title: regex pattern to match alternating subpatterns
body: <p>I'm trying to devise a regex pattern (in PHP) which will allow for any alternation of two subpatterns. So if pattern A matches a group of three letters, and B matches a group of 2 numerals, all of these would be OK:</p>
  
  <pre>
  aaa
  aaa66bbb
  66
  67abc
  12abc34def56ghi78jkl
  </pre>
  
  <p>I don't mind which subpattern starts or ends the sequence, just that after the first match, the subpatterns must alternate. I'm totally stumped by this - any advice will be gratefully received!</p>