Palindrome Checker

What does the Palindrome Checker do?

The Palindrome Checker tells you whether a word, phrase, or sentence reads the same forwards and backwards. Paste in your text, hit the button, and the answer comes back instantly: yes or no.

Type "racecar" and you get a yes. Type "hello" and you get a no. Type "A man, a plan, a canal: Panama" and you also get a yes, because the checker is smart enough to ignore the spaces, commas, colons, and capital letters that would otherwise trip up a naive comparison.

That last part is what makes the tool worth bookmarking. A first-attempt palindrome check is usually written as "does the string equal its reverse?" — which works for single words like kayak and falls apart the moment punctuation or mixed case enters the picture. The checker handles the cleanup for you.

What is a palindrome, exactly?

A palindrome is a sequence that reads identically in both directions. The word comes from the Greek palindromos, meaning "running back again." Palindromes show up in three flavors:

• Single-word palindromes — level, radar, civic, noon, refer. These are the easy case. You can verify them by hand in a second.
• Multi-word palindromes — "Taco cat", "No lemon, no melon", "Was it a car or a cat I saw?" These only work once you ignore spaces and punctuation. The letters alone form the palindrome.
• Sentence-length palindromes — "A man, a plan, a canal: Panama", "Madam, I'm Adam", "Doc, note, I dissent. A fast never prevents a fatness. I diet on cod." These are the showpieces. Constructing one that also makes grammatical sense is genuinely hard.

One thing worth nailing down: a palindrome is about letters, not words. "Was it a cat I saw" is a palindrome because reading the letters from right to left gives you the same sequence. The word order reversed would be "saw I cat a it Was", which is not the same thing.

Famous palindromes worth knowing

People have been collecting palindromes for thousands of years. The oldest known one is the Sator Square, a Latin word grid found in the ruins of Pompeii dating to before 79 AD. Here are the ones that show up most often in puzzles, trivia, and conversation:

The longest English palindrome ever published is generally credited to Peter Norvig (yes, the AI researcher), who in 2002 generated a 21,012-word palindrome by computer. Before that, the record was held by Dan Hoey at 540 words. These are constructed palindromes — built rather than discovered — and they read more like word salad than prose. The reigning naturally-readable palindrome is usually said to be Demetri Martin's poem "Dammit I'm Mad," which is 224 words long and makes a passable amount of sense throughout.

How the checker decides

The algorithm is short and worth understanding because it shows up in entry-level coding interviews more than almost any other problem. There are two reasonable ways to do it.

Approach 1: reverse and compare. Strip non-letter characters, lowercase everything, build a reversed copy, and check equality. That's three operations and reads cleanly:

• Clean: "A man, a plan, a canal: Panama" becomes "amanaplanacanalpanama".
• Reverse: "amanaplanacanalpanama" (which happens to be the same).
• Compare: equal, so it's a palindrome.

Approach 2: two pointers from the ends. Put one finger at the start of the cleaned string and one at the end. Compare the characters. If they match, move both fingers inward and repeat. If they ever disagree, stop — it's not a palindrome. If the fingers meet in the middle without a mismatch, it is.

Two-pointers is more memory-efficient because it never builds a second copy of the string. For a five-letter word the difference is meaningless; for a 50,000-character input it matters. The Palindrome Checker uses the reverse-and-compare version because it runs in your browser on text small enough that clarity wins over micro-optimization.

When you'll use it

People reach for a palindrome checker for several distinct reasons, and the same tool serves all of them:

• Homework and puzzles — A surprising number of grade-school worksheets ask kids to find palindromes in lists of words. Pasting the list and checking each one beats writing them out backwards on scratch paper.
• Crossword and word-game prep — Palindrome clues show up in the New York Times crossword multiple times a year. If you suspect an answer is a palindrome, the checker confirms it before you commit to ink.
• Coding interview practice — "Write a function to check if a string is a palindrome" is the canonical warm-up question. Before writing the code, it helps to have a reference implementation to test against. Paste your test cases here, see the expected output, then go validate your own function.
• Verifying you wrote one correctly — Constructing a palindrome is harder than it looks. After you arrange your phrase, the checker confirms whether the spaces and punctuation lined up the way you thought they did.
• Naming things — Brands, products, and characters with palindromic names are memorable. Otto, Anna, Hannah, Nan. If you're picking a name and want to know whether it's symmetric, this is the fastest check.
• Date trivia — Palindromic dates (like 02/02/2020 or 12/02/2021 in DD/MM/YYYY) are mild internet events. The checker validates whether a candidate date qualifies.

Edge cases the checker handles

A few patterns trip up casual palindrome tests but work fine here:

• Mixed case — "Racecar", "RACECAR", and "racecar" all return yes. The checker lowercases before comparing.
• Punctuation — Commas, periods, colons, semicolons, exclamation points, question marks, apostrophes, and quotes are all stripped out before the comparison. "Madam, I'm Adam." is the same as "madam im adam" for checking purposes.
• Spaces — Removed. "taco cat", "tacocat", and "t a c o c a t" all check as palindromes.
• Digits — Kept. "12321" is a numeric palindrome and returns yes. "A1B2B1A" works because the letters and digits both line up symmetrically.
• Empty input — Returns no answer rather than yes. An empty string is technically a palindrome (it reads the same in both directions because it's nothing), but reporting "yes" on no input is more annoying than helpful.

What the checker does not handle: Unicode normalization edge cases. If you paste a string with combining accents (where é is encoded as e plus a separate accent character), the cleanup might miss them. For everyday English, Spanish, French, and similar text this isn't a problem. For Arabic, Hebrew, or text with stacked diacritics, results may vary.

A worked example, step by step

Take the classic: "A man, a plan, a canal: Panama".

1. Lowercase everything: "a man, a plan, a canal: panama"
2. Remove non-letter, non-digit characters: "amanaplanacanalpanama"
3. Reverse the cleaned string: "amanaplanacanalpanama"
4. Compare: the two strings are identical.
5. Result: yes, it's a palindrome.

Now try one that fails: "A man, a plan, a canal: Suez".

1. Lowercase: "a man, a plan, a canal: suez"
2. Clean: "amanaplanacanalsuez"
3. Reverse: "zeuslanacanalpanama"
4. Compare: not equal.
5. Result: no, not a palindrome. (Joke aside — the canal-construction history doesn't override the math.)

Related text tools

The Palindrome Checker fits into a small family of text-analysis micropps that all work the same way: paste, see a result, no signup.

• Reverse Text — If you want to see the literal reversed version of your input (rather than just yes/no), reverse-text shows you the mirror image. Handy for testing palindromes by eye and for novelty uses like signature reversal.
• Character Counter — Useful for spotting whether a candidate palindrome has an odd or even letter count, which affects whether there's a single middle character or two.
• Case Converter — If you're working from a source that's all caps or mixed case and want to normalize it before another tool sees it.
• Vowel Counter — A related novelty check for the vowel-to-consonant ratio in a phrase.
• Word Counter — When you want a full readout of words, characters, sentences, and paragraphs alongside the palindrome check.

Frequently asked questions

Does the Palindrome Checker count spaces and punctuation?

No. The checker strips out spaces, commas, periods, apostrophes, colons, and other punctuation before comparing. This is the standard convention for sentence palindromes — without it, "A man, a plan, a canal: Panama" wouldn't qualify. If you want a strict character-by-character check, you'd need to verify that manually.

Is the check case-sensitive?

No. The checker lowercases everything before comparing, so "Racecar", "RACECAR", and "racecar" all return the same answer. This matches how palindromes are conventionally judged in English.

Does my input get saved or sent anywhere?

No. The check runs entirely in your browser in JavaScript. Nothing is uploaded to any server. Close the tab and the input is gone — useful if you're checking something you'd rather not have logged.

What's the longest palindrome the checker can handle?

Far longer than you'll ever paste. The algorithm runs in linear time, and your browser can hold strings of millions of characters in memory without trouble. Even Peter Norvig's 21,012-word computer-generated palindrome would check in a fraction of a second.

Are numeric palindromes supported?

Yes. "12321", "1001", and "7" all return yes. The checker keeps digits during the cleanup step, so any sequence of letters and numbers that reads the same forwards and backwards qualifies. This makes the tool useful for checking palindromic dates, license plates, and ID numbers.

Does the empty string count as a palindrome?

Technically yes — an empty string reads the same in both directions because there's nothing to read. But the checker returns no answer for empty input, since reporting "yes" on a blank textbox is more confusing than useful.

Why does the checker say my phrase isn't a palindrome when I think it should be?

The most common reason is a typo. Palindromes are unforgiving — one missing or extra letter and the symmetry breaks. The second most common reason is a non-letter character that looks like a letter (a smart quote, an em-dash, a non-breaking space). The checker treats these as punctuation and strips them, but if your source uses unusual characters, paste the result through a plain text editor first to normalize it.