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Excludes a and b.

From

Cover of Troubleshooting OpenVPN

Note

ha@ha ~ $ cat test a b aa bb ab ba i ii abacab backdoor cab test

%TRab(aba)+.,?=

A B

ha@ha ~ $ ha@ha ~ $ grep [^ab] test i ii abacab backdoor cab test

%TRab(aba)+.,?=

A B ha@ha ~ $