Deleting rows with special character

Dear all,
maybe a simple problem but I found no solution for my problem.
I have a matrix Y with 23 000 rows and 220 colums. The entries are "A", "B" or "C".
I want to extract all rows (as a matrix ) of the matrix Y where all entries of a row are (for example) "A".
Is there any solution? I tried the stringr- package but i doesn't work out.
All the best and thanks!
Peter