for a simple question I would prefer a simple solution (maybe not so perfect but easy to understand also to non-regexperts). This does exactly "match y or yes in case-insensitive manner":

while(<>) { 
  print "yes\n" if /(y|yes)/i;
}
[download]

update: above will match everything with a 'y' inside as ikegami observed. Below code does what I meant (this time tested before posting)

foreach (qw( YES YE Y yes ye y ys es s e Yeeessseeseeesssssess)){
  print "$_ = yes\n" if /^(y|yes)$/i;
}
[download]