title: alphanumeric regular expression in R
body: <p>I am trying to use [:alnum:] as explained on ?regex</p>
  
  <p>Anyone knows why   </p>
  
  <pre><code>grepl("^([a-zA-Z0-9])+([;])", x="dj5sads;adsa")
  </code></pre>
  
  <p>returns TRUE, but </p>
  
  <pre><code>grepl("^([:alnum:])+([;])", x="dj5sads;adsa")    
  </code></pre>
  
  <p>returns FALSE?</p>