Comment search modification

How can I stop comment search from searching before I have completed entering my search term?

For example, I would like to search for "n.b."

No sooner have I entered "n" than search begins looking, wasting time flagging every "n" that appears in every word of every comment.

This is really annoying, and I don't see a preference in the "search" category to prevent this premature search behavior.

Anyone?