DB<1> $a="abcdabcd"

   DB<2> p $a =~ /[0-9a-f]{4|8}/

   DB<3> p $a =~ /[0-9a-f]{4}/
1
   DB<4> p $a =~ /[0-9a-f]{8}/
1

##</code><code>##

   1: BRANCH(15)
   2:   ANYOF[0-9a-f](13)
  13:   EXACT <{4>(18)
  15: BRANCH(18)
  16:   EXACT <8}>(18)
  18: END(0)