RegExp validation for password explained code example

Example: RegExp validation for password explained

r'^
  (?=.*[A-Z])       // should contain at least one upper case
  (?=.*[a-z])       // should contain at least one lower case
  (?=.*?[0-9])      // should contain at least one digit
  (?=.*?[!@#\$&*~]) // should contain at least one Special character
  .{8,}             // Must be at least 8 characters in length  
$