title: difference between [0-9] n times and [0-9]{n} in R regexp
body: <p>Both are supposed to the best of my knowledge to be the same but I actually see a difference, look at this minimal example from <a href="https://stackoverflow.com/questions/28725115/r-regex-capture-numbers-in-string-and-replacing-them-in-another-column-captu/28725655">this question</a>:</p>
  
  <pre><code>a&lt;-c("/Cajon_Criolla_20141024","/Linon_20141115_20141130",
  "/Cat/LIQUID",
  "/c_puertas_20141206_20141107",
  "/C_Puertas_3_20141017_20141018",
  "/c_puertas_navidad_20141204_20141205")
  
  sub("(.*?)_([0-9]{8})(.*)$","\\2",a)
  [1] "20141024"    "20141130"    "/Cat/LIQUID" "20141107" "20141018"   
  [6] "20141205"   
  
  sub("(.*?)_([0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9])(.*)$","\\2",a)
  [1] "20141024"    "20141115"    "/Cat/LIQUID" "20141206" "20141017"   
  [6] "20141204" 
  </code></pre>
  
  <p>What am I missing? Or is this a bug?</p>