I think there is a very good and a very bad side to this, although the bad side can be mitigated.

The Bad side

The bad side that I see, as others have pointed out, is that someone may pose a pure regex question, when they should actually be asking a more general question. A simple example would be something like why doesn't this regex match this deeply nested xml fragment - In this case someone could easily recognise the real problem, move the node and point the monk to XML::Twig or similar. Not all non-re questions would be as easily identified though, and people might just hedge their bets in phrasing questions thus removing the benefit of having an re specific area.

The Good side

On the other hand, a big plus that I see is that a regex specific area would be relevant to non-Perl people. I can envision regex.perlmonks.org becomming the defacto place for all regex QA, thus helping raise the profile of perlmonks and Perl (which nicely segues to the node: "Popularity of Perl vs. availability of Perl developers").

On balance I think it's worth a try. If it turns out to be a bad idea, all the nodes can just be re-homed into SoPW and the area removed.