title: Regex - how to match two or four same letters but not three
body: <p>Let's say I have input string of:</p>
  
  <pre><code>"some random text RR some Text RRR RRRR" 
  </code></pre>
  
  <p>and what I need is to match only double R or quadruple R BUT NOT triple R. How would I do that using a regular expression?</p>
  
  <p>EDIT:
  what if the input string is "somerandomtextRRsomeTextRRRanytextRRRRanytext" and I want only match double or quadruple R?</p>