title: regex to validate strings with numbers and letters
body: <p>I need help to create a regex to validate one string, the string must be 12 characters, they can all be numbers or they can be 11 numbers and a letter, the letter can be anywhere between those numbers.</p>
  
  <p>Some examples:</p>
  
  <p>20710117540C</p>
  
  <p>00A109822346</p>
  
  <p>005109822346</p>
  
  <p>K05109822346</p>
  
  <p>// error cases</p>
  
  <p>KY0510982234</p>
  
  <p>KY05109822345</p>
  
  <p>5505109822345</p>
  
  <p>Thanks!</p>