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)$/
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