title: Extract letters from a string in R
body: <p>I have a character vector containing variable names such as <code>x &lt;- c("AB.38.2", "GF.40.4", "ABC.34.2")</code>. I want to extract the letters so that I have a character vector now containing only the letters e.g. <code>c("AB", "GF", "ABC")</code>. </p>
  
  <p>Because the number of letters varies, I cannot use <code>substring</code> to specify the first and last characters.</p>
  
  <p>How can I go about this?</p>