title: .Net Regex for Comma Separated string with a strict format
body: <p>I've got a string that I need to verify for validity, the later being so if:</p>
  
  <ul>
  <li>It is completely empty</li>
  <li>Or contains a comma-separated string that MUST look like this: 'abc,def,ghi,jkl'.</li>
  </ul>
  
  <p>It doesn't matter how many of these comma separated values are there, but if the string is not empty, it must adhere to the comma (and only comma) separated format with no white-spaces around them and each value may only contain ascii a-z/A-z.. no special characters or anything.</p>
  
  <p>How would I verify whether strings adhere to the rules, or not?</p>