Palindrome Checker

“Is this a palindrome?” hides three smaller questions, and all three are checkboxes here. Does spacing count? Does punctuation? Does capitalisation? The Palindrome Checker opens with all three ignored, the literary convention that lets A man a plan a canal Panama qualify, and each one can be switched back on for the strict answer.

The verdict banner is only half the output. Beneath it a character mirror lays the cleaned string out as tiles, green where a character matches its counterpart from the far end and red where it does not, so a near miss shows you precisely which pair broke it. When the answer is no, a third panel names the longest palindromic run buried inside the string and gives its length.

Six sample chips sit at the foot, five of them palindromes and one that is not, so the failure state can be inspected without inventing an input. Everything is computed in the page as you type.

The Controls and the Two Result Panels

• Ignore case lowercases before the other two filters run, which matters because the cleaned string quoted in the banner is the lowercased one.
• Ignore punctuation discards everything outside letters, digits and whitespace, so Dennis sinned and Dennis, sinned reduce to the same candidate.
• Ignore spaces runs last and removes all whitespace, which is what turns a phrase into a single word to test.
• The Character Mirror renders only for cleaned strings between 1 and 80 characters. Paste a paragraph and the verdict still appears while the tiles do not, because 400 tiles would tell you nothing.
• The Longest Palindromic Substring panel shows only when the verdict is negative and the run is at least 2 characters, so it never competes with a green banner.

The Algorithm Behind the Longest Substring

The search expands around every centre. At each position it tries a centre on the character, then a centre in the gap before the next one, and walks outwards while the two ends agree. That covers odd-length and even-length runs in a single pass and costs O(n²) at worst, which is nothing on the 80-character strings this panel appears for. Tap the rotator engine chip and it answers rotator at 7 characters, having discarded the shorter otato and tat on the way.

What Happens When You Turn the Filters Off

• Untick all three and Racecar fails, because the leading R no longer matches the trailing r. The mirror shows 2 red tiles at the ends and green throughout the middle.
• An empty or whitespace-only field produces no verdict at all. A non-empty trimmed string is required, so clearing the box removes every panel rather than showing a false negative.
• Digits always participate. 2002 qualifies, and 20/02 qualifies too once punctuation is ignored.
• Accented letters are casualties of Ignore punctuation, which keeps only unaccented letters, digits and whitespace, so a word opening on À loses that letter before the check even runs.
• Red tiles always come in pairs, since a mismatch at one position is mirrored at its counterpart. The exact centre of an odd-length string compares against itself and is therefore always green.