Zend certified PHP/Magento developer

Prepending Text To Every Line Within Capture Group

What I have:

X
(
1
2
3
)
BREAK

Y
(
4
5
)
BREAK

Z
(
6
7
8
9
)
BREAK

What I’d like:

X,1
X,2
X,3

Y,4
Y,5

Z,6
Z,7
Z,8
Z,9

I’d prefer to figure out how to do this using RegEx in Notepad++.
As I understand it, I’d need something like this in the Find text box:

^(.*?)((.*?)).*?BREAK

with Search Mode being Regular Expression and . matches newline selected.

But I don’t know what to enter in the Replace section of the Find/Replace input box.

How can I specify that I want to replace every instance of rn between the two brackets with the previously captured character (X, Y or Z)?