title: R REGEX Match - at least 1 lowercase letter, 1 number, and no special characters at 8 length
body: <p>I'm trying to create a grepl regex in R to match strings that:</p>
  
  <ol>
  <li>Contain 1 or more lowercase letters</li>
  <li>Contain 1 or more numbers</li>
  <li>Only allow lowercase letters (a-z) or numbers, i.e. no spaces, no special
  characters, no other punctuation</li>
  <li>The string must be exactly 8 characters long</li>
  </ol>
  
  <p>However, my attempt so far doesn't yield any luck:</p>
  
  <pre><code>grepl("((?=.*[[:lower:]])(?=.*[[:digit:]])[[:alpha:]]{8})", x, perl=TRUE)
  </code></pre>
  
  <p>Any ideas where I'm going wrong?</p>
  
  <p>Examples of inclusion cases would be: <code>xxxxxxx8, 1234567x, ab12ef78</code></p>
  
  <p>Examples of exclusion cases would be: <code>x!3d5f78, x23456789, Ab123456</code></p>