How to split a string while ignoring the case of the delimiter?

There's no easy way to accomplish this using string.Split. (Well, except for specifying all the permutations of the split string for each char lower/upper case in an array - not very elegant I think you'll agree.)

However, Regex.Split should do the job quite nicely.

Example:

var parts = Regex.Split(input, "aa", RegexOptions.IgnoreCase);

In your algorithm, you can use the String.IndexOf method and pass in OrdinalIgnoreCase as the StringComparison parameter.

If you don't care about case, then the simplest thing to do is force the string to all uppercase or lowercase before using split.

stringbits = datastring.ToLower().Split("aa")

If you care about case for the interesting bits of the string but not the separators then I would use String.Replace to force all the separators to a specific case (upper or lower, doesn't matter) and then call String.Split using the matching case for the separator.

strinbits = datastring.Replace("aA", "aa").Replace("AA", "aa").Split("aa")