title: gsub error turning upper to lower case in R
body: <p>I would like to recode some identifiers, from upper case to lower case. </p>
  
  <p>I am not sure what the issue is here. </p>
  
  <pre><code>n = c('AFD.434', 'BSD.23', 'F234.FF')
  gsub(pattern = '[[:upper:]]', replacement = '[[:lower:]]', n)
  
  [1] "[[:lower:]][[:lower:]][[:lower:]].434" "[[:lower:]][[:lower:]][[:lower:]].23"  "[[:lower:]]234.[[:lower:]][[:lower:]]"
  </code></pre>
  
  <p>Any advice?</p>