Here is my shell script:

echo ${BUILD_NUMBER};
echo ${SERVER};

sed -e 's/NSString *SERVER = @"[A-Z0-9]*";/NSString *SERVER = @"${SERVER}";/g'

Here is the output when the build runs. Notice the second ${SERVER} is not replaced.

+ echo 98
98
+ echo CI1
CI1
+ sed -e 's/NSString *SERVER = @"[A-Z0-9]*";/NSString *SERVER = @"${SERVER}";/g'