i had this doubt before and got it cleared here.... this is what i learned....

/m modifier are used to to let $ and ^ match before and after a \n respectively for eg if you do  /^foo/m it would not only match at the beginning but also right after a newline character (ie the second line)...

now for /s modifier it is necessary to know what '.' does usually. it doesn't match the newlines normally ie you have to force it to match the newlines as well for eg  /foo.bar/s would match foo and bar on different lines...