Fix "unterminated `y' command" errors which happens when running GNU sed

versions between 4.0.9 (approximate) and 4.1.5.  Escaping the square brackets
within the y command prevents the problem altogether.  See the following links
for more information on the y command bug:

  http://osdir.com/ml/editors.sed.user/2006-02/msg00012.html
  http://osdir.com/ml/editors.sed.user/2006-02/msg00013.html