Atbash cipher

The Atbash cipher reflects the alphabet onto itself: 'a' maps to 'z', 'b' to 'y', 'c' to 'x', and so on (uppercase maps the same way, 'A' to 'Z'). Apply Atbash to a string, transforming every letter while preserving its case and leaving all other characters unchanged. For example, 'abc xyz' becomes 'zyx cba', and 'Hello!' becomes 'Svool!'. Applying the cipher twice returns the original text.