You parsed it incorrectly. (I parsed broquaint's regex incorrectly). By just adding whitespace I get the following:

/ C (?: 800 ) |
  (?: (?: 35 |
          29 )
      (?: (?: 50 ) |
          (?: 00XL )
      )
  ) /x
[download]

Which shows that some (?:) were completely useless. This imposes zero runtime overhead so it only really matters because it makes it harder for the next programmer to read. In removing the useless (?:) groups I get.

/ C800 |
   (?: 35 | 29 )
   (?: 50 | 00XL )
   /x
[download]