Close. You missed the "unless they already have a target" condition specified by the OP. For example,
<a href="http://example.com" target="foo">here</a>
shouldn't change, but it becomes
<a href="http://example.com" target="_target">here</a>

Update: Added example.

In reply to Re^2: Regexp: Match anything except a certain word by ikegami
in thread Regexp: Match anything except a certain word by fraktalisman

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