title: stringr equivalent to grep
body: <p>Is there an <em>stringr</em> equivalent to base R's <code>grep</code> function?</p>
  
  <p>I want to have the index of the string that matches. Example:</p>
  
  <pre><code>grep("F|Y", LETTERS)
  [1]  6 25
  </code></pre>
  
  <p>With <em>stringr</em> my workaround would be using <code>which</code> as follows:</p>
  
  <pre><code>which(str_detect(LETTERS, "F|Y"))
  [1]  6 25
  </code></pre>