title: Regex finding aa but not aaa with grep
body: <p>Can't figure out the regex pattern for matching aa but not aaa.</p>
  
  <pre><code>x &lt;- c("ab", "abc", "abcc", "abccc", "abcccc", "abccccc")
  
  grep(pattern="c{2,3}", x, value=TRUE, perl=TRUE)
  ## [1] "abcc"    "abccc"   "abcccc"  "abccccc"
  
  grep(pattern="^((?!c{4,}).)*$", x, value=TRUE, perl=TRUE)
  ## [1] "ab"    "abc"   "abcc"  "abccc"
  </code></pre>
  
  <p>But what's the pattern to yield:</p>
  
  <pre><code>grep(pattern=..., x, value=TRUE, perl=TRUE)
  ## [1] "abcc"  "abccc"
  </code></pre>