For example, a search for posts by rjlittlefield, containing diffraction blurring without the quotes, returns 75 replies after this post, but only 52 when quotes are added as in "diffraction blurring" .
A search for "diffraction blurring" AND ("underlying issue" OR "pretty happy") quickly finds those two old posts (and a couple of new posts, including this one).
Words or phrases can be excluded by using the minus sign, as in grating -diffraction .
There are some oddities in the facility. As one example, I had expected that grating AND NOT diffraction would also work to exclude a word. But no, that gives the same result as "grating" AND "diffraction", with NOT excluded as too short, too common. As a second example, specifying the query as "diffraction blurring" AND ("underlying issue" OR "pretty happy") provides a result page that says
but then actually specifying that form as a new query finds fewer matches!Searched query: +"diffraction blurring" +("underlying issue" "pretty happy")
I plan to stick with the AND/OR/parenthesis notation that I rashly imagine I understand.
This improved search facility is provided by switching the forum configuration from "phpBB Native Fulltext" to "MySQL Fulltext". I do not know exactly what that means, under the covers, but both facilities are provided -- selectable one at a time -- by the standard phpBB distribution.