title: R regex to find two words same string, order and distance may vary
body: <p>I want to create a single regex (if possible) to search through strings and determine if two words occur in the same string.  I know I can use two <code>grepl</code> statements (as seen below) but am wanting to use a single regex to test for this condition.  The more efficient the regex the better.</p>
  
  <p>I want to find strings that contain both "man" and "dog" case insensitive.</p>
  
  <pre><code>x &lt;- c(
      "The dog and the man play in the park.",
      "The man plays with the dog.",
      "That is the man's hat.",
      "Man I love that dog!",
      "I'm dog tired"
  )
  
  ## this works but I want a single regex
  grepl("dog", x, ignore.case=TRUE)  &amp; grepl("man", x, ignore.case=TRUE) 
  </code></pre>