The Four Reversal Modes
Reverse characters flips the entire string end to end, turning "hello" into "olleh". Reverse words keeps each word spelled normally but reverses their order, turning "the quick fox" into "fox quick the". Reverse lines keeps each line's content untouched but reverses which line comes first, useful for flipping a list or a block of text top-to-bottom. Upside-down flip swaps each character for a Unicode lookalike glyph that visually resembles the same letter rotated 180 degrees (a becomes ɐ, e becomes ǝ, and so on), then reverses the character order too, so the result reads correctly when the whole line is mentally flipped upside down, the effect used in novelty "upside-down text" social posts.
Handling Emoji and Accented Characters Correctly
A naive character reversal that just flips an array of individual Unicode code points breaks any character actually made of more than one code point acting as a single visual unit, a combining accent mark (like an accented letter written as a base letter plus a separate combining diacritic), an emoji with a skin-tone modifier, or a multi-person ZWJ emoji sequence like a family or couple emoji. Reversing those constituent code points independently scrambles them: an accent ends up floating before its letter instead of on top of it, and a modified or joined emoji falls apart into its separate pieces. Correct reversal needs to treat each of these as one indivisible unit (a grapheme cluster) and reverse the order of whole units, not their raw code points.
How the Upside-Down Flip Works
This mode swaps each character for a different, unrelated Unicode character that just happens to look like the original rotated 180 degrees, it isn't actual image rotation or a font effect, since plain text can't be visually rotated. Not every character has a good upside-down lookalike (some letters, like m or w, only approximate the flipped shape of their counterpart), and characters with no defined mapping pass through unchanged rather than being dropped, so the flipped text stays fully readable even where the illusion isn't perfect.
Common Uses
Upside-down or reversed text is commonly used for novelty social media posts, puzzles, and simple visual ciphers. Reversing text is also a common step in checking whether a string is a palindrome (reads the same forwards and backwards), and reversing line order is useful for flipping a chronological log or list into the opposite order without needing to manually reorder each line.
Limitations
The upside-down flip only covers a fixed set of common Latin letters, digits, and punctuation; characters outside that set (most accented letters, non-Latin scripts, and many symbols) are left unchanged rather than approximated, since there's no good upside-down lookalike glyph available for them in Unicode. Reversal generally isn't reversible into the exact original byte sequence when combined with the flip mode, since flip substitutes different characters entirely rather than just reordering the original ones. Use it to reverse text online as a backwards text generator, an upside down text generator and flip text generator, a way to reverse words online as a reverse text generator, a mirror text online tool, an upside down text converter, and to reverse letters online for any string.