is it a valid command.

is it a valid command

Serial.readStringUntil('#' || '@'); actually i am trying to read a string and on the basis of the last char i'll take a decision so pls tell me is this valid or not

Have read the Reference material?
Have you tried it?

AFAIK the answer is NO - you can only search for a single character.

You may be interested in the examples in Serial Input Basics - simple reliable ways to receive data. If you have room to receive the complete message then you can search it to your heart’s content.

If you don’t have space to receive the whole message then you will need to read it one character at a time checking each one.

…R

It will compile ok, and it will read a string until it encounters an ASCII control-A.