If you are interested in details of regain or if you want to fix a bug, you'll need the sourcecode.
There are several possibilites to get the sourcecode. You can either download the zipped source of a particular version or you can get the newest developer version directly from the SVN repository.
The zipped sourcecode of every version is available at the files section of the sourceforge project site of regain.
In order to load the source directly from the SVN repository, you have to install a SVN client first.
After that, you can check out the source with the following command:
svn co https://regain.svn.sourceforge.net/svnroot/regain regain
This creates the directory
regain including the whole source.
You can browse the SVN directory at sourceforge.