title: regex for multiple imei validation
body: <p>i am making a regex for imei, but i want it should optionally take multiple imei, example
  123456789123456 this will be accepted and if commo i.e , will be at the end then it should also allow comma but only after the 15th digit not before the 15th digit,
  i want it should validate this</p>
  
  <p>123456789123456 </p>
  
  <p>and if comma is added it should only at after the 15th digit and if comma added it should validate like</p>
  
  <p>123456789123456,123456789123456,123456789123456</p>
  
  <p>a comma between all the 15th digit will be accepted not before 15th digit
  such multiple imei
  i had made this
  ^[0-9]{15,15}|[,]$
  but it is not working as it allows comma , before the 15th digit, which i dont want, how can i modify my regex? or i want to change the whole regex?
  i am using such code</p>
  
  <pre><code>         Regex regex = new Regex("^[0-9]{15,15}|[,]$");
          if (regex.IsMatch(textBox2.Text))
          {
              return false;
          }
          else
          {
              return true;
          }
  </code></pre>