Palindrome Checker

A palindrome reads the same forwards and backwards: level, racecar, “Was it a car or a cat I saw?”. This checker tells you whether your text is one, ignoring spaces, punctuation, capital letters and accents the way word-game and puzzle rules usually do. It can check a whole list line by line, find the longest palindrome hidden inside a longer text, and turn any word into a palindrome.

How to use this tool

1. Type or paste a word, phrase or sentence, or a list with one item per line.
2. Choose whether to check the whole text, each line, or make a palindrome.
3. Decide what to ignore: spaces and punctuation, case and accents.
4. Read the answer, the text as it was compared and the same text backwards.

Formula

Palindrome if c₁ = cₙ, c₂ = cₙ₋₁, … for the n characters left after normalising

Normalising removes spaces and punctuation, lowers the case and strips accents, depending on your choices. Characters are compared as whole user-perceived characters, so Korean syllables such as 기러기, accented letters and emoji count as one each. The longest palindrome inside the text is found with Manacher’s algorithm, which checks every possible centre in linear time, and is shown in its normalised form. Making a palindrome appends the reverse of the text, either sharing the last letter (race → racecar) or not (ab → abba).

Example

“A man, a plan, a canal: Panama” becomes amanaplanacanalpanama after removing spaces, punctuation and capitals: 21 letters that read the same backwards, so the answer is Yes. In “forgeeksskeegfor”, the longest palindrome inside is geeksskeeg (10 letters).