//i is equally acceptable.

That's why qr/(?<=ss)x/ui fails in versions of Perl without experimental (and buggy) variable look-behinds. /u is added by use feature qw( unicode_strings ); since 5.14, and thus by use v5.14; and higher.

Also, 

$ 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]"

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In reply to Re^2: uc and German eszett "ß" (Unicode standard) by ikegami
in thread uc and German eszett "ß" by LanX

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