How to use the palindrome checker
- Type or paste a word, number or sentence into the box.
- Leave Ignore accents ticked to treat an accented letter and its plain form as the same, or untick it to compare them as different letters.
- Read Yes or No at the top, with the letters and digits compared forward and backward.
- Look at the longest palindrome inside the text and the list of palindrome words of three or more letters.
Formula
clean = letters and digits only, in lowercase
palindrome when clean[i] = clean[last - i] for every i
longest inside = grow a mirror outward from every position and keep the longest
Spaces, punctuation and capital letters are ignored, so "A man, a plan, a canal: Panama" is a palindrome. Letters are compared as whole characters, so emoji and letters from other alphabets are handled safely.
Three texts
"A man, a plan, a canal: Panama" becomes the 21 letters amanaplanacanalpanama, which read the same in both directions, so the answer is Yes. The whole text is also the longest palindrome inside it.
"Hello, world" becomes helloworld, which backward is dlrowolleh, so the answer is No. The longest palindrome inside is "owo", with 3 characters. A number such as 12321 is also a palindrome, and the French phrase "Ésope reste ici et se repose" is one only if accents are ignored.
Words, phrases and numbers
A palindrome reads the same backward as forward. Single words such as "level" and "racecar" count, and so do phrases once spaces and punctuation are removed. Numbers can be palindromes too, such as 12321 or dates written as digits.
Single characters and empty strings are trivially palindromes, but the page asks for at least one letter or digit before it gives an answer.
How the search inside text works
To find the longest palindrome inside a text, the page treats every letter and every gap between two letters as a possible center, then grows outward while both sides match. This is fast even for long texts. To flip text yourself and compare, try reverse text, and to count the words in a passage, use the word counter.
Assumptions and limits
- Only letters and digits are compared. Spaces, punctuation and symbols are ignored.
- Case is ignored. Accents are ignored unless you turn the option off.
- Text is limited to 20,000 characters.
- Palindrome words must have at least three letters to be listed, so single letters are not reported.
Frequently asked questions
What is a palindrome?
It is a word, number or phrase that reads the same forward and backward, such as "level", 12321 or "Was it a car or a cat I saw?" once spaces, capital letters and punctuation are ignored.
Does punctuation matter?
Not here. All punctuation and spaces are removed before the comparison, which is the usual way to test sentences.
Why does the page show a longest palindrome inside the text?
Even when the whole text is not a palindrome, it may contain one. For "Hello, world" the longest palindrome inside the letters is "owo".
Are accented letters treated the same as plain ones?
By default yes, so é and e match. Untick the accent option to compare them as different letters.