More code BUT definitely more optimal than 99% of the solutions here. Optimal is the way !!

Nice simple 1 liner BUT certainly not optimal.

Most solutions here parse through the entire collection and are far from optimal. What if the collection contained a file with 10 trillion numbers and the offending number was 4th on the list or worse 1st on the list. Why parse a collection that big only to discover the offending number was 1st on the list ?

Rather a few more lines of code.

Nice solution. Good one. I'm totally rubbish at regex and used Linq instead. Your solution actually taught me a thing or two I could not find anywhere.

An extra test case would rule out some of the solutions. So if you input "jack and jill" the answer would be "Jack And Jill". But if I input "jack aNd jill" the answer should still be "Jack And Jill". Some of the solutions dont pass this test. Or am I understanding the requirement incorrectly ?

This does not work if the input is "jack aNd jill". it does not convert the N to a lower case letter.