 as in <DATA>) The actual file which i am working on is similar but is larger in size, and has around 8000 lines. What is the solution to this problem? awk hasn't helped me so far.