Palindrome Checker

A palindrome reads the same in both directions: kayak, racecar, 12321. For sentences we ignore everything except letters and digits, so "A man, a plan, a canal: Panama!" counts. The starter only works for lowercase single words.

isPalindrome("kayak")                           // true
isPalindrome("Kayak")                           // true
isPalindrome("A man, a plan, a canal: Panama!") // true
isPalindrome("No 'x' in Nixon")                 // true
isPalindrome("hello")                           // false
isPalindrome("123 45")                          // false
isPalindrome("")                                // true (nothing to mismatch)

Constraints

• Compare only the letters a–z and digits 0–9, case-insensitively. Everything else is ignored.
• An empty string (or one with only punctuation) counts as a palindrome.
• Always return a real boolean: true or false.