Parsing OPTARG inside case with regular expression and [[ ]]'s - regex

Question

So, I'm setting up a bash script and want to parse arguments to certain flags using getopts. For a minimal example, consider the a script which has a flag, -M, and it takes y or n as an argument. If I use the following code:
#!/bin/bash
# minimalExample.sh
while getopts "M:" OPTION;
do
case ${OPTION} in
M)
RMPI=${OPTARG}
if ! [[ "$RMPI" =~ "^[yn]$" ]]
then
echo "-M must be followed by either y or n."
exit 1
fi
;;
esac
done
I get the following:
$ ./minimalExample.sh -M y
-M must be followed by either y or n.
FAIL: 1
However, if I use the following code instead
#!/bin/bash
# minimalExample2.sh
while getopts "M:" OPTION;
do
case ${OPTION} in
M)
RMPI=${OPTARG}
if [ -z $(echo $RMPI | grep -E "^[yn]$") ]
then
echo "-M must be followed by either y or n."
exit 1
else
echo "good"
fi
;;
esac
done
I get:
$ ./minimalExample2.sh -M y
good
Why doesn't minimalExample.sh work?

Answer 1

quoting regexp in this context forces a string comparison.
change to
if ! [[ "$RMPI" =~ ^[yn]$ ]]
check following post for more details,
bash regex with quotes?

Answer 2

Why do you need regex here at all? -M y is not the same as -M n, is it? So you definitely will use some statement (case or if) to distinguish one from another.
#!/bin/bash
while getopts "M:" OPTION; do
case ${OPTION} in
M)
case ${OPTARG} in
y)
# do what must be done if -M y
;;
n)
# do what must be done if -M n
;;
*)
echo >&2 "-M must be followed by either y or n."
exit 1
;;
;;
esac
done
Please note >&2 – error messages should be output to STDERR, not to STDOUT.