in reply to Regex negative word

Just for fun, I decided to try using (?(cond)ptrue[|pfalse]) , a regexp feature I learned just yesterday while reading the Perl Pocket Reference. However, reading perldoc perlre, I noted something the Perl Pocket Reference didn't: this feature is "highly experimental". That's probably why it wasn't used by anyone in the first place. Since this isn't in the first place, no harm done. :-)
/^(?(?!cat).*\bdog)$/