UNIX for Beginners Questions & Answers

Hello Everyone,
I am very new to the world of regular expressions. I am trying to use grep/sed for the following:

Input file is something like this and there are multiple such files:

Code:

abc
1
2
3
4
5
***END***
abc
6
7
8
9
***END***
abc
10
11
12
***END***

I need to be able to edit the text in all of these files so that all the files look like this:

Code:

abc
1,2,3,4,5
***END***
abc
6,7,8,9
***END***
abc
10,11,12
***END***

Kindly help me. Your help is much appreciated!

Moderator's Comments:

Kindly do wrap your samples into CODE TAGS, as per forum rules.