Help - How to unwind an SRS address?

Hi,

Please can someone give me a regex or *simple* instructions on how to
reconstruct the real senders email address from an SRS one (pref
without installing anything)?

And yes - I've looked at the docs, but there's loads and loads of
detail - *so* much that I can't figure out easily how to do this with
any confidence that it's going to work properly or indefinitely!

Many thanks to anyone who can help!

Kind Regards,
Chris Drake