title: Match letters in R regex
body: <p>Suppose I run the following</p>
  
  <pre><code>txt &lt;- "client:A, field:foo, category:bar"
  grep("field:[A-z]+", txt, value = TRUE, perl = TRUE)
  </code></pre>
  
  <p>Based on <a href="http://regexr.com" rel="nofollow noreferrer">regexr.com</a> I expected I would get <code>field:foo</code>, but instead I get the entire string. Why is this?</p>