I need to validate password entered by user and check if the password contains at least one uppercase and one lowercase char in Dart.

I wrote this String extension:

```dart
extension StringValidators on String {
  bool containsUppercase() {
    // What code should be here?
  }

  bool containsLowercase() {
    // What code should be here?
  }
}

```

And use it like this:

```dart
final text = passwordTextController.text;
final isValid = text.containsUppercase() && text.containsLowercase();

```

Is there any regexp for this purpose? Or it should be plain algorithm? Please help me to find out the elegant way. Thanks!