Palindrome Checker

How to use

1. 1Pick a mode: Text for words, phrases, and sentences; Number for positive integers; Word order for phrases whose word sequence mirrors itself.
2. 2In Text mode, set the normalization toggles. Case-insensitive treats A and a as equal; Letters and digits only ignores punctuation and whitespace; Ignore diacritics treats é and e as equal.
3. 3Paste or type your input. Or click any sample button to load a canonical palindrome (racecar, A man a plan a canal Panama, 12321, and others).
4. 4Read the verdict card for the strict and normalized answers. The mirror visualization below shows each character pair and highlights mismatches in red.
5. 5Inspect the longest palindromic substring panel for the longest palindrome contained anywhere in the normalized input, with its position and length.
6. 6Use Copy summary to grab a paste-ready text block with the input, the normalized text, the reversed normalized text, and the verdict for both rules.

About this tool

Palindrome Checker decides whether a word, phrase, sentence, or positive integer reads the same forwards and backwards. It runs every comparison in your browser using two parallel rules so you can see both answers at once: the strict rule, which compares the raw input character by character including spaces, case, and punctuation, and the normalized rule, which folds case, ignores punctuation and whitespace, and optionally folds diacritics so 'A man, a plan, a canal: Panama' qualifies the way it is traditionally read aloud. Three modes are exposed. Text mode handles free-form prose with togglable normalization options. Number mode accepts positive integers (digits 0 to 9 only) and tolerates thousand separators (commas, spaces, underscores) so values copied from spreadsheets or code literals work without manual cleanup. Word-order mode splits the input on whitespace, strips punctuation and case inside each token, and checks whether the sequence of *words* reads the same in reverse order, which is the form Stephen Sondheim and similar wordplay puzzles use. Beyond the yes-or-no answer, the page renders a character-by-character mirror so you can see exactly which pairs match and which break the symmetry, computes the longest palindromic substring inside the normalized text using Manacher's linear-time algorithm (which is the optimal known method for this problem and works on Unicode code points, not just ASCII), and surfaces statistics including the raw character count, the compared character count after normalization, the number of mismatched pairs in both strict and normalized comparisons, the length of the longest palindrome contained in the input, and a symmetry score expressed as the share of mirrored pairs that match. A built-in sample library covers the canonical cases (racecar, level, madam, A man a plan a canal Panama, Was it a car or a cat I saw, No x in Nixon, the numeric palindrome 12321, the four-digit palindrome 9009 which is the largest palindromic product of two two-digit numbers, and a Sondheim-style word-order palindrome) so the tool is also useful as a reference. Useful for English language students confirming a textbook example, writers checking a tagline or brand name, recreational mathematicians counting palindromic primes and palindromic products, coding interview preparation where the longest palindromic substring is a classic question, accessibility writers testing pangrams that double as palindromes, kids learning about word play, and anyone curious whether a name reads the same in reverse. Nothing leaves your device; the input is processed and discarded in memory.

Free to use. Works in your browser. No signup, no login.