Comma delimited row into multiple rows, repeat first value

Shell Programming and Scripting
1

i am building a database to keep track of unix groups.

Using the command "ypcat group"

I get an output similar to the following

group1:GROUP:9999:user1,user2,user3
groupA:GROUP:1111:usera,userb,userc

I want to convert this output so it looks like this

group1:user1
group1:user2
group1:user3
groupA:usera
groupA:userb
groupA:userc

using two sed commands i've gotten to this point

ypcat group | sed 's#:.*:.*:#:\n#g' | sed 's#,#\n#g'
This gets me to here:
group1:
user1
user2
user3
groupA:
usera
userb
userc

I know there is a way to preappend the group lines to the other lines with the hold function in sed but i can't seem to get it right. There is also probably an awk command to accomplish this.
The sed command should look similar to this but it has no affect:

sed '#:#{hd}G'
2 
$
$ cat f28
group1:GROUP:9999:user1,user2,user3
groupA:GROUP:1111:usera,userb,userc
$
$
$ perl -lne 'do {foreach (split /,/,$2) {print $1,":",$_}} if /^(.*?):.*:(.*?)$/' f28
group1:user1
group1:user2
group1:user3
groupA:usera
groupA:userb
groupA:userc
$
$

tyler_durden

3

With awk:

ypcat group |
  awk -F'[:,]' '{
    for (i = 3; ++i <= NF;)
      print $1, $i
    }' OFS=: