I'd like to see 2 improvements in the "data preview" screen, regarding how whitespace is displayed.
One problem is that leading and trailing spaces are invisible. A possible fix for this would be to put a colored background on the data itself. A fixed-width font would also be helpful.
Another problem is that line breaks are not displayed at all (but do appear in the CSV output).
It's impossible to create a correct regex without knowing the details of the whitespace. I often have to go wading through the page source.