How to parse data in a text file? [closed]

Question

I had a question about altering a text file. I have data in the following format.

1 a,b,c
2 d,e
3 a, l, m,n

Note that some of them have spaces after the coma

Now I have to resort the data in the following format

1 a
1 b
1 c
2 d 
2 e
3 a
3 l
3 m 
3 n

Now my plan is to first parse the data, and hold a count of how many data elements there are for each number. (Ex. for the number 1, there are 3 elements). Then I would move down the data by 3 spaces, starting from the bottom. Kinda like moving an array. (The main concern here is space more than time complexity)

Then I would enter each data element at the top.

My first question is, how exactly would I parse the data in a txt file and indicate delimiters such as the coma and the possible spaces.

Second, having done this, how would I just move down a whole line inside a text file down one?

I am very sorry if my question is convoluted. I will gladly clarify anything. Thank you so much for your time! Please let me know if there is anything i need to add. Also, I'm comfortable in both java and c. So advice on either language is useful.