s/\t(?=[\t\n])/\t\\N/g;

[download]

But this type of simple regex doesn't handle quoted values that contain adjacent tabs, of course.

To also handle the first field being empty: 

s/(^|\t)(?=[\t\n])/$1\\N/g;

[download]

- tye        

In reply to Re: Regexp: Overlapping matches (?=) by tye
in thread Regexp: Overlapping matches by Anonymous Monk

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