How To Reverse Text

Reversing text means writing its characters in the opposite order, so the last becomes the first and the first becomes the last. The operation is its own inverse: reversing a string twice returns the original.

There are three different things people mean by reversing text, and they give different results. Reversing characters flips the whole string, reversing words keeps each word intact and flips their order, and reversing lines flips the order of the lines.

Reversing characters is the strict version. The word stressed becomes desserts, and stop becomes pots. Because the letters are rearranged rather than reordered as units, the result is often unreadable but occasionally, by luck, forms another real word.

A palindrome is a string that reads the same both ways, so reversing it changes nothing. racecar, level and madam are palindromes, and so is the sentence a man a plan a canal panama once the spaces and capitals are removed.

Reversing words is more useful for text. It produces an anastrophe, the figure of speech Yoda is famous for, and it is also a quick way to check that a list is in the order you think it is.

Reversing lines is a practical editing tool. It turns a chronological list into a reverse-chronological one, and it is the fastest way to check whether the last line of a document is really the last.

The subtlety is that a character is not always what it appears to be. A string is stored as a sequence of code units, and reversing those units blindly can split a character in half.

Emoji are the clearest example. Most emoji sit above the basic multilingual plane and are stored as two code units called a surrogate pair. Reversing the two halves separately produces two broken halves that no longer render as the original symbol.

Accented letters have a related problem. If a letter is stored as a base letter followed by a combining accent mark, reversing the units puts the accent before the letter it belongs to, and the accent may then attach to the wrong character.

The fix is to reverse by code point or by grapheme cluster rather than by code unit. Reversing by code point keeps surrogate pairs together, and reversing by grapheme cluster also keeps combined accents and joined emoji intact.

Reversing is not the same as reversing a file or a byte sequence. Bytes have their own order, and reversing the bytes of a UTF-8 string produces mojibake rather than readable reversed text.

A common use is the mirror or upside-down text seen on novelty sites, where each character is replaced by a look-alike and the order is reversed. That is two operations combined, a substitution and a reversal, not a reversal alone.

The calculator reverses the text by character, word or line, and handles surrogate pairs correctly so that emoji survive the round trip. It reports the character counts of the input and the output.