title: Filter only rows that contain exact two strings in a column
body: <p>I have a data.frame as follow:</p>
  <p><code>df = data.frame(sp_name = c(&quot;Xylopia brasiliensis&quot;, &quot;Xylosma tweediana&quot;, &quot;Zanthoxylum fagara subsp. lentiscifolium&quot;, &quot;Schinus terebinthifolia var. raddiana&quot;, &quot;Eugenia&quot;), value = c(1, 2, 3, 4, 5))</code></p>
  <p>Here's the deal: I am only interested in subsetting/filtering the rows from the df that contain <strong>exactly</strong> two words (in my case, Xylopia brasiliensis and Xylosma tweediana). How can I proceed? I'm failing miserably in using the <code>filter</code> function from <code>tidyverse</code></p>
  <p>Thanks already.</p>