Somthing you might find interesting:

Regex.Replace(modifiedInput, "(oo)", "00", RegexOptions.IgnoreCase);

can be written like:

Regex.Replace(modifiedInput, "(?i)oo", "00");

This can also be applied to a regexp group:

Regex.Replace(modifiedInput, "(?i:foo) (bar)", "00");

making only the "foo" part case insensitive.
See for info: https://msdn.microsoft.com/en-us/library/yd1hzczs.aspx

Hope this helps,
Kind regards,
Maurice

"public static BigInteger[] PowersOfTwo(int n)" is public static method with name PowersOfTwo that takes "int n" as argument and has "BigInteger[]" return type, which is array of BigInteger