I would be a little nervous of that composed regex approach. I think it requires quite a bit of confidence to be able to split regex like that, and to be certain that when re-combined the full expression remains valid. [Note: not every regex variant supports in-line comments.]
I prefer just to accept that regex can be complex, but also can be extremely useful. I would prefer just to precede them with a few lines of comment, describing the pattern they are trying to match, and any exceptions that I had to account for. I think it is pointless trying to describe them in detail because, if they ever needed revising, I know I will have to start from scratch anyway />/>
On the topic of useful tools, I use rubular. Has support for capturing groups and allows me to see how my pattern does against various lines of text.