Indeed. My take on the matter is ... what this person wants to get is:   “this-or-that part of ‘a domain name.’” And it so happens that the “obvious way” to get there is... “well, (obviously) I have to (figure out how to) write a regular expression” before I'll be able to “get that.”

And I'm reaching over and hitting the Pause button and saying... “oh, really?” I'll bet that there's a very good CPAN-module already out there which will give you the answer that you want. I'll wager that in fact you don't have to tackle regular-expressions in order to get the result you want, because somebody out there has already done it and has done it very well.

Dictum Ne Agas — “Do Not Do A Thing Already Done.”