Palindrome Checker

Classic palindrome examples

Categories of palindrome

• Character palindromes. The default, same letters forwards and backwards, ignoring spaces.
• Word palindromes. “I did, did I”, each word mirrored as a unit.
• Semordnilaps. Words that form a different word when reversed: “stressed” → “desserts.”
• Numeric palindromes. Any digit sequence that reads the same both ways: 1991, 1881.
• Genomic palindromes. DNA sequences where a region reads the same on both strands (relevant to restriction enzymes).

Programming the check

The core logic in pseudocode:

function isPalindrome(s):
    normalized = lowercase(s)
    normalized = removeAll(normalized, /[^\p{L}\p{N}]/)   // keep letters and digits, any script
    return normalized == reverse(normalized)

This is a standard interview warm-up question. Variations add edge cases: ignore unicode combining marks, handle empty strings, support emoji, respect grapheme clusters.

Common variations on the rule

• Strict mode: count every character including spaces and punctuation. "aba" passes; "a man a plan" fails.
• Alphanumeric only: strip everything that is not a letter or digit. Most casual palindrome checks use this.
• Unicode normalisation: some checkers fold accented forms (NFKD) before comparing, so “café” would match “éfac”.

This tool uses the alphanumeric-only approach: it lowercases, keeps every Unicode letter and digit (any script), and drops everything else. It does not fold accents, so an accented letter must line up with the same accented letter for the text to count as a palindrome.