The length of a tab is not defined across applications, so it is not necessarily the same size as 8 spaces. A tab is a single character, usually expressed in most programming languages as "\t".

There are several characters which could come under the heading "white-space", that includes tabs and spaces, but also new-lines "\n", carriage-returns "\r", form-feeds "\f" and sometimes more. If you wish to include all white-space in you regular expression then use the \s short-cut as described by Nikhil Jain, otherwise just use s/\t/,/g. I'm guessing that
I don't want to replace a single character character or any non tab space with comma means you need \t, not \s.