Grammer Syntax must be in all upper case for this to work.

ANTLR_Grammer_Case_Insensitive.cs

public class InsenstiveStringStream : ANTLRStringStream 
{
    public InsenstiveStringStream(char\[\] data, int numberOfActualCharsInArray) : base(data, numberOfActualCharsInArray) {}
 
    public CaseInsensitiveStringStream() {}
 
    public CaseInsensitiveStringStream(string input) : base(input) {}
 
    public override int LA(int i) 
    {
        if (i == 0) return 0;
        if (i < 0) i++;
        if (((p + i) - 1) >= n) return (int) CharStreamConstants.EOF;
        return Char.ToUpperInvariant(data\[(p + i) - 1\]);
    }
}