Procmail regexp question

I'd like to filter messages with one string but not another in the
,subject line.  For example, with "cat" but not "dog", so
        "Subject: My cat"
is accepted but
        "Subject: My cat hates dogs"
and
        "Subject: My dog"
        "Subject: My goat"
are not.

It's not hard in the full regexp.  For example see:

http://stackoverflow.com/questions/2078915/a-regular-expression-to-exclude-a-word-string

However, procmail's regexp is limited, so how to do it?

Jon
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