$ perl -Mre=debug -e'qr/[abc\xDF]/ui'
Compiling REx "[abc\xDF]"
~ tying lastbr BRANCH (4) to ender TAIL (15) offset 11
~ tying lastbr BRANCH (1) to ender END (16) offset 15
Final program:
   1: BRANCH (4)
   2:   EXACTFUP <ss> (16)
   4: BRANCH (FAIL)
   5:   ANYOF[ABCabc] (16)
  15: TAIL (16)
  16: END (0)
minlen 1
Freeing REx: "[abc\xDF]"