public static bool Foo(ReadOnlySpan<char> span)
{
    for (int i = 0; i < span.Length; i++)
    {
        if (span[i] == '%' && (uint)(i + 2) < (uint)span.Length)
        {
            if (span[i + 1] == 'F' && // Bounds check
                span[i + 2] == 'F')
            {
                return true;
            }
        }
    }
    
    return false;
}