# [dc], 39
?dsiZdsl[[]r1-d0<m]dsmxklixzll-2%B*C1+P
Input string is read from STDIN and should be in the form `[yynynynynyyn]`.
dc is not known for its string handling, but we have just enough here to get this to work. The approach here is to count the `n`s, and output `y` if even or `n` if odd. This is done by executing the input string as a macro. `dc` will output `'y' (0171) unimplemented` for all the `y`s and attempt to pop strings and print them for all the `n`s. So first we make sure we have plenty (total input string length) of empty strings `[]` on the stack to pop. Then we execute the input string and see how many `[]` are left on the stack. The original string length is subtracted from this to give the (-ve) total number of `n`s. The rest is arithmetic to do mod 2 and have the output come out right as ASCII `y` or `n`.
?dsi # Read input string, duplicate, store in register i
Zdsl # Get input length, duplicate, store in register l
[ ] # define macro to:
[] # push empty string
r # swap empty string and remaining length
1- # subtract 1 from length
d0 # duplicate and compare with 0
<m # if >0 recursively call this macro again
dsmx # duplicate macro, store in register m and execute
k # discard left-over 0
lix # load input string and execute as macro
z # get stack length
ll- # load string length and subract
2% # mod 2 (result is -ve because difference is -ve)
B* # multiply by 11 ('y' - 'n')
C1+ # add 121 ('y')
P # print result as ASCII char
[Try it online!][TIO-jhi0hjr2]
[dc]: https://www.gnu.org/software/bc/manual/dc-1.05/html_mono/dc.html
[TIO-jhi0hjr2]: https://tio.run/##S0n@/98@pTgzKqU4Jzo6tshQN8XAJjc2pTi3Ijsns6IqJ0fXSNVJy9lQO@D//@jKyjworMyLBQA "dc – Try It Online"