preg match< / p >

function is_password_valid($password) {
  if (preg_match("/^(?=.*[A-Z])(?=.*[a-z])(?=.*\d)[a-zA-Z\d]{8,}$/",$password)) {
    echo "password valid";
  } else {
    echo "password not valid";
  }
}

is_password_valid("Password123");