Or without a capture: 

perl -wMstrict -le
"for (@ARGV) {
  print;
  s{ (?<! \\) $ }'\\'xms;
  print
  }
" with\ sans
with\
with\
sans
sans\

[download]

In reply to Re^2: Check last character of a string by Anonymous Monk
in thread Check last character of a string by theroninwins

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